Chemistry help?

Question

Calculate the value for K for the reac-on: HCN(aq) + F–(aq)<-> CN–(aq) + HF(aq)

(Ka for HCN is 6.2×10–10; Ka for HF is 7.2×10–4.)

The answer is in green but I need to know how to do it  

K = 8.6 × 10–7 

 

Answer 1

Step 1:

The chemical reaction is .

Consider that the reaction takes place in two elementary steps.

Reaction 1: .

Here acid ionization constant is .

Reaction 2: .

Here acid ionization constant is .

Step 2:

The chemical reaction is .

k_a for [HCN] is .

k_a for [HF] is .

Equilibrium constant :

Solution:

.