Determine the [H3O+] and pH of a 0.200 M solution of formic acid.

Answer 1

Write chemical equation, and then use ICE table since formic acid is a weak acid:

HCO2H(aq) + H2O(l) —> HCOO-(aq) + H3O+(aq)

I 0.200M 0 0

C -x +x +x

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E 0.200M - x x x

Calculate [H3O+] using Ka equation (we can approximate “x” in denominator to be zero. You find your Ka value from a chart of Ka values):

Ka = ([HCOO-][H3O+])/[HCO2H]

Ka = (x²)/(0.2-x)

Ka = (x²)/0.2

1.77 x 10^-4 = (x²)/0.2

sqrt(1.77 x 10^-4 x 0.2) = x

5.95 x 10^-3 = x

Since [H3O+] = x, [H3O+] = 5.95 x 10^-3 M

Calculate pH:

pH = -log[H3O+]

pH = -log(5.95 x 10^-3)

pH = 2.23