Calculating the Equilibrium Constant
Infer K from equilibrium measurements after converting amounts to concentrations or partial pressures.
AP Chemistry Topic 7.4 · CED printed p. 132 / PDF p. 139. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Writing Q
- Amount–volume conversion
Now we can measure K rather than guess it. First establish that the mixture has reached equilibrium at a recorded temperature; then measure the concentrations or partial pressures of the species in its expression. Write the balanced equation and expression before entering numbers. A coefficient of two means you square the whole measured concentration, including its numerical value. If the measurement begins as moles, divide by that sample’s solution or container volume first. For Kp use each gas’s partial pressure. The total pressure alone is insufficient unless the composition is also known. Different initial mixtures can reach different equilibrium concentrations but should give the same K at the same temperature. That repeatability is useful evidence for the model. A systematic concentration error can affect K disproportionately because of the powers, so report the measured precision and check whether replicate results are consistent.
Let's work through it
- For A(g) + B(g) ⇌ C(g), equilibrium analysis of a 2.00 L flask finds 0.400 mol A, 0.600 mol B, and 0.240 mol C.
- Convert all three amounts: [A] = 0.200 M, [B] = 0.300 M, [C] = 0.120 M. The common volume still matters because there are two concentration factors below and one above.
- Kc = 0.120/(0.200 × 0.300) = 2.00.
- Label this Kc at the measured temperature; do not assume the same value at a new temperature.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
May you insert equilibrium moles directly into Kc?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
May you insert equilibrium moles directly into Kc?
Worked answer and reason
Only after converting to concentrations; equal volumes do not always cancel.
The exponent total may differ on the two sides.
Retrieval 2
Two runs have different final concentrations but the same temperature. Must K differ?
Worked answer and reason
No. Their equilibrium quotients should match.
K constrains a ratio, not a unique set of concentrations.
Catch the tempting mistake
- Any constant sensor reading gives the correct K.
- Confirm reversible equilibrium and use simultaneous measurements at a controlled temperature.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 7 problem 4 · ap application · data
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For the teacher
- Ask students how a 5% underestimate of a squared denominator concentration biases K: it biases K high.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 7.4.A
- Calculate K_c or K_p based on experimental observations of concentrations or pressures at equilibrium.
- 7.4.A.1
- Equilibrium constants can be determined from experimental measurements of the concentrations or partial pressures of the reactants and products at equilibrium.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 — Required topic-numbered learning objectives and essential knowledge, all continuations and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
- OpenStax Chemistry 2e §13.2, Equilibrium Constants — Pressure/concentration quotients, pure-phase activities, and the distinction between composition and rate.
Glossary and deeper reading · 2 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Mixture
A mixture contains two or more substances whose proportions can vary. Its components retain their chemical identities until a chemical reaction changes them. For example, water and ethanol can form mixtures with many different compositions.
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Chemical equilibrium
Chemical equilibrium is a dynamic state in which forward and reverse reaction rates are equal. Macroscopic composition remains constant in a closed system under fixed conditions, but reactant and product concentrations need not be equal. For example, a reversible reaction can reach equilibrium with much more reactant than product.