Q: A sample of gas has a density of 2.17 g/L at 2.0 atm and room temperature. What is the molar mass of the gas? (Use $$PV = nRT$$ and $$d = \frac{PM}{RT}$$, $$R = 0.0821\ \text{L atm mol}^{-1}\text{K}^{-1}$$, $$T = 298\ \text{K})$$

$$M = \frac{dRT}{P} = \frac{2.17 \times 0.0821 \times 298}{2.0} = 26.5\ \text{g/mol}$$

Q: A mixture of 502 mL of N$$_2 at 1.04 $$atm and 344 mL of SF$$_6 at 0.85 $$atm is placed in a 250.0 mL flask. What is the partial pressure of SF$$_6 in $$the flask after mixing?

$$P_{SF_6$} = \frac{0.85 \times 344}{250} = 1.17\ \text{atm}$$

Question 1

Which of the following statements best describes the behavior of gases according to the kinetic molecular theory?

Accepted Answer

Gas molecules move in constant, random motion and experience elastic collisions with the container walls.

Question 2

A weather balloon is released at sea level with a volume of 10.0 L at 27°C and a pressure of 645 torr. It ascends to 22,600 ft where the pressure is 96 mmHg and the temperature is -30°C. What is the new volume of the balloon? (Assume ideal gas behavior.)

Accepted Answer

$$V_2$$ = \frac{$$P_1$$ $$V_1$$ $$T_2$$}{$$P_2$$ $$T_1$$} = \frac{645 	imes 10.0 	imes 243}{96 	imes 300} = 54.4\ 	ext{L}$$

Question 3

Which of the following is NOT an assumption of the ideal gas law?

Accepted Answer

Gas molecules lose energy during collisions with the container walls.

Question 4

At STP, what is the molar volume of an ideal gas?

Accepted Answer

$$22.4\ 	ext{L}$$

Question 5

A sample of gas has a density of 2.17 g/L at 2.0 atm and room temperature. What is the molar mass of the gas? (Use $$PV = nRT$$ and $$d = \frac{PM}{RT}$$, $$R = 0.0821\ 	ext{L atm mol}^{-1}	ext{K}^{-1}$$, $$T = 298\ 	ext{K})$$

Accepted Answer

$$M = \frac{dRT}{P} = \frac{2.17 	imes 0.0821 	imes 298}{2.0} = 26.5\ 	ext{g/mol}$$

Question 6

Which of the following conditions would cause a real gas to deviate most from ideal behavior?

Accepted Answer

High pressure and low temperature

Question 7

Which gas law describes the relationship between pressure and volume at constant temperature?

Accepted Answer

Boyle's Law: $$P_1$$ $$V_1$$ = $$P_2$$ $$V_2$$

Question 8

A mixture of 502 mL of N$$_2 at 1.04 $$atm and 344 mL of SF$$_6 at 0.85 $$atm is placed in a 250.0 mL flask. What is the partial pressure of SF$$_6 in $$the flask after mixing?

Accepted Answer

$$P_{SF_6$} = \frac{0.85 	imes 344}{250} = 1.17\ 	ext{atm}$$

Question 9

Which of the following best explains why the molar volume of CO$$_2$$ and NH$$_3 at $$STP is significantly different from that of He and N$$_2$$?

Accepted Answer

CO$$_2$$ and NH$$_3$$ have stronger intermolecular forces, causing deviations from ideal gas behavior.

Question 10

Which equation would you use to determine the molecular speed of a gas molecule at a given temperature?

Accepted Answer

$$u = \sqrt{\frac{3RT}{M}}$$

Question 11

A 1.0 g sample of H$$_2$$ and O$$_2$$ has a total pressure of 4.279 atm at 298 K in a 1.25 L flask. What is the number of grams of each element in the container? (Assume no reaction occurs.)

Accepted Answer

H$$_2$$: 0.071 g, O$$_2$$: 0.929 g

Question 12

Which of the following best describes the effect of increasing the value of 'a' in the van der Waals equation?

Accepted Answer

It corrects for the attractive forces between gas molecules.

Question 13

Which of the following is the correct van der Waals equation for real gases?

Accepted Answer

$$\left(P + \frac{a n^2$}{V^2$}\right)(V - nb) = nRT$$

Question 14

A mixture of gases contains 2.0 moles of N$$_2$$ and 6.0 moles of H$$_2. $$What is the mole fraction of N$$_2$$?

Accepted Answer

$$X_{N_2$} = \frac{2.0}{2.0 + 6.0} = 0.25$$

Question 15

If the temperature of a gas is doubled while the pressure is halved, what happens to the volume? (Assume constant n.)

Accepted Answer

The volume increases by a factor of 4.

Question 16

Which of the following best describes Dalton's Law of Partial Pressures?

Accepted Answer

The total pressure of a mixture of gases is equal to the sum of the partial pressures of each gas.

Question 17

A sample of gas at 1.0 atm and 273 K occupies 22.4 L. If the pressure is increased to 2.0 atm at constant temperature, what is the new volume?

Accepted Answer

$$V_2$$ = \frac{$$P_1$$ $$V_1$$}{$$P_2$$} = \frac{1.0 	imes 22.4}{2.0} = 11.2\ 	ext{L}$$

Question 18

Which of the following gases would diffuse fastest at room temperature?

Accepted Answer

H$$_2$$

Chapter 6: Gases – Properties, Laws, and Applications

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