## General Chemistry: Principles and Modern Applications (10th Edition)

$[HCOO^-] = 0.77M$
1. Calculate $[H_3O^+]$: $[H_3O^+] = 10^{-pH}$ $[H_3O^+] = 10^{- 4.06}$ $[H_3O^+] = 8.7 \times 10^{- 5}M$ 2. Write the $K_a$ equation, and find the $HCOO^-$ concentration: $K_a = \frac{[H_3O^+][HCOO^-]}{[HCOOH]}$ $1.8 \times 10^{-4} = \frac{8.7 \times 10^{-5}*[HCOO^-]}{0.366}$ $\frac{1.8 \times 10^{-4}}{8.7 \times 10^{-5}} = \frac{[HCOO^-]}{0.366}$ $2.1 * 0.366 = [HCOO^-]$ $[HCOO^-] = 0.77M$