What is the pH of a 0.250 M aqueous solution of formic acid?
K_a = 1.8 × 10^-4

Question

What is the pH of a 0.250 M aqueous solution of formic acid? K_a = 1.8 × 10^-4 A)11.8 B)2.2 C)0.60 D)5.4 E)8.6

Quizplus · Accepted Answer

The pH of a solution can be calculated using the formula pH = -log[H+], where [H+] is the concentration of hydrogen ions in the solution. For weak acids like formic acid, we use the acid dissociation constant (Ka) to find the concentration of hydrogen ions. Given that the Ka of formic acid is 1.8 × 10^-4, we can set up an equation using the formula Ka = [H+][A-]/[HA], where [HA] is the initial concentration of the acid, and [H+] = [A-] for the dissociation of one mole of HA into H+ and A-. Assuming x is the concentration of H+ ions that dissociate, we have 1.8 × 10^-4 = x^2 / 0.250. Solving for x gives us the concentration of H+ ions, and taking the negative logarithm of this concentration gives us the pH. The calculation yields a pH close to 2.2, making option B the correct answer.

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