You have been observing an insect that defends itself from enemies by secreting a caustic liquid. Analysis of the liquid shows it to have a total concentration of formate plus formic acid (Ka =1.8 x 10^-4) of 1.45 M; the concentration of formate ion is 0.015 M. What is the pH of the secretion?

pH = 1.76

Explanation:

The mixture of formate and formic acid produce a buffer. The pH of the buffer is obtained using H-H equation as follows:

pH = pKa + log [A-] / [HA]

Where pH is the pH of the buffer:

pKa = -log Ka = 3.74

[A-] is molar concentration of conjugate base (Formate) = 0.015M

[HA] is molar concentration of weak acid (Formic acid) = 1.45M - 0.015M = 1.435M

Replacing:

pH = 3.74 + log [0.015M] / [1.435M]