Question detail
Which of the following correctly describes how to calculate the mole fraction of a gas in a mixture?
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Equilibrium constant Kp for homogeneous systems (A-level only)
Question
- A. Mole fraction = moles of gas / total moles of gas in the mixture
- B. Mole fraction = total moles of gas / moles of gas
- C. Mole fraction = pressure of gas / total pressure of gas
- D. Mole fraction = total pressure of gas / pressure of gas
Answer
Mole fraction = moles of gas / total moles of gas in the mixture
Explanation
The correct option is Mole fraction = moles of gas / total moles of gas in the mixture. Mole fraction = moles of gas / total moles of gas in the mixture is the best answer because it directly supports the AQA A-Level Chemistry objective to define partial pressure in a gas mixture. This reasoning is anchored to Kp and partial pressure (A-level only) in Equilibrium constant Kp for homogeneous systems (A-level only), and it separates partial pressure from similar A-Level ideas rather than relying on a vague recall statement. Other options are weaker if they use the wrong evidence, calculation, mechanism, observation, unit, or conclusion for this subtopic.
Common mistake
Misunderstanding Partial Pressure
Students often confuse partial pressure with total pressure, thinking they are the same concept.
To define partial pressure in a gas mixture, use the formula: P_partial = X * P_total, where P_partial is the partial pressure of the gas, X is the mole fraction of the gas, and P_total is the total pressure of the gas mixture. For example, if the total pressure is 2 atm and the mole fraction of gas A is 0.3, then P_partial = 0.3 * 2 atm = 0.6 atm. Thus, the partial pressure of gas A is 0.6 atm. Keep the correction anchored to Kp and partial pressure (A-level only) and the objective: Define partial pressure in a gas mixture.
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