calculating Kc from Kp at 450 Celsius

Question

Consider the reaction below:

2 SO₃(g) ⇌ 2 SO₂(g) + O₂(g)

The Kp is 1.81 × 10⁻⁵ at 450.0°C.

Calculate Kc at the same temperature.

(R = 0.08206 L･atm/mol･K)

I know the equation for this is Kp=Kc(RT)^n, but I'm not sure what should be used for R or n.

Answer 1

R is the universal gas constant, and you can use the value 0.0821 L-atm/mol-K, as noted in the problem statement.

The other variable is Δn, or the change in total moles between reactants and products. In this reaction, there are 2 mols of reactants (2 mol SO₃) and 3 mols of products (2 mol SO₂, 1 mol O₂). Δn is 1 mol.

Applying this to the equation that you identified:

K_c = K_p / (RT)^Δn = 1.81 x 10^-5 / (0.0821 L-atm/mol-K * 723 K)¹

= 3.049 x 10^-7