Acetic acid, (HC₂H₃O₂)is a weak acid (K_a = 1.8 $\times$ 10^-⁵).Calculate the pH of a 15.1 M HC₂H₃O₂ solution. A)-1.18 B)3.57 C)1.78 D)1.18 E)12.22

Question

Quizplus · Accepted Answer

Using the equation for the acid dissociation constant (Ka) of acetic acid:

Ka = [H+][C2H3O2-]/[HC2H3O2]

We can derive an expression for the pH of the solution:

Ka = [H+][C2H3O2-]/[HC2H3O2]
1.8 x 10^-5 = x^2 / (15.1 - x)

where x is the concentration of H+ (or [H+]).

Using the quadratic formula and ignoring x when compared to the initial concentration (15.1 M), we find that:

[H+] = 1.05 x 10^-3 M

pH = -log[H+] = 1.78

Therefore, the answer is C.

Acetic Acid, (HC2H3O2)is a Weak Acid (Ka = 1 ×\times× 10-5)