Question

100 ml solution (I) of buffer containing 0.1(M) HA and 0.2 (M) $A^{-}$, is mixed with another solution (II) of 100 ml containing 0.2(M) HA and 0.3(M) $A^{-}$. After mixing what is the pH of resulting solution ?
Given $p{K}_a$ of HA = 5

• A. $5-log\frac{5}{3}$
• B. $5+log\frac{5}{3}$
• C. $5+log\frac{2}{5}$
• D. $5-log\frac{5}{2}$

Answer 1

The correct option is B $5+log\frac{5}{3}$

After mixing total moles of
$A^{-}=100\times 0.2\times {10}^{-3}+100\times 0.3\times {10}^{-3}$
$=100\times {10}^{-3}\times 0.5$ moles
After mixing total moles of
HA $=100\times 0.1\times {10}^{-3}+100\times 0.2\times {10}^{-3}$
$=100\times 0.3\times {10}^{-3}$ moles
After mixing resulting pH $=5+log\frac{5}{3}$