Exam #1 - Bohr Prep

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Statistics

Breakdown

Acids & Bases 3/3

Atomic & Molecular Structure 2/3

Chemical Equilibria 0/2

Chemical Kinetics 1/2

Gases 2/2

Laboratory Techniques 2/2

Liquids & Solids 1/3

Nuclear Reactions 1/1

Oxidation Reduction Reactions 0/2

Periodic Properties 1/2

Solutions 2/3

Stoichiometry & General Concepts 2/3

Thermodynamics & Thermochemistry 1/2

Summary

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Question 1 of 30

1. Question
Which of the following acids is strongest? The acid is followed by its \( pK_a \) value
- A. \( HClO_2 \quad pK_a= 1.96 \)
- B. \( HF \quad pK_a= 3.14 \)
- C. \( HCN \quad pK_a= 9.21 \)
- D. \( HClO_3 \quad pK_a< 1 \)
- E. \( HCOOH \quad pK_a= 3.75 \)
Correct

The \( pK_a \) is a measure of the strength of acids and the higher value means the weaker acid. According to the value of \( pK_a \), \( HClO_3 \) is the Strong acid and completely dissociate in aq solution.

Incorrect

The \( pK_a \) is a measure of the strength of acids and the higher value means the weaker acid. According to the value of \( pK_a \), \( HClO_3 \) is the Strong acid and completely dissociate in aq solution.
Question 2 of 30

2. Question
Which diagram shows the titration of \( H_3PO_4 \)?
- A.
- B.
- C.
- D.
- E.
Correct

The acid and base equivalence point is a point at which the moles of the titrant and analyte are equal. The first equivalence point of H₃PO₄ corresponds to the conversion of H₃PO₄ to H₂PO₄^– Ka1and the second equivalence point corresponds to the conversion of H₂PO₄^– to HPO₄ ^2- Ka2and the third equivalence point corresponds to the conversion HPO₄^2- to PO₄^3-. Ka3

Incorrect

The acid and base equivalence point is a point at which the moles of the titrant and analyte are equal. The first equivalence point of H₃PO₄ corresponds to the conversion of H₃PO₄ to H₂PO₄^– Ka1and the second equivalence point corresponds to the conversion of H₂PO₄^– to HPO₄ ^2- Ka2and the third equivalence point corresponds to the conversion HPO₄^2- to PO₄^3-. Ka3
Question 3 of 30

3. Question
Which of the following acids will have the strongest conjugate base?
- A. \( HCl \quad pK_a < 1 \)
- B. \( H_3PO_4 \quad pK_a = 2.12 \)
- C. \( HClO_2 \quad pK_a = 1.96 \)
- D. \( HF \quad pK_a = 3.14 \)
- E. \( HCN \quad pK_a = 9.21 \)
Correct

The acid with the strongest conjugate base will be the weakest acid of the group. According to the value of pK_a, HCN is the only weak acid listed among the choices.

Incorrect

The acid with the strongest conjugate base will be the weakest acid of the group. According to the value of pK_a, HCN is the only weak acid listed among the choices.
Question 4 of 30

4. Question
Which of the following compounds is polar?
- A. CCl4
- B. PCl5
- C. CO2
- D. H2CO
- E. CH4
Correct

Formaldehyde is a polar compound. One end of the molecule has slightly positive charge. The other end has lightly negative charge.
Carbon dioxide in a non-polar compound. Both end of the molecule are slightly negative in charge.

Incorrect

Formaldehyde is a polar compound. One end of the molecule has slightly positive charge. The other end has lightly negative charge.
Carbon dioxide in a non-polar compound. Both end of the molecule are slightly negative in charge.
Question 5 of 30

5. Question
Which of the following is the correct bond angle found in BH3?
- A. \( 120^o \)
- B. \( 109.5^o \)
- C. \( 180^o \)
- D. \( 107^o \)
- E. \( 105^o \)
Correct

As can be seen, the BH3 bond angle will be 120 degrees since it has a trigonal planar molecular geometry.

