Q: A weather balloon contains 44.0 L of helium at a certain temperature and pressure. If the amount of helium is increased from 2.0 moles to 5.0 moles, what is the final volume, assuming temperature and pressure remain constant?

$$V_2$$ = $$V_1$$ \times \frac{$$n_2$$}{$$n_1$$} = 44.0\ \text{L} \times \frac{5.0}{2.0} = 110\ \text{L}$$

Q: At STP, what is the volume occupied by 4.00 g of methane ($$CH_4$$)? (Molar mass of $$CH_4 = 16.05$$\ \text{g/mol}$$)

$$\text{Moles of } CH_4$ = \frac{4.00\ \text{g}}{16.05\ \text{g/mol}} = 0.249\ \text{mol}$$; $$V = 0.249\ \text{mol} \times 22.4\ \text{L/mol} = 5.58\ \text{L}$$

Q: A scuba diver uses a gas mixture with a total pressure of 8.00 atm. If the partial pressure of oxygen is 1280 mmHg, what is the partial pressure of helium in atm? (1 atm = 760 mmHg)

$$P_{\text{O}_2$$} = \frac{1280\ \text{mmHg}}{760\ \text{mmHg/atm}} = 1.68\ \text{atm}$$; P$$_{\text{He}} = 8.00\ \text{atm} - 1.68\ \text{atm} = 6.32\ \text{atm}$$

Q: Which of the following is the correct molar mass for $$C_2H_6O ($$ethanol)? (Atomic masses: C = 12.01, H = 1.01, O = 16.00)

$$2 \times 12.01 + 6 \times 1.01 + 1 \times 16.00 = 24.02 + 6.06 + 16.00 = 46.08\ \text{g/mol}$$

Q: A gas mixture contains 3.0 atm of nitrogen, 2.0 atm of oxygen, and 1.0 atm of argon. What is the total pressure of the mixture?

$$P_{\text{total}} = 3.0\ \text{atm} + 2.0\ \text{atm} + 1.0\ \text{atm} = 6.0\ \text{atm}$$

Question 1

A sample of nitrogen gas occupies 2.50 L at 1.00 atm and 300 K. If the pressure is increased to 2.00 atm and the temperature is raised to 400 K, what will be the new volume of the gas? (Assume the amount of gas remains constant.)

Accepted Answer

$$V_2$$ = \frac{$$P_1$$ $$V_1$$ $$T_2$$}{$$T_1$$ $$P_2$$} = \frac{(1.00\ 	ext{atm})(2.50\ 	ext{L})(400\ 	ext{K})}{(300\ 	ext{K})(2.00\ 	ext{atm})} = 1.67\ 	ext{L}$$

Question 2

A weather balloon contains 44.0 L of helium at a certain temperature and pressure. If the amount of helium is increased from 2.0 moles to 5.0 moles, what is the final volume, assuming temperature and pressure remain constant?

Accepted Answer

$$V_2$$ = $$V_1$$ 	imes \frac{$$n_2$$}{$$n_1$$} = 44.0\ 	ext{L} 	imes \frac{5.0}{2.0} = 110\ 	ext{L}$$

Question 3

Which of the following statements best describes Avogadro’s Law?

Accepted Answer

The volume of a gas is directly proportional to the number of moles of gas, provided temperature and pressure are constant.

Question 4

At STP, what is the volume occupied by 4.00 g of methane ($$CH_4$$)? (Molar mass of $$CH_4 = 16.05$$\ 	ext{g/mol}$$)

Accepted Answer

$$	ext{Moles of } CH_4$ = \frac{4.00\ 	ext{g}}{16.05\ 	ext{g/mol}} = 0.249\ 	ext{mol}$$; $$V = 0.249\ 	ext{mol} 	imes 22.4\ 	ext{L/mol} = 5.58\ 	ext{L}$$

Question 5

A scuba diver uses a gas mixture with a total pressure of 8.00 atm. If the partial pressure of oxygen is 1280 mmHg, what is the partial pressure of helium in atm? (1 atm = 760 mmHg)

Accepted Answer

$$P_{	ext{O}_2$$} = \frac{1280\ 	ext{mmHg}}{760\ 	ext{mmHg/atm}} = 1.68\ 	ext{atm}$$; P$$_{	ext{He}} = 8.00\ 	ext{atm} - 1.68\ 	ext{atm} = 6.32\ 	ext{atm}$$

Question 6

Which of the following is the correct molar mass for $$C_2H_6O ($$ethanol)? (Atomic masses: C = 12.01, H = 1.01, O = 16.00)

Accepted Answer

$$2 	imes 12.01 + 6 	imes 1.01 + 1 	imes 16.00 = 24.02 + 6.06 + 16.00 = 46.08\ 	ext{g/mol}$$

Question 7

A 0.500 mol sample of gas occupies 11.2 L at STP. Which law or concept does this observation best illustrate?

Accepted Answer

The molar volume of a gas at STP is 22.4 L per mole.

Question 8

A gas mixture contains 3.0 atm of nitrogen, 2.0 atm of oxygen, and 1.0 atm of argon. What is the total pressure of the mixture?

Accepted Answer

$$P_{	ext{total}} = 3.0\ 	ext{atm} + 2.0\ 	ext{atm} + 1.0\ 	ext{atm} = 6.0\ 	ext{atm}$$

Question 9

If the temperature of a gas is doubled (in Kelvin) while the pressure is kept constant, what happens to the volume?

Accepted Answer

The volume doubles.

Question 10

A 0.180 L sample of helium gas is at 0.800 atm and 29°C. What temperature (in °C) is required for the gas to have a volume of 90.0 mL and a pressure of 3.20 atm? (Use the combined gas law.)

Accepted Answer

$$T_2$$ = \frac{$$P_2$$ $$V_2$$ $$T_1$$}{$$P_1$$ $$V_1$$} = \frac{(3.20\ 	ext{atm})(0.0900\ 	ext{L})(302\ 	ext{K})}{(0.800\ 	ext{atm})(0.180\ 	ext{L})} = 603\ 	ext{K};\ $$T_2 = 603 - 273 = 330$$\ ^\circ	ext{C}$$

Question 11

Which of the following is NOT a correct statement about gases at STP?

Accepted Answer

1 mole of any gas occupies 24.0 L at STP.

Question 12

A hospital stores oxygen in a 50.0 L tank at 25°C and 150 atm. If the oxygen is used until the pressure drops to 1.00 atm at the same temperature, what volume would the released oxygen occupy at 1.00 atm?

Accepted Answer

$$V_2$$ = \frac{$$P_1$$ $$V_1$$}{$$P_2$$} = \frac{150\ 	ext{atm} 	imes 50.0\ 	ext{L}}{1.00\ 	ext{atm}} = 7500\ 	ext{L}$$

Question 13

Which of the following best explains why scuba divers use a mixture of helium and oxygen rather than pure oxygen at high pressures?

Accepted Answer

Pure oxygen at high pressures can be toxic to humans, so helium is used to dilute the oxygen and reduce the risk of oxygen toxicity.

Question 14

A gas cylinder contains 5.00 moles of $$CO_2 at $$STP. What is the total volume of the gas?

Accepted Answer

$$V = n 	imes 22.4\ 	ext{L} = 5.00 	imes 22.4 = 112\ 	ext{L}$$

Question 15

Which of the following is the correct sequence of steps to determine the molar mass of a compound?

Accepted Answer

Obtain the molar mass of each element, multiply by the number of moles (subscript), then add the masses for each element.

Gas Laws, Molar Mass, and Dalton’s Law: Study Notes for GOB Chemistry

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