Question

Calculate the change in pH of one litre of buffer solution containing $0.10$ mol each of $N{H}_3$ & $N{H}_{4}Cl$ upon addition of $\left(i\right)$ $0.02$ mol of dissolved gaseous $HCl$ $\left(ii\right)$ $0.02$ mol of dissolved $NaOH$. Assume no change in solution volume. $(K_{b}forN{H}_3=1.8\times {10}^{-5})$

• A. $0.1761$, decreases
• B. $0.206$, decreases
• C. $0.1761$, increases
• D. $0.206$, increases

Answer 1

The correct option is A $0.1761$, decreases

According to the question: $\left(i\right)pH=4.57\left(ii\right)pH=4.92$
$pH=p{k}_b+log\frac{base}{acid}pH=-log(1.8\times {10}^{-5})+log\left(0.1/1\right)\left(0.1/1\right)pH=-log\left(0.000018\right)+log1pH=4.74+0=4.74\left(i\right)Acid=0.1+0.02=0.12;Base=0.1-0.02=0.08pH=4.74+log\left(\frac{0.08}{0.12}\right)=4.74-0.17=4.57$

$\left(ii\right)Base=0.1+0.02=0.12;Acid=0.1-0.02=0.08pH=4.74+log\left(\frac{0.12}{0.08}\right)=4.74+0.18=4.92$