Question

Weak acids partially dissociate in solution and establish equilibrium with H⁺

HA ⇌ H⁺ + A^-

The equilibrium constant, called K_a, is a measure of the extent of the dissociation and the strength of the weak acid.

                             

Calculate the concentration of H⁺, [H⁺], in the following acid solutions.

1.      0.2 M HF, K_a = 7.2 × 10^-4

2.      0.2 M CH₃COOH, K_a = 1.8 × 10^-5

3.      0.2 M HCN, K_a = 6.2 × 10^-10

4.      0.2 M HCl (strong acid)

Answer 1

1. HF H⁺ + F^-

Its equilibrium constant K_a will be-

   

where, [...] represents the concentration

From the equilibrium reaction we can see that, [H⁺] = [F^-]

   [ concentration of HF given is 0.2 M]

  

  

The concentration of H⁺ in the 0.2 M HF acid solution is 0.012 M.

[Note:- Follow the same procedure for question no. 2 & 3]

4. Since, HCl is a strong acid. So it dissociates completely.

For strong acid, HCl H⁺ + Cl^-

So, the concentration of HCl and H⁺ will be same.

[HCl] = [H⁺] = 0.2 M [ concentration of HCl given is 0.2 M]

Ans: The concentration of H⁺ in the HCl acid solution is 0.2 M.