Hydrocyanic acid,HCN,is a weak acid whose K_a value is 4.0 10^-10.What is the pH of a 0.10 M solution of HCN?

Question

Quizplus · Accepted Answer

The first step is to write the expression for the ionization of HCN:

HCN + H2O ⇌ H3O+ + CN-

The Ka value for this reaction is:

Ka = ([H3O+][CN-])/[HCN]

Since HCN is a weak acid, we can assume that its initial concentration is the same as the equilibrium concentration:

[HCN] = 0.10 M

Now we need to find the equilibrium concentrations of the H3O+ and CN- ions. At equilibrium, we can assume that the concentration of H3O+ is equal to the concentration of CN-:

[H3O+] = [CN-] = x

Substituting these values into the Ka expression gives:

4.0 x 10^-10 = x^2/0.10

Solving for x gives:

x = 2.0 x 10^-6

Therefore, the pH of the solution is:

pH = -log[H3O+] = -log(2.0 x 10^-6) = 5.2

So the answer is (B).

Hydrocyanic Acid,HCN,is a Weak Acid Whose Ka Value Is 4

Hydrocyanic Acid,HCN,is a Weak Acid Whose K_a Value Is 4