MCAT General Chemistry Question 118: Answer and Explanation
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Question: 118
13. A gaseous mixture contains nitrogen and helium and has a total pressure of 150 torr. The nitrogen particles comprise 80 percent of the gas, and the helium particles make up the other 20 percent of the gas. What is the pressure exerted by each individual gas?
- A. 100 torr nitrogen, 50.0 torr helium
- B. 120 torr nitrogen, 30.0 torr helium
- C. 30.0 torr nitrogen, 150 torr helium
- D. 50.0 torr nitrogen, 100 torr helium
Correct Answer: B
Explanation:
The partial pressure of each gas is found by multiplying the total pressure by the mole fraction of the gas. Because 80 percent of the molecules are nitrogen, the mole fraction of nitrogen gas is equal to 0.80. Similarly, for helium, the mole fraction is 0.20. To find the pressure exerted by nitrogen, multiply the total pressure (150 torr) by 0.80 to obtain 120 torr of nitrogen. The remainder, 30 torr, is attributable to helium.