Incorrect

As can be seen, the BH3 bond angle will be 120 degrees since it has a trigonal planar molecular geometry.
Question 6 of 30

6. Question
Which of the following compounds is an ionic solid?
- A. CSCl – NH3
- B. MgBr2 – NH3
- C. CSCl – MgBr2
- D. MgBr2
- E. NH3
Correct

In chemistry, an ionic compound is a chemical compound composed of ions held together by electrostatic forces termed ionic bonding. Ionic Solids are solids composed of oppositely charged ions. They consist of positively charged cations and negatively charged anions .When Ionic Solids are dissolved in water the cations and the anions separate, they become free to move about in the water allowing the solution to conduct electrical current.
For cesium chloride, you could draw a simple diagram showing the arrangement of the chloride ions around each cesium ion.

The above mentioned content is applied about MgBr2 too.

Incorrect

In chemistry, an ionic compound is a chemical compound composed of ions held together by electrostatic forces termed ionic bonding. Ionic Solids are solids composed of oppositely charged ions. They consist of positively charged cations and negatively charged anions .When Ionic Solids are dissolved in water the cations and the anions separate, they become free to move about in the water allowing the solution to conduct electrical current.
For cesium chloride, you could draw a simple diagram showing the arrangement of the chloride ions around each cesium ion.

The above mentioned content is applied about MgBr2 too.
Question 7 of 30

7. Question
A mixture of O₂, SO₂, and SO₃ contains 0.077 M SO₂, 0.077 M SO₃ and 0.087M O₂ in a 1 L container. What is the equilibrium constant of the following reaction?

2SO₂(g)+O₂(g)⇌2SO₃(g)
- A. 1.80
- B. 0.90
- C. 1.15
- D. 0.090
- E. 0.115
Correct

The equilibrium constant is calculated by the equation below

K_c = [SO₃]² / [SO₂]² [O₂]

Kc= [0.077]²/ [0.077]² [0.087] Kc= 1.15

Incorrect

The equilibrium constant is calculated by the equation below

K_c = [SO₃]² / [SO₂]² [O₂]

Kc= [0.077]²/ [0.077]² [0.087] Kc= 1.15
Question 8 of 30

8. Question
If the K_sp of Fe(OH)₃ is \( 4\times 10^{-38} \), calculate the molar solubility of Fe(OH)₃ in a pH = 5.0 buffer solution.
- A. 4×10 ^-11
- B. 4×10 ^-14
- C. 1.6×10 ^-14
- D. 1.6×10 ^-11
- E. 1.0×10 ^-14
Correct

we must first calculate the [OH¯]. To do this, we will use the pH and acid base concepts. Then, we will use the K_sp expression to calculate the K_sp

Using pOH = -log ₁₀ ([OH^–])    pH + pOH = 14.0

pOH = (14.0 – 5.0) = 9.0 and [OH^–] = 1× 10-9 M

The solubility equilibrium and product are

Fe(OH)₃(s)⇌ Fe³⁺(aq) + 3OH^–(aq)   K_sp = [Fe3⁺][OH^–]³

\( [Fe^{3+}] =\frac{[Ksp]}{[OH-]3}= 4 \times 10^{-38}/( 1\times 10^{-9})^3 = 4×10^{-11} M \)

Incorrect

we must first calculate the [OH¯]. To do this, we will use the pH and acid base concepts. Then, we will use the K_sp expression to calculate the K_sp

Using pOH = -log ₁₀ ([OH^–])    pH + pOH = 14.0

pOH = (14.0 – 5.0) = 9.0 and [OH^–] = 1× 10-9 M

The solubility equilibrium and product are

Fe(OH)₃(s)⇌ Fe³⁺(aq) + 3OH^–(aq)   K_sp = [Fe3⁺][OH^–]³

\( [Fe^{3+}] =\frac{[Ksp]}{[OH-]3}= 4 \times 10^{-38}/( 1\times 10^{-9})^3 = 4×10^{-11} M \)
Question 9 of 30

9. Question
Regarding the following reaction :

\( N_2 O_5 (g)→4 NO_2 (g)+ O_2 (g) \)

During 80 seconds, 9.2 g of \( NO_2 \) gas is produced. What is the average rate of \( NO_2 \) in terms of mole/min?
- A. 0.17
- B. 0.21
- C. 0.15
- D. 0.12
- E. 0.10
Correct

\[ R_{NO2}=\frac{(9.2/46 mol)}{(80/60 min)}=0.15mole.min^{(-1)} \]

Incorrect

\[ R_{NO2}=\frac{(9.2/46 mol)}{(80/60 min)}=0.15mole.min^{(-1)} \]
Question 10 of 30

10. Question
What is the overall order of a particular reaction that has a rate constant of 1.3 × 10^-4M^-1 S^-1?
- A. 1
- B. 3
- C. 4
- D. 2
- E. 0
Correct

The unit for K is M^-1 S^-1, so the rate of the reaction is calculated according to the equation below: Rate= K [A]². The overall reaction is a second-order reaction.

