Question

$HCOOH\rightleftharpoons HCO{O}^{-}+H^{+};K_a=1.7\times {10}^{-2}$
The ionization of formic acid is represented above. Calculate $\left[H^{+}\right]$ of a solution initially containing $0.1M$ $HCOOH$ and $0.05M$ $HCOONa$:

• A. $8.5\times {10}^{-5}M$
• B. $3.4\times {10}^{-2}M$
• C. $4.1\times {10}^{-3}M$
• D. $1.8\times {10}^{-2}M$

Answer 1

The correct option is B $3.4\times {10}^{-2}M$

$HCOOH\rightleftharpoons {HCOO}^{(-)}+H^{+}{K}_a=1.7\times {10}^{-2}$

The mixture of $0.1M$ $HCOOH$ and $0.05M$ $HCOONa$ acts as buffer.

$\therefore$ $pH={pK}_a+log\frac{\left[HCOONa\right]}{\left[HCOOH\right]}=-log({10}^{-2}\times 1.7)+log\left(\frac{0.05}{0.1}\right)$

$\therefore$ $\left[H^{+}\right]=3.4\times {10}^{-2}M$