Question

Calculate ${\mathrm{CH}}_3{\mathrm{COO}}^{-}$ concentration in $\mathrm{HCl}$ solution. If the dissociation constant of 0.01M ${\mathrm{CH}}_3\mathrm{COOH}$ is $1.8x{10}^{-5}$?

Answer 1

Calculating ${\mathrm{CH}}_3{\mathrm{COO}}^{-}$ concentration in $\mathrm{HCl}$ solution

1. Step 1: Dissociation of 0.01M $\mathrm{HCl}$ and ${\mathrm{CH}}_3\mathrm{COOH}$
   $\mathrm{HCl}\to {\mathrm{Cl}}^{–}+{\mathrm{H}}^{+}$
   ${\mathrm{CH}}_3\mathrm{COOH}\leftrightharpoons {\mathrm{CH}}_3{\mathrm{COO}}^{–}+{\mathrm{H}}^{+}$
2. Step 2: Initial concentration
   $\underset{0.01\mathrm{M}}{{\mathrm{CH}}_3\mathrm{COOH}}\leftrightharpoons \underset{0}{{\mathrm{CH}}_3{\mathrm{COO}}^{–}}+\underset{0}{{\mathrm{H}}^{+}}$
3. Step 3: Determination of concentration of ${\mathrm{CH}}_3{\mathrm{COO}}^{-}$
   The value of the dissociation constant $\left(K_a\right)=1.8\times {10}^{-5}$
   $$\begin{gathered} K_a=\frac{\left[C{H}_{3}CO{O}^{–}\right]\left[H^{+}\right]}{\left[C{H}_{3}COOH\right]} \\ \left[C{H}_{3}CO{O}^{–}\right]=K_a\frac{\left[C{H}_{3}COOH\right]}{[H+]} \\ \left[C{H}_{3}CO{O}^{–}\right]=\frac{(1.8x{10}^{-5}x0.01)}{0.1} \\ \left[C{H}_{3}CO{O}^{–}\right]=1.8x\mathit{}{10}^{-6} \end{gathered}$$

Therefore, the concentration of ${\mathrm{CH}}_3{\mathrm{COO}}^{-}$ is $1.8\times {10}^{-6}$mol L^-1.