Incorrect

The unit for K is M^-1 S^-1, so the rate of the reaction is calculated according to the equation below: Rate= K [A]². The overall reaction is a second-order reaction.
Question 11 of 30

11. Question
If a student intends to prepare 50ml of a 0.100 M solution of HCl from 37% hydrochloric acid, how many liters of HCL does the student need?
- A. 0.25
- B. 0.04
- C. 0.42
- D. 0.35
- E. 0.44
Correct

\[ 50 ml \times \frac{0.1mol}{1lit} \times \frac{1lit}{1000ml} \times \frac{100gr}{37 gr} \times \frac{37 gr}{1 mol} \times \frac{1ml}{1.18gr}= 0.42 ml \]

Incorrect

\[ 50 ml \times \frac{0.1mol}{1lit} \times \frac{1lit}{1000ml} \times \frac{100gr}{37 gr} \times \frac{37 gr}{1 mol} \times \frac{1ml}{1.18gr}= 0.42 ml \]
Question 12 of 30

12. Question
Which of the following is NOT correct about solutions?
- A. A solution is a special type of heterogeneous mixture composed of two or more substances.
- B. A solute is a substance dissolved in another substance, known as a solvent.
- C. The mixing process of a solution happens at a scale where the effects of chemical polarity are involved, resulting in interactions that are specific to solvation.
- D. The solution assumes the phase of the solvent when the solvent is the larger fraction of the mixture, as is commonly the case.
- E. The concentration of a solute in a solution is the mass of that solute expressed as a percentage of the mass of the whole solution.
Correct

A solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. The mixing process of a solution happens at a scale where the effects of chemical polarity are involved, resulting in interactions that are specific to solvation. The solution assumes the phase of the solvent when the solvent is the larger fraction of the mixture, as is commonly the case. The concentration of a solute in a solution is the mass of that solute expressed as a percentage of the mass of the whole solution. The term aqueous solution is when one of the solvents is water.

Incorrect

A solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. The mixing process of a solution happens at a scale where the effects of chemical polarity are involved, resulting in interactions that are specific to solvation. The solution assumes the phase of the solvent when the solvent is the larger fraction of the mixture, as is commonly the case. The concentration of a solute in a solution is the mass of that solute expressed as a percentage of the mass of the whole solution. The term aqueous solution is when one of the solvents is water.
Question 13 of 30

13. Question
What is the concentration of nitrate (NO3-) in each of the following aqueous solutions respectively?

1) 0.25 M KNO₃

2) 0.10 M Al (NO₃)₃

3) 0.20 M Ca(NO₃)₂
- A. 0.25 M, 0.30 M, and 0.40 M
- B. 0.35 M, 0.56 M, and 0.40 M
- C. 0.12 M, 0.30 M, and 0.45 M
- D. 0.25 M, 0.65 M, and 0.78 M
- E. 0.42 M, 0.77 M, and 0.45 M
Correct

\[ NO_3^{-1}=\frac{0.25 \text{ mol } KNO3}{L} \times \frac{1 { \text{ mol }} NO3^{-1}}{\text{mol }KNO3}=0.25 \text{ M} \]

\[ NO_3^{-1}=\frac{0.10 \text{ mol Al} (NO3)3}{L} \times \frac{3 { \text{ mol }} NO3^{-1}}{\text{mol Al }(NO3)3}=0.30 \text{ M} \]

\[ NO_3^{-1}=\frac{0.20 \text{ mol Ca} (NO3)2}{L} \times \frac{2 { \text{ mol }} NO3^{-1}}{\text{mol Ca }(NO3)2}=0.40 \text{ M} \]

Incorrect

\[ NO_3^{-1}=\frac{0.25 \text{ mol } KNO3}{L} \times \frac{1 { \text{ mol }} NO3^{-1}}{\text{mol }KNO3}=0.25 \text{ M} \]

\[ NO_3^{-1}=\frac{0.10 \text{ mol Al} (NO3)3}{L} \times \frac{3 { \text{ mol }} NO3^{-1}}{\text{mol Al }(NO3)3}=0.30 \text{ M} \]

\[ NO_3^{-1}=\frac{0.20 \text{ mol Ca} (NO3)2}{L} \times \frac{2 { \text{ mol }} NO3^{-1}}{\text{mol Ca }(NO3)2}=0.40 \text{ M} \]
Question 14 of 30

