Question

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

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

