Question

Calculate the molar concentration of a solution of acetic acid (HOAc) that has a pH of $4.00(K_a=1.8\times {10}^{-5})$

• A. $1.0\times {10}^{-3}$
• B. $1.0\times {10}^{-6}$
• C. $5.56\times {10}^{-4}$
• D. $3.0\times {10}^{-2}$

Answer 1

The correct option is C $5.56\times {10}^{-4}$

Given, $pH=-log\left[H^{+}\right]=4$
$\left[H^{+}\right]={10}^{-4}M,\left[OA{c}^{-}\right]={10}^{-4}M$
$K_a=1.8\times {10}^{-5}$
$HOAc\left(aq\right)+H_{2}O\left(l\right)\rightleftharpoons H_3{O}^{+}\left(aq\right)+OA{c}^{-}\left(aq\right)K_a=\frac{\left[H_3{O}^{+}\right]\left[OA{c}^{-}\right]}{\left[HOAc\right]}or1.8\times {10}^{-5}=\frac{({10}^{-4}{)}^2}{\left[HOAc\right]}\Rightarrow \left[HOAc\right]=5.56\times {10}^{-4}M$