14. Question
If 0.50 mol of an ideal gas occupies a volume of 2.50 l at 25oC, calculate the pressure of the gas.
- A. 7 atm
- B. 4.9 atm
- C. 3.2 atm
- D. 4.5 atm
- E. 6.2 atm
Correct

The pressure of an ideal gas can be calculated by the equation below

PV= nRT

At first, we have to convert 25⁰C to kelvin T= 25.0 + 273= 298 K

V= 2.50 l , n= 0.50 mol , \( R= 0.0821 \frac{l\text { } atm}{mol\text { } K} \)

\[ P=\frac{n RT}{V}=\frac{0.50mol \times 0.0821 \frac{l\text { } atm}{mol\text { } K} \times 298K}{2.50l}=4.89 \]

Rounding off 4.89 to 4.9 atm.

Incorrect

The pressure of an ideal gas can be calculated by the equation below

PV= nRT

At first, we have to convert 25⁰C to kelvin T= 25.0 + 273= 298 K

V= 2.50 l , n= 0.50 mol , \( R= 0.0821 \frac{l\text { } atm}{mol\text { } K} \)

\[ P=\frac{n RT}{V}=\frac{0.50mol \times 0.0821 \frac{l\text { } atm}{mol\text { } K} \times 298K}{2.50l}=4.89 \]

Rounding off 4.89 to 4.9 atm.
Question 15 of 30

15. Question
What will be the volume of an ideal gas at a constant temperature when the pressure of the gas is increased from 0.8 atm to 1.6 atm and its initial volume is 1.5 L?
- A. 1.5 l
- B. 1.2 l
- C. 1 l
- D. 0.75 l
- E. 0.85 l
Correct

The pressure of a gas is inversely proportional to its volume when the temperature is constant. If the temperature of a system remains the same over a given time. It is used in Boyle’s law

P₁V₁= P₂V₂

P₁ and V₁=initial pressure and volume

P₂ and V₂= final pressure and volume

0.8(atm) × 1.5 (l) = 1.6 (atm) × X (l) X= 0.75 l

Incorrect

The pressure of a gas is inversely proportional to its volume when the temperature is constant. If the temperature of a system remains the same over a given time. It is used in Boyle’s law

P₁V₁= P₂V₂

P₁ and V₁=initial pressure and volume

P₂ and V₂= final pressure and volume

0.8(atm) × 1.5 (l) = 1.6 (atm) × X (l) X= 0.75 l
Question 16 of 30

16. Question
Which of the following elements will have the greatest reactivity with chlorine?
- A. Be
- B. Sr
- C. Ba
- D. Mg
- E. Ca
Correct

Metals have fewer electrons in their outer shell compared to non-metals and hence in reactions, they give away their outermost electrons to the non-metals, as it is easier to get rid of few electrons than many. As we work our way down group 2, each successive element has an increasing number of shells and hence an increased atomic radius, so the outermost electrons experience less attraction to the nucleus and can be transferred to the non-metal element more easily.

Incorrect

Metals have fewer electrons in their outer shell compared to non-metals and hence in reactions, they give away their outermost electrons to the non-metals, as it is easier to get rid of few electrons than many. As we work our way down group 2, each successive element has an increasing number of shells and hence an increased atomic radius, so the outermost electrons experience less attraction to the nucleus and can be transferred to the non-metal element more easily.

Question 17 of 30

17. Question

Which of the following descriptions is incorrect for the chemical tool mentioned for it?

	Description	Tool
1	Measure the volume of a liquid	Graduated cylinder
2	Guide liquid or powder into small opening	Funnel
3	Use for mixing or heating chemicals	Test tube
4	Measure mass accurately	Flask
5	Transport a measured volume of liquid	pipette

A. 1
B. 2
C. 3
D. 4
E. 5

Correct

Flask is commonly used for containing liquid and performing mixing, heating, cooling, precipitation, condensation, and other processes.

Incorrect

Flask is commonly used for containing liquid and performing mixing, heating, cooling, precipitation, condensation, and other processes.

