A solution of HCOOH ( K _a = 1.8×10^{- 4}) and HCOO^- was submitted for chemical analysis. The results were:
[HCOOH] = 0.050 M, [HCOO^-] = 0.15 M. Calculate the pH.

Question

A solution of HCOOH ( K _a = 1.8×10^{- 4}) and HCOO^- was submitted for chemical analysis. The results were: [HCOOH] = 0.050 M, [HCOO^-] = 0.15 M. Calculate the pH.

Quizplus · Accepted Answer

The correct answer is A because the pH of the solution can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]), where pKa is the negative logarithm of the acid dissociation constant, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the acid. In this case, pKa = -log(1.8×10^-4) = 3.74, [A-] = 0.15 M, and [HA] = 0.050 M. Substituting these values into the equation gives pH = 3.74 + log(0.15/0.050) = 4.22.

A Solution of HCOOH ( K a = 1

A Solution of HCOOH ( K _a = 1