Find the concentration of [H3O+] in the following acids:

Question

a) The [H3O+] in a solution of 0.20 mol/L methanoic acid is a.bcde mol/L

b) The [H3O+] of a solution of 1.76 mol/L benzoic acid in water is a.bcdef mol/L

Answer 1

a) methanoic acid (HCOOH) is a weak acid with a Ka = 1.8x10-4 (Wikipedia)

HCOOH ==> H+ + COO-

Ka = [H+][COO-] / [HCOOH]

1.8x10-4 = (x)(x) / 0.20 - x (assume x is small and ignore it in the denominator)

x2 = 3.6x10-5

x = 6x10-3 mol/L = [H+] Note: this is only about 3% of 0.2 mol/L so above assumption was valid.

[H3O+] = 6.000 mol/L

b) Do the same as for (a) but use the Ka for benzoic acid (look it up).