Question 18 of 30

18. Question
Determine A, B and C in the following phase diagram.
- A. A: Gas B: Liquid C: Solid
- B. A: Solid B: Liquid C: Gas
- C. A: Gas B: Solid C: Liquid
- D. A: Liquid B: Solid C: Gas
- E. A: Liquid B: Gas C:Solid
Correct

The stability of phases can be predicted by the chemical potential, in that the most stable form of the substance will have the minimum chemical potential at the given temperature and pressure. This can be summarized in a phase diagram like the one shown below.

Incorrect

The stability of phases can be predicted by the chemical potential, in that the most stable form of the substance will have the minimum chemical potential at the given temperature and pressure. This can be summarized in a phase diagram like the one shown below.
Question 19 of 30

19. Question
Which of the following is the strongest of the inter-molecular forces?
- A. Metallic bond
- B. Ionic bond
- C. Polar covalent bond
- D. Nonpolar covalent bond
- E. A and B
Correct

Incorrect
Question 20 of 30

20. Question
In which salt [that is crystallized in the NaCl system] the difference of measured and experimental values of lattice energy is higher than the others?
- A. NaCl
- B. KI
- C. NaF
- D. CsF
- E. B and C
Correct

By decreasing the covalent property of ionic composition, the experimental value of lattice energy will be more. It has more difference with its measured value because the covalent property of CsF is less than the others. Therefore we can expect that the lattice energy of CsF is higher than the others.

Incorrect

By decreasing the covalent property of ionic composition, the experimental value of lattice energy will be more. It has more difference with its measured value because the covalent property of CsF is less than the others. Therefore we can expect that the lattice energy of CsF is higher than the others.
Question 21 of 30

21. Question
What is the missing particle in the following nuclear reaction?

²³⁹₉₃Np → ²³⁹₉₄Pu + X
- A. ¹₀n
- B. ⁴₂He
- C. ⁰₁e
- D. ¹₁H
- E. ⁰_-1e
Correct

All nuclear reactions must be balanced in mass number and atomic number. To balance the equation above for mass, charge, and mass number, the second nucleus on the right side must have atomic number 0 and mass number -1; it is therefore also ⁰_-1e.

Incorrect

All nuclear reactions must be balanced in mass number and atomic number. To balance the equation above for mass, charge, and mass number, the second nucleus on the right side must have atomic number 0 and mass number -1; it is therefore also ⁰_-1e.
Question 22 of 30

22. Question
Which of the following metals is the strongest reducing agent?
- A. Mg²⁺ + 2e^- → Mg E_reduction= -2.372 V
- B. Zn²⁺ + 2e → ZnE_reduction = -0.763 V
- C. Li⁺ + e^- → Li E_reduction = -3.0401 V
- D. Fe ²⁺ + 2e^- → Fe E_reduction = -0.440 V
- E. Cu²⁺ + 2e^- →Cu _Ereduction = 0.337 V
Correct

Reduction potential (E_h) is the measurement of the tendency of an environment to oxidize substrates. Strong reducing agents can be said to have high electron-transfer potential. Therefore Li is the strongest reducing agent.

Incorrect

Reduction potential (E_h) is the measurement of the tendency of an environment to oxidize substrates. Strong reducing agents can be said to have high electron-transfer potential. Therefore Li is the strongest reducing agent.
Question 23 of 30

23. Question
Which of the following statements is INCORRECT regarding electrolytic cells?
- A. It drives a non-spontaneous redox reaction.
- B. Reduction occurs at the cathode.
- C. Anions are attracted to the cathode.
- D. The cations gain electrons from the cathode.
- E. The anions give up electrons to the anode.
Correct

Fig. An electrolytic cell

An electrolytic cell is an electrochemical cell that drives a non-spontaneous redox reaction through the application of electrical energy. They are often used to decompose chemical compounds.

Galvanic cells compared to electrolytic cells:
In an electrolytic cell, the current is passed through the cell by an external voltage, causing an otherwise nonspontaneous chemical reaction to proceed. In a galvanic cell, the progress of a spontaneous chemical reaction causes an electric current to flow. An equilibrium electrochemical cell is at the state between an electrolytic cell and a galvanic cell. The tendency of a spontaneous reaction to push a current through the external circuit is exactly balanced by an external voltage that is called a counter electromotive force or counter e.m.f. so that no current flows. If this counter voltage is increased the cell becomes an electrolytic cell and if it is decreased the cell becomes a galvanic cell.

