Question

What will be the $pH$ of the solution, if $0.01$ moles of HCl is dissolved in a buffer solution containing $0.02$ moles of propanoic acid $(K_a=1.34\times {10}^{-5})$ and $0.0152$ moles of salt, at ${25}^o$C?
$\left[log\right(0.173)=-0.76]$

• A. $3.11$
• B. $4.11$
• C. $5.11$
• D. $6.11$

Answer 1

Answer: (B)

$4.11$

The buffer solution has weak propanoic acid and its salt that is it's conjugate base

$C{H}_{3}CHO{O}^{-}+H^{+}\rightleftarrows C{H}_{3}C{H}_{2}COOH+H_{2}O$

$n_{HCl}=0.01$

Number of moles of propanoic acid $=0.02$

Moles of conjugate base after addition of $HCl=0.0152-0.01=0.0052$

moles of propanoic acid after addition of $HCl=0.02+0.01=0.03$

$pH=p{K}_a+\log \left[\frac{base}{acid}\right]$

$p{K}_a=-\log K_a=-\log 1.34\times {10}^{-5}$

$p{k}_a=4.87$

$pH=4.87+\log \left[\frac{0.0052}{0.03}\right]=4.87+\log \left(0.133\right)$

$pH=4.87-0.76$

$pH=4.11$