Question detail
A reaction at equilibrium has the following partial pressures: P(N2) = 0.5 atm, P(H2) = 1.0 atm, and P(NH3) = 2.0 atm. Calculate Kp for the reaction N2(g) + 3H2(g) ⇌ 2NH3(g). What are the units of Kp?
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. atm^-2
- B. atm^2
- C. atm^-1
- D. none
Answer
atm^-2
Explanation
The correct option is atm^-2. atm^-2 is the best answer because it directly supports the AQA A-Level Chemistry objective to calculate Kp and determine appropriate units. 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 units 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
Misidentifying the Units of Kp
Students often write the equilibrium constant Kp as a dimensionless quantity, ignoring that it is expressed in terms of pressure units such as atm or bar.
Kp must be expressed with the same pressure units as the partial pressures used in the equilibrium expression, e.g. atm, bar, or kPa. When all partial pressures are in atm, Kp is given in atm⁰⁰ (dimensionless), but the convention is to state the units explicitly as atm or bar to avoid confusion. Keep the correction anchored to Kp and partial pressure (A-level only) and the objective: Calculate Kp and determine appropriate units.
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