Incorrect

Fig. An electrolytic cell

An electrolytic cell is an electrochemical cell that drives a non-spontaneous redox reaction through the application of electrical energy. They are often used to decompose chemical compounds.

Galvanic cells compared to electrolytic cells:
In an electrolytic cell, the current is passed through the cell by an external voltage, causing an otherwise nonspontaneous chemical reaction to proceed. In a galvanic cell, the progress of a spontaneous chemical reaction causes an electric current to flow. An equilibrium electrochemical cell is at the state between an electrolytic cell and a galvanic cell. The tendency of a spontaneous reaction to push a current through the external circuit is exactly balanced by an external voltage that is called a counter electromotive force or counter e.m.f. so that no current flows. If this counter voltage is increased the cell becomes an electrolytic cell and if it is decreased the cell becomes a galvanic cell.
Question 24 of 30

24. Question
According to the periodic law, when elements are arranged in order of increasing _____________, there is a periodic repetition of their _____________ properties.
- A. atomic number, chemical
- B. atomic mass, chemical
- C. atomic mass, physical and chemical
- D. atomic number, physical and chemical
- E. atomic charge, chemical
Correct

When Mendeleev’s table is ordered by the atomic number, the discrepancies within this table disappear. Tellurium has an atomic number of 52, while iodine has an atomic number of 53. So even though tellurium does indeed have a greater atomic mass than iodine, it is properly placed before iodine in the periodic table. Mendeleev and Moseley are credited with being most responsible for the modern periodic law: when elements are arranged in order of increasing atomic number, there is a periodic repetition of their chemical and physical properties. The result is the periodic table as we know it today. Each new horizontal row of the periodic table corresponds to the beginning of a new period because a new principal energy level is being filled with electrons. Elements with similar chemical properties appear at regular intervals, within the vertical columns called groups.

Incorrect

When Mendeleev’s table is ordered by the atomic number, the discrepancies within this table disappear. Tellurium has an atomic number of 52, while iodine has an atomic number of 53. So even though tellurium does indeed have a greater atomic mass than iodine, it is properly placed before iodine in the periodic table. Mendeleev and Moseley are credited with being most responsible for the modern periodic law: when elements are arranged in order of increasing atomic number, there is a periodic repetition of their chemical and physical properties. The result is the periodic table as we know it today. Each new horizontal row of the periodic table corresponds to the beginning of a new period because a new principal energy level is being filled with electrons. Elements with similar chemical properties appear at regular intervals, within the vertical columns called groups.
Question 25 of 30

25. Question
Which of the following is correct about electronegativity?
- A. Electronegativity is a measure of the ability of an atom to attract the electrons when the atom is part of a compound.
- B. Electronegativities generally increase as you move from left to right across a period in the periodic table.
- C. Electronegativities generally decrease as you move from top to bottom down a group in the periodic table.
- D. Fluorine has the highest electronegativity value.
- E. All of the above
Correct

Electronegativities generally increase from left to right across a period. This is due to an increase in nuclear charge. Alkali metals have the lowest electronegativities, while halogens have the highest. Because most noble gases do not form compounds, they do not have electronegativities. Note that there is little variation among the transition metals. Electronegativities generally decrease from top to bottom within a group due to the larger atomic size.

Incorrect

Electronegativities generally increase from left to right across a period. This is due to an increase in nuclear charge. Alkali metals have the lowest electronegativities, while halogens have the highest. Because most noble gases do not form compounds, they do not have electronegativities. Note that there is little variation among the transition metals. Electronegativities generally decrease from top to bottom within a group due to the larger atomic size.
Question 26 of 30

26. Question
An unknown compound contains 2.82g Na, 4.35g Cl and 7.83g O. What is the empirical formula of the compound?
- A. NaCl₂O
- B. NaClO₄
- C. Na₂ClO₂
- D. NaClO
- E. NaClO₃
Correct

\[ \text{mol }Na=\frac{2.82 \text{ g }Na}{23 \text{ g }/mol}=0.1226 \]

\[ \text{mol }Cl=\frac{4.35 \text{ g }Cl}{35.45 \text{ g }/mol}=0.1227 \]

\[ \text{mol }O=\frac{7.83 \text{ g }O}{16 \text{ g }/mol}=0.4894 \]

We should divide the numbers above by the lowest number of moles.

