# Equilibrium Constant Formula

The equilibrium constant of a chemical reaction is the value of the reaction quotient when the chemical reaction has reached the equilibrium state. The value of equilibrium constant depends on ionic strength and temperature but independent of the analytical concentrations of the reactants and products in the chemical reaction.

If the proportion of reactants and products are same, this reaction can be called as an equilibrium reaction. If A and B denote the reactants, C and D are products; then an equilibrium reaction can be expressed as A + B —> C + D. You can calculate the equilibrium constant  Kc from this equation.

The equilibrium constant formula is

Where A, B, C and D are molar concentrations

a, b, c and d are respective coefficients

Example 1

Determine the equilibrium constant of the following equation.

2HNO3 —> N2O5 + H2O

The molar concentration of HNO3, N2O5 and H2O are is 0.05, 0.3 and 0.1

Solution

The given equation is

2HNO3 —> N2O5 + H2O

Considering molar concentrations of every reactant and product,

The equilibrium constants formula is given by

Kc = [C]c [D]d / [A]a [B]b

= [0.3]1 [0.1]1 / [0.05]2

Kc = 12

Example 2

Determine the equilibrium constant of the below equation.

Na2O + H2O —> 2NaOH

Molar concentrations are

NaOH = 0.23M

Na2O = 0.015M

H20 = 0.1M

Solution

The given equation is

Na2O + H2O —> 2NaOH

Considering molar concentrations of NaOH, Na2O and H2O

The equilibrium constants formula is given by

Kc = [C]c [D]d / [A]a [B]b

= (0.23)2 / (0.015)1 (0.1)1

Kc = 35.266