An aqueous mixture of hydrocyanic acid and ammonia has initial concentrations of 0.100 M HCN (aq) and 0.140 M NH3 (aq). At equilibrium, the CN - (aq) concentration is 0.055 M. Calculate Kc for the reaction

Kc = 0.791

Explanation:

Step 1: Data given

Hydrocyanic acid = HCN

ammonia = NH3

initial concentrations of HCN = 0.100 M

initial concentrations of NH3 = 0.140 M

At equilibrium, the CN - (aq) concentration is 0.055 M.

Step 2: The balanced equation

HCN (aq) + NH3 (aq) ↔ CN - (aq) + NH4 + (aq)

Step 3: The concentrations at the equilibrium

For 1 mol HCN we need 1 mol NH3 to produce 1 mol CN - and 1 mol NH4+

[CN-] = [NH4+] = 0.055 M

[HCN] = 0.100 - 0.055 = 0.045 M

[NH3] = 0.140 - 0.055 = 0.085 M

Step 4: Calculate Kc

Kc = [CN-][NH4+] / [HCN][NH3] = (0.055 * 0.055) / (0.045 * 0.085) = 0.791

Kc = 0.791