Q: A 1.80 g sample of an unknown gas occupies 625 mL at 1.05 atm and 35.0 $$^\circC. $$What is the molar mass of the gas? (Use $$R = 0.08206 L $$atm mol$$^{-1} K$^{-1})$$

$$M = \frac{mRT}{PV} = \frac{1.80 \times 0.08206 \times 308.15}{1.05 \times 0.625} = 69.5 g/$$mol

Q: A reaction occurs in two steps: $$A \rightarrow B$$, $$\Delta H_1$ = +35.0 kJ$$; $$B \rightarrow C$$, $$\Delta H_2$ = -82.0 kJ. $$What is $$\Delta H$$ for $$A \rightarrow C$$?

$$\Delta H = +35.0\ \text{kJ} + (-82.0\ \text{kJ}) = -47.0\ \text{kJ}$$

Question 1

Which of the following statements best explains why the boiling points of methane (CH$$_4$$), ethane (C$$_2H$_6$$), propane (C$$_3H$_8$$), and butane (C$$_4H$_{10}) $$generally increase with increasing molar mass?

Accepted Answer

Larger molecules have more electrons, leading to stronger London dispersion forces.

Question 2

A 1.80 g sample of an unknown gas occupies 625 mL at 1.05 atm and 35.0 $$^\circC. $$What is the molar mass of the gas? (Use $$R = 0.08206 L $$atm mol$$^{-1} K$^{-1})$$

Accepted Answer

$$M = \frac{mRT}{PV} = \frac{1.80 	imes 0.08206 	imes 308.15}{1.05 	imes 0.625} = 69.5 g/$$mol

Question 3

Which of the following best describes the trend in first ionization energy across a period from left to right in the periodic table?

Accepted Answer

It generally increases due to increasing nuclear charge and decreasing atomic radius.

Question 4

A gas occupies 2.80 L at 745 torr. What volume will it occupy at 1.15 atm if temperature and amount of gas remain constant? (1 atm = 760 torr)

Accepted Answer

$$P_1V_1 = P_2V_2 \Rightarrow V_2$ = \frac{P_1V_1}{P_2$} = \frac{745 	imes 2.80}{1.15 	imes 760} = 2.40 L$$

Question 5

Which of the following best explains why a gas cannot be liquefied by pressure alone above its critical temperature?

Accepted Answer

Above the critical temperature, the kinetic energy of the molecules is too high for intermolecular forces to bring them together into a liquid.

Question 6

Which of the following is the correct ground-state electron configuration for selenium (Se, $$Z=34$$)?

Accepted Answer

$$1s^2$$\ $$2s^2$$\ $$2p^6$$\ $$3s^2$$\ $$3p^6$$\ $$4s^2$$\ $$3d^{10}$$\ $$4p^4$$

Question 7

Which of the following best describes the predominant intermolecular force in a pure sample of HF?

Accepted Answer

Hydrogen bonding, due to the interaction between hydrogen and highly electronegative fluorine atoms.

Question 8

A reaction occurs in two steps: $$A \rightarrow B$$, $$\Delta H_1$ = +35.0 kJ$$; $$B \rightarrow C$$, $$\Delta H_2$ = -82.0 kJ. $$What is $$\Delta H$$ for $$A \rightarrow C$$?

Accepted Answer

$$\Delta H = +35.0\ 	ext{kJ} + (-82.0\ 	ext{kJ}) = -47.0\ 	ext{kJ}$$

Question 9

Which of the following best explains why elements in the same group of the periodic table often have similar chemical properties?

Accepted Answer

They have the same number of valence electrons, leading to similar reactivity.

Question 10

Which of the following is the correct net ionic equation for the reaction of aqueous barium chloride with aqueous sodium sulfate?

Accepted Answer

$$\mathrm{Ba$$^{2+}$$(aq) + SO_4$^{2-}(aq) \rightarrow $$BaSO_4$$(s)}$$

Question 11

Which of the following statements about the relationship between the sign of $$\Delta H$$ and the nature of a process is correct?

Accepted Answer

A positive $$\Delta H$$ indicates an endothermic process; a negative $$\Delta H$$ indicates an exothermic process.

Question 12

Which of the following best describes the effect of strong intermolecular forces on vapor pressure, boiling point, and heat of vaporization?

Accepted Answer

Strong intermolecular forces result in low vapor pressure, high boiling point, and high heat of vaporization.

Question 13

A sample contains 8.00 g of oxygen atoms within 25.0 g of a compound. What is the mass percent of oxygen in the compound?

Accepted Answer

$$\frac{8.00\ 	ext{g}}{25.0\ 	ext{g}} 	imes 100\% = 32.0\%$$

Question 14

Which of the following is NOT a unit of pressure?

Accepted Answer

Joule (J)

Question 15

Which of the following best describes the difference between a physical change and a chemical change at the particle level?

Accepted Answer

A physical change alters the arrangement or state of particles without changing their identity, while a chemical change results in the formation of new substances with different particles.

Question 16

Which of the following is the correct electron configuration for the oxide ion, O$$^{2-}$$?

Accepted Answer

$$1s^2$$\ $$2s^2$$\ $$2p^6$$

Question 17

Which of the following best explains why real gases deviate most strongly from ideal behavior at high pressure and low temperature?

Accepted Answer

At high pressure and low temperature, intermolecular forces and molecular volumes become significant.

Question 18

Which of the following is the correct balanced equation for the formation of exactly 1 mol of liquid water from its elements in their standard states?

Accepted Answer

$$\mathrm{H_2$(g) + \frac{1}{2}O_2$(g) \rightarrow H_2O(l)}$$

CHEM 141 Final Exam Study Guide – Step-by-Step General Chemistry Guidance

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