Q: A 2.00 L flask contains 0.50 mol of N$$_2$$ and 0.25 mol of O$$_2 at 300 K. $$What is the total pressure in the flask? (Use $$R = 0.0821 L$$·atm·mol$$^{-1}$$·K$$^{-1})$$

$$P_T = \frac{(0.50 + 0.25) \times 0.0821 \times 300}{2.00} = 6.16$$ atm

Q: If the mole fraction of CO$$_2 in a $$gas mixture is 0.20 and the total pressure is 1.5 atm, what is the partial pressure of CO$$_2$$?

$$P_{CO_2$} = 0.20 \times 1.5 = 0.30$$ atm

Question 1

Which statement best describes Dalton's Law of Partial Pressures for a mixture of nonreacting gases?

Accepted Answer

The total pressure is the sum of the partial pressures of each gas present.

Question 2

Given a mixture of O$$_2$$, N$$_2$$, Ar, and CO$$_2 in a $$container, which equation correctly expresses the total pressure according to Dalton's Law?

Accepted Answer

$$P_T = P_{O_2$} + P_{N_2$} + P_{Ar} + P_{CO_2$}$$

Question 3

A 2.00 L flask contains 0.50 mol of N$$_2$$ and 0.25 mol of O$$_2 at 300 K. $$What is the total pressure in the flask? (Use $$R = 0.0821 L$$·atm·mol$$^{-1}$$·K$$^{-1})$$

Accepted Answer

$$P_T = \frac{(0.50 + 0.25) 	imes 0.0821 	imes 300}{2.00} = 6.16$$ atm

Question 4

If the mole fraction of CO$$_2 in a $$gas mixture is 0.20 and the total pressure is 1.5 atm, what is the partial pressure of CO$$_2$$?

Accepted Answer

$$P_{CO_2$} = 0.20 	imes 1.5 = 0.30$$ atm

Question 5

Which of the following best explains why nonreacting gases in a mixture behave independently?

Accepted Answer

Each gas exerts pressure as if it were the only gas present, because gas molecules do not interact chemically.

Question 6

A mixture contains 0.01 mol He, 0.015 mol Ne, 0.025 mol Ar, and 0.05 mol Kr. What is the mole fraction of Ne?

Accepted Answer

$$X_{Ne} = \frac{0.015}{0.1} = 0.15$$

Question 7

A sample of gas is collected over water at 25°C. The total pressure is 760 mmHg and the vapor pressure of water at 25°C is 24 mmHg. What is the pressure of the dry gas?

Accepted Answer

$$P_{gas} = 760 - 24 = 736$$ mmHg

Question 8

Which of the following is the correct mathematical form of Dalton's Law for a gas mixture?

Accepted Answer

$$P_T = \sum_{i} P_i$$

Question 9

If a 6.19 g sample of PCl$$_5 is $$placed in a 2.00 L flask and completely vaporized at 252°C, what is the pressure in the flask? (Molar mass of PCl$$_5 = 208.2 g/$$mol, $$R = 0.0821 L$$·atm·mol$$^{-1}$$·K$$^{-1}$$, $$T = 525 K)$$

Accepted Answer

$$n = \frac{6.19}{208.2} = 0.0297$$ mol; \; P = \frac{nRT}{V} = \frac{0.0297 	imes 0.0821 	imes 525}{2.00} = 0.64$ atm

Question 10

In a mixture of gases, which variable is used to express the ratio of moles of a particular gas to the total moles present?

Accepted Answer

Mole fraction ($$X_i)$$

Question 11

A mixture of two gases, A and B, has partial pressures $$P_A = 0.75$$ atm and $$P_B = 1.50$$ atm. What is the total pressure?

Accepted Answer

$$P_T = 0.75 + 1.50 = 2.25$$ atm

Question 12

Which real-world scenario best illustrates Dalton's Law of Partial Pressures?

Accepted Answer

Collecting a gas over water, where the total pressure is the sum of the gas pressure and water vapor pressure.

Question 13

If the partial pressure of N$$_2 in $$air is 0.78 atm and the total atmospheric pressure is 1.00 atm, what is the mole fraction of N$$_2$$?

Accepted Answer

$$X_{N_2$} = \frac{0.78}{1.00} = 0.78$$

Question 14

A gas mixture contains 2.0 mol O$$_2$$, 3.0 mol N$$_2$$, and 1.0 mol CO$$_2. $$What is the mole fraction of O$$_2$$?

Accepted Answer

$$X_{O_2$} = \frac{2.0}{2.0 + 3.0 + 1.0} = \frac{2.0}{6.0} = 0.33$$

Question 15

Which of the following is NOT an assumption of Dalton's Law of Partial Pressures?

Accepted Answer

Gases in the mixture must have the same molar mass.

Question 16

A mixture of gases is in a 5.0 L container at 400 K. The mixture contains 0.2 mol H$$_2$$ and 0.3 mol N$$_2. $$What is the total pressure? ($$R = 0.0821 L$$·atm·mol$$^{-1}$$·K$$^{-1})$$

Accepted Answer

$$P_T = \frac{(0.2 + 0.3) 	imes 0.0821 	imes 400}{5.0} = 3.28$$ atm

Question 17

If a gas mixture contains 0.40 mol of O$$_2$$ and 0.60 mol of N$$_2$$, and the total pressure is 2.0 atm, what is the partial pressure of O$$_2$$?

Accepted Answer

$$P_{O_2$} = \frac{0.40}{0.40 + 0.60} 	imes 2.0 = 0.8$$ atm

Dalton’s Law of Partial Pressures and Gas Mixtures

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