The K_a for acetic acid is 1.8 x 10^-5.Which of the following statements best describes the pH of a 0.010 M solution of acetic acid? A)The pH is greater than 0 but less than 2. B)The pH is exactly 2. C)The pH is greater than 2 but less than 7. D)The pH is exactly 7. E)The pH is greater than 7 but less than 12.

Question

Quizplus · Accepted Answer

The equation for the dissociation of acetic acid is: CH3COOH ⇌ CH3COO- + H+, so the Ka expression is:

Ka = [CH3COO-][H+]/[CH3COOH]

At equilibrium, let x be the concentration of H+ and CH3COO-, so the concentration of CH3COOH will be 0.010 M - x. Then,

Ka = x^2/(0.010 - x)

Since the Ka value is small, we can assume that x is much smaller than 0.010. Then,

Ka = x^2/0.010

x = sqrt(Ka x 0.010)

x = sqrt(1.8 x 10^-5 x 0.010)

x = 1.34 x 10^-3 M

So, the pH of the solution is:

pH = -log[H+]

pH = -log(1.34 x 10^-3)

pH = 2.87

Therefore, the pH is greater than 2 but less than 7. Choice C is the best answer.

The Ka for Acetic Acid Is 1

The K_a for Acetic Acid Is 1