\[ \text{ for } Na=\frac{0.1226}{0.1226}=1 \\ \text{ for } Cl=\frac{0.1227}{0.1226} \approx 1 \\ \text{ for } O=\frac{0.4894}{0.1226}=4 \]

The empirical formula is NaClO₄

Incorrect

\[ \text{mol }Na=\frac{2.82 \text{ g }Na}{23 \text{ g }/mol}=0.1226 \]

\[ \text{mol }Cl=\frac{4.35 \text{ g }Cl}{35.45 \text{ g }/mol}=0.1227 \]

\[ \text{mol }O=\frac{7.83 \text{ g }O}{16 \text{ g }/mol}=0.4894 \]

We should divide the numbers above by the lowest number of moles.

\[ \text{ for } Na=\frac{0.1226}{0.1226}=1 \\ \text{ for } Cl=\frac{0.1227}{0.1226} \approx 1 \\ \text{ for } O=\frac{0.4894}{0.1226}=4 \]

The empirical formula is NaClO₄
Question 27 of 30

27. Question
What is the atomic mass of element X when 3.625 g of element X combined with 1.476 g of oxygen to form X₂O₃?
- A. 56.8 amu
- B. 61.5 amu
- C. 59 amu
- D. 63 amu
- E. 57.5 amu
Correct

\[ \text{ mass of }X =\text{ mass of oxygen } \times \frac{1}{\text{ molar mass of O }} \times \frac{\text{ 2 mol X} }{\text{ 3 mol O}} \times \text{molar mass of X} \]

We should substitute 3.625 g for mass of X and 1.476 g for mass of O

\[ 3.625 g = 1.476 g × \frac{1}{16 g/mol} × 2/3 × \text{ molar mass of X } \\ \text{ Molar mass } X \approx 59 \]

Incorrect

\[ \text{ mass of }X =\text{ mass of oxygen } \times \frac{1}{\text{ molar mass of O }} \times \frac{\text{ 2 mol X} }{\text{ 3 mol O}} \times \text{molar mass of X} \]

We should substitute 3.625 g for mass of X and 1.476 g for mass of O

\[ 3.625 g = 1.476 g × \frac{1}{16 g/mol} × 2/3 × \text{ molar mass of X } \\ \text{ Molar mass } X \approx 59 \]
Question 28 of 30

28. Question
A compound of carbon and hydrogen contains 92.3% C and has a molar mass of 78.1 g/mol. What is its molecular formula?
- A. C₆H₁₂
- B. C₅H₁₂
- C. C₄H₁₂
- D. C₆H₆
- E. C₅H₁₀O
Correct

We start with a 100-g sample.

We divide each element’s percentage (converted to grams) by its atomic mass:

\[ C:\frac{92.3 g}{12g/mol}=7.69 \\ H:\frac{7.7 g}{1g/mol}=7.7 \]

To establish a whole-number ratio of carbon to hydrogen, divide each factor by the smallest factor. The ration of atoms is 1 to 1

\[ C:\frac{7.69}{7.69}=1 \\ H:\frac{7.7}{7.69}\approx 1 \]

The empirical formula is CH. Since the molecular mass of the compound is 78.1 amu, the sum of the mass of 1C and 1H in atomic mass units (12amu + 1amu = 13amu) must be equal to 78.1 amu. To find this number, divide 78.1 amu by 13 amu:

\( \frac{78.1}{13}=6 \text{ so the molecular formula is }C_6H_6 \)

Incorrect

We start with a 100-g sample.

We divide each element’s percentage (converted to grams) by its atomic mass:

\[ C:\frac{92.3 g}{12g/mol}=7.69 \\ H:\frac{7.7 g}{1g/mol}=7.7 \]

To establish a whole-number ratio of carbon to hydrogen, divide each factor by the smallest factor. The ration of atoms is 1 to 1

\[ C:\frac{7.69}{7.69}=1 \\ H:\frac{7.7}{7.69}\approx 1 \]

The empirical formula is CH. Since the molecular mass of the compound is 78.1 amu, the sum of the mass of 1C and 1H in atomic mass units (12amu + 1amu = 13amu) must be equal to 78.1 amu. To find this number, divide 78.1 amu by 13 amu:

\( \frac{78.1}{13}=6 \text{ so the molecular formula is }C_6H_6 \)
Question 29 of 30

