Equilibrium Constant Examples: 45 Problems with Answers

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Equilibrium Constant.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

The ratio of product concentrations to reactant concentrations at equilibrium, each raised to its stoichiometric coefficient.

K is the scoreboard at the end of the game — it tells you which side won. Large K means products dominate; small K means reactants dominate.

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How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Equilibrium Constant starts by naming reactants and products, then checks conservation with a balanced equation.

Common stuck point: Students often know a formula related to equilibrium constant but skip the recognition step: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation?

Worked Examples

Example 1

medium

For

N_2O_4 \rightleftharpoons 2NO_2

[N_2O_4] = 0.040

M and

[NO_2] = 0.040

M at equilibrium. Find

K_c

Answer

K_{c} = 0.040

First step

K_c = [NO_2]^2 / [N_2O_4]

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Example 2

medium

For

A \rightleftharpoons B

K_1 = 4

. If reaction 2 is

B \rightleftharpoons A

, what is

K_2

Example 3

hard

For

H_2 + I_2 \rightleftharpoons 2HI

, a

1.0

L vessel initially holds

0.50

mol

H_2

and

0.50

mol

I_2

. At equilibrium

[HI] = 0.80

M. Find

K_c

Example 4

hard

For

A \rightleftharpoons B

K_c = 4

. Start with

[A]_0 = 1.0

M and no B. Find

[A]

and

[B]

at equilibrium.

Example 5

hard

For

A \rightleftharpoons 2B

with

K_c = 1.0

and starting with

[A]_0 = 1.0

M and no

B

, set up and solve for

[B]

at equilibrium.

Example 6

hard

For

2SO_2 + O_2 \rightleftharpoons 2SO_3

, at equilibrium

[SO_2] = 0.20

[O_2] = 0.10

[SO_3] = 0.80

M. Find

K_c

Example 7

challenge

Phosgene decomposes:

COCl_2 \rightleftharpoons CO + Cl_2

K_c = 0.090

T

. A

1.0

L flask holds

1.0

mol

COCl_2

initially. Find

[CO]

at equilibrium.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy

Write the equilibrium expression

K_c

for

A \rightleftharpoons B

Example 2

easy

In the expression for

2A \rightleftharpoons B

, what power does

[A]

take?

Example 3

easy

A reaction has a very large

K

. Do products or reactants dominate at equilibrium?

Example 4

easy

Should pure solids appear in an equilibrium constant expression?

Example 5

easy

Write

K_c

for

N_2 + 3H_2 \rightleftharpoons 2NH_3

Example 6

easy

Does adding more reactant change the value of

K

(at constant temperature)?

Example 7

easy

K = 1

, what does that say about products and reactants at equilibrium?

Example 8

easy

Write

K_c

for the heterogeneous equilibrium

CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g)

Example 9

medium

At equilibrium

[A]=2.0

[B]=4.0

M for

A \rightleftharpoons B

. Find

K_c

Example 10

medium

For

H_2 + I_2 \rightleftharpoons 2HI

, equilibrium gives

[H_2]=0.10

[I_2]=0.20

[HI]=0.40

M. Find

K_c

Example 11

medium

A reaction has

Q = 5

and

K = 2

. Which direction does the reaction shift?

Example 12

medium

For

2SO_2 + O_2 \rightleftharpoons 2SO_3

, write

K_c

and state its concentration dependence on

[O_2]

Example 13

medium

K_{forward} = 4

, what is

K

for the reverse reaction?

Example 14

medium

A reaction

A \rightleftharpoons 2B

has

K = 9

. If

[A] = 1.0

M at equilibrium, find

[B]

Example 15

medium

Two reactions are added: reaction 1 has

K_1 = 2

, reaction 2 has

K_2 = 3

. What is

K

for the overall reaction?

Example 16

medium

A reaction has

Q = 1

and

K = 5

. Which direction does it shift?

Example 17

medium

Write the equilibrium expression

K_c

for

CO + 2H_2 \rightleftharpoons CH_3OH

Example 18

challenge

For

N_2 + 3H_2 \rightleftharpoons 2NH_3

, equilibrium gives

[N_2]=1.0

[H_2]=2.0

[NH_3]=4.0

M. Compute

K_c

Example 19

challenge

A 1.0 L vessel starts with 1.0 mol

H_2

and 1.0 mol

I_2

. At equilibrium 0.20 mol

H_2

remains for

H_2 + I_2 \rightleftharpoons 2HI

. Find

K_c

Example 20

challenge

Reaction 1:

A \rightleftharpoons B

K_1 = 4

. Reaction 2:

B \rightleftharpoons C

K_2 = 5

. Find

K

for

A \rightleftharpoons C

and for

C \rightleftharpoons A

Example 21

easy

What is the symbol for the reaction quotient (not at equilibrium)?

Example 22

easy

Write

K_c

for

3A \rightleftharpoons B

Example 23

easy

A reaction has

K = 1 \times 10^{-8}

. Are products or reactants favored at equilibrium?

Example 24

easy

Write

K_c

for

H_2(g) + I_2(g) \rightleftharpoons 2HI(g)

Example 25

easy

Does a catalyst change the value of

K

Example 26

medium

For

A \rightleftharpoons 2B

[A] = 0.50

M and

[B] = 1.0

M at equilibrium. Find

K_c

Example 27

medium

Reaction has

K = 10

. If

Q = 100

, which direction does it shift?

Example 28

medium

Write

K_c

for the heterogeneous equilibrium

2NaHCO_3(s) \rightleftharpoons Na_2CO_3(s) + H_2O(g) + CO_2(g)

Example 29

medium

Reaction

A \rightleftharpoons B

has

K = 8

300

K. After increasing

T

K

becomes

4

. Is the forward reaction endothermic or exothermic?

Example 30

medium

For

2A \rightleftharpoons B

K_c = 25

and

[A] = 0.20

M at equilibrium. Find

[B]

Example 31

medium

Reaction

A \rightleftharpoons B

has

K = 2

. If you instead write

2A \rightleftharpoons 2B

, what is

K

Example 32

medium

500

K_c = 0.50

for

A \rightleftharpoons B

. Starting with

[A] = 1.0

[B] = 0

M, in which direction does the reaction proceed?

Example 33

medium

For

A + B \rightleftharpoons C

K_c = 50

. At equilibrium

[A] = 0.10

M and

[C] = 0.50

M. Find

[B]

Example 34

medium

For a reaction with

\Delta n_{gas} = 0

, how are

K_p

and

K_c

related?

Example 35

hard

For

A \rightleftharpoons 2B

K_c = 0.040

. Starting with pure

A

0.10

M and pure

B

0.20

M, predict the direction of net change.

Example 36

hard

For

N_2O_4 \rightleftharpoons 2NO_2

in the gas phase at

T

K_c = 0.36

and

RT = 24.5

L·atm/mol. Find

K_p

Example 37

hard

Two reactions:

R_1

A \rightleftharpoons B

K_1 = 4

R_2

2B \rightleftharpoons C

K_2 = 9

. Find

K

for

2A \rightleftharpoons C

Example 38

challenge

For

A \rightleftharpoons B

K = 4.0

T_1 = 300

K and

K = 1.0

T_2 = 400

K. State whether the forward reaction is exothermic or endothermic, and justify with Le Châtelier.

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Related Concepts

Chemical Equilibrium Concentration Le Chatelier's Principle

Background Knowledge

These ideas may be useful before you work through the harder examples.

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