Question

If first dissociation of $X(OH{)}_3$ is $100\text{\%}$ whereas second dissociation is $50\text{\%}$ and third dissociation is negligible then the $pH$ of $4\times {10}^{-3}MX(OH{)}_3$ is :

• A. $11.78$
• B. $10.78$
• C. $2.5$
• D. $2.22$

Answer 1

Answer: (A)

$11.78$

For polyprotic base:

$\left[O{H}^{-}\right]=C{\alpha }_1+C{\alpha }_2=C({\alpha }_1+{\alpha }_2)=4\times {10}^{-3}\times 1.5=6\times {10}^{-3}$

So, $\left[H^{+}\right]=\frac{{10}^{-14}}{6\times {10}^{-3}}=1.67\times {10}^{-12}$

$pH=-log\left(1.67\times {10}^{-12}\right)=11.78$