29. Question
Calculate ΔH for NO₂ by using the following data.

NO (g) + O₃ (g) → NO₂(g) + O₂(g)     ΔH= – 198.9 KJ

O₃ (g) → 3/2 O₂ (g)     ΔH= – 142.3 KJ

O₂ (g) → 2O (g)    ΔH= + 495.0 KJ
- A. -430.9 KJ
- B. +350.3 KJ
- C. -495.0 KJ
- D. -103.2 KJ
- E. -304.1 KJ
Correct

The enthalpy of formation (or heat of formation) ∆H_fis the enthalpy change associated with forming 1 mole of NO₂   from its constituent elements in their standard state

NO (g) + O (g) → NO₂(g)

We can use Hess’s law to calculate the heat of formation of NO_2.

The law says: if a reaction is carried out in a number of steps, ∆H for the overall reaction is the sum of ∆H for each individual step. We want NO2 on the product side, so we reverse the second reaction. The sign of ΔH is also changed. The third reaction multiply ½

NO _(g) + O_{3 (g)} → NO_{2 (g)} + O_{2 (g)}          ∆H = -198.9 kJ

3/2 O_{2 (g)} → O_{3 (g)}                               ∆H = +142.3 kJ

O _(g) → ½ O_{2 (g)}                                 ∆H = -495.0/2 = -247.5 kJ

overall      NO _(g) + O _(g) → NO_{2 (g)}

∆H = (-198.9) + 142.3 -274.5 = -304.1 kJ

Incorrect

The enthalpy of formation (or heat of formation) ∆H_fis the enthalpy change associated with forming 1 mole of NO₂   from its constituent elements in their standard state

NO (g) + O (g) → NO₂(g)

We can use Hess’s law to calculate the heat of formation of NO_2.

The law says: if a reaction is carried out in a number of steps, ∆H for the overall reaction is the sum of ∆H for each individual step. We want NO2 on the product side, so we reverse the second reaction. The sign of ΔH is also changed. The third reaction multiply ½

NO _(g) + O_{3 (g)} → NO_{2 (g)} + O_{2 (g)}          ∆H = -198.9 kJ

3/2 O_{2 (g)} → O_{3 (g)}                               ∆H = +142.3 kJ

O _(g) → ½ O_{2 (g)}                                 ∆H = -495.0/2 = -247.5 kJ

overall      NO _(g) + O _(g) → NO_{2 (g)}

∆H = (-198.9) + 142.3 -274.5 = -304.1 kJ
Question 30 of 30

30. Question
Which of the following processes indicate an increase in entropy?

a) N₂ _(s)+ 3H₂_(g) → 2NH₃_(g)

b) NaClO₃ _(s) → Na⁺_(aq) + ClO₃^–_(aq)

c) COCl₂ ₍_g₎→ CO_(g) + Cl_{2 (g)}
- A. a and b
- B. a and c
- C. b and c
- D. a
- E. c
Correct

Entropy is roughly related to the disorder, and gases are more disordered than the other states.

Reaction 1: The number of particles in the system decreases, i.e. there are four moles of gas reactants and only 2 moles of gas products.so the entropy is on the decline.

Reaction 2: The number of particles in the system increases, i.e. the single reactant dissociates into two ion particles. In addition, the ions in the ionic solid are organized in a rigid lattice structure whereas the ions in aqueous solutions are free to move randomly through the solvent. So the entropy is in a decrease.

The number of particles in the system increases. i.e. there is one mol of gas reactant and 2 moles of gas products. The disorder is increased more.

Incorrect

Entropy is roughly related to the disorder, and gases are more disordered than the other states.

Reaction 1: The number of particles in the system decreases, i.e. there are four moles of gas reactants and only 2 moles of gas products.so the entropy is on the decline.

Reaction 2: The number of particles in the system increases, i.e. the single reactant dissociates into two ion particles. In addition, the ions in the ionic solid are organized in a rigid lattice structure whereas the ions in aqueous solutions are free to move randomly through the solvent. So the entropy is in a decrease.

The number of particles in the system increases. i.e. there is one mol of gas reactant and 2 moles of gas products. The disorder is increased more.

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General Chemistry Exam #1

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You will have 30 minutes to answer 30 questions.

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You will have the test results and score back immediately after submitting them.

You are able to access the periodic table through the ‘Exhibit’ button.

If this is your first time, read the General Chemistry page.

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