AP Chemistry Equilibrium Constant Kc and Kp

Calculating Kc from concentrations, Kp from partial pressures, and K from an ICE table.

The equilibrium constant expression itself is short. The actual tested skill is substituting the right numbers into it correctly, whether those numbers are given directly as equilibrium values, or have to be worked out first from starting concentrations.

The Equilibrium Constant Expression

For aA + bB ⇌ cC + dD: K_c = [C]^c[D]^d / ([A]^a[B]^b) using molar concentrations, or the identical pattern using partial pressures for K_p. Only gases and aqueous species appear in the expression; pure solids and liquids are left out entirely.

Worked Example 1: Calculating Kc

N₂(g) + 3H₂(g) ⇌ 2NH₃(g). At equilibrium: [N₂] = 0.50 M, [H₂] = 0.30 M, [NH₃] = 0.20 M. Find K_c.

K_c = [NH₃]² / ([N₂][H₂]³) = (0.20)² / [(0.50)(0.30)³] = 0.0400 / 0.0135 = 2.96

Worked Example 2: Calculating Kp

2SO₂(g) + O₂(g) ⇌ 2SO₃(g). At equilibrium: P_SO2 = 0.20 atm, P_O2 = 0.15 atm, P_SO3 = 0.60 atm. Find K_p.

K_p = P_SO3² / (P_SO2² × P_O2) = (0.60)² / [(0.20)²(0.15)] = 0.360 / 0.00600 = 60.0

Worked Example 3: Finding K From an ICE Table

H₂(g) + I₂(g) ⇌ 2HI(g). Starting with 1.00 M H₂ and 1.00 M I₂ and no HI, the reaction proceeds until 0.78 M of H₂ has been consumed. Find K_c.

Equilibrium row: [H₂] = 1.00 − 0.78 = 0.22 M, [I₂] = 1.00 − 0.78 = 0.22 M, [HI] = 2(0.78) = 1.56 M (the coefficient of 2 applies to the change in HI).

K_c = [HI]² / ([H₂][I₂]) = (1.56)² / [(0.22)(0.22)] = 2.4336 / 0.0484 = 50.3

See the full ICE table guide for the general setup method, including when the simplifying approximation can replace this kind of direct substitution.

Common Kc and Kp Mistakes

Trying to convert between Kc and Kp. That conversion isn't tested on the AP exam, calculate whichever one the problem's given data actually supports.
Including a pure solid or liquid in the expression. Only gases and aqueous species belong in K; solids and liquids are left out by convention.
Forgetting to raise each concentration to its coefficient's power. A coefficient of 3 means that concentration is cubed in the expression, not multiplied by 3.
Mixing up the change in a species with the change in x. If a coefficient is 2, that species' change is 2x, not x, exactly the same coefficient-scaling rule used in every ICE table.

This same equilibrium-constant method applies across the rest of AP Chemistry too -- for every other free tool and guide on this site, start from the AP Chem Score Calculator.

Frequently Asked Questions

What is the difference between Kc and Kp?

Kc uses molar concentrations of each species; Kp uses partial pressures, and applies only to gas-phase reactions. They describe the same equilibrium, written in two different sets of units.

What is the equilibrium constant expression?

For aA + bB ⇌ cC + dD, K = [C]^c[D]^d / ([A]^a[B]^b) for Kc (using concentrations), or the same pattern using partial pressures for Kp. Products go on top, reactants on the bottom, each raised to its coefficient from the balanced equation.

Do I need to convert between Kc and Kp?

No. Converting between Kc and Kp is not assessed on the AP Chemistry exam. You need to calculate Kc directly from concentrations, or Kp directly from partial pressures, whichever the problem gives you, not convert one into the other.

Do pure solids and liquids appear in the K expression?

No. Only gases and aqueous species appear in a K expression. A pure solid or pure liquid's concentration doesn't change as the reaction proceeds, so by convention it's treated as 1 and left out of the expression entirely.

How do I find K if I only know starting concentrations, not equilibrium concentrations?

Set up an ICE table: Initial concentrations, the Change each species undergoes (in terms of x), and the resulting Equilibrium concentrations. Once you know the actual amount that reacted, substitute the Equilibrium row into the K expression.

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