I got pH= 1.84, is that correct?

Question

What is the pH of a 1.15 M solution of methanoic acid? Ka = 1.8 x 10^-4?

Answer 1

YES...the pH is 1.84. Nice job.

pH = -log [H+]. Find [H+].

methanoic acid = HCOOH (formic acid)

HCOOH ==> H+ + COO-

Ka = 1.8x10-4 = [H+][COO-] / [HCOOH]

1.8x10^-4 = x² / 1.15 - x (assume x is small relative to 1.15 and ignore it to avoid using quadratic)

1.8x10^-4 = x² / 1.15

x² = 2.05x10^-4

x = 1.43x10^-2 = [H⁺] (note: this is small relative to 1.15 so the above assumption was valid)

pH = -log 1.43x10^-2 = 1.84