Question

How much sodium acetate should be added to $0.1\,M$ solution of $ CH_{3}COOH $ to give a solution of $pH \,5.5$. ( $ pK_{a} $ of $ CH_{3}COOH=4.5 $ ) ?

• A. 0.1 M
• B. 0.2 M
• C. 1.0 M
• D. 10.0 M

Answer 1

Answer: (C)

$pH = p K_{a}+\log \frac{\text { [salt] }}{\text { [acid] }}$
Here, $pH =5.5, p K_{a}=4.5$
Concentration of $CH _{3} COOH =0.1 M$
$pH = pK _{a}+\log \frac{\left[ CH _{3} COONa \right]}{\left[ CH _{3} COOH \right]}$
$5.5 =4.5+\log \frac{\left[ CH _{3} COONa \right]}{0.1} $
$1.0 =\log \frac{\left[ CH _{3} COONa \right]}{0.1} $
$10 =\frac{\left[ CH _{3} COONa \right]}{0.1} $
$\left[ CH _{3} COONa \right] =10 \times 0.1=1.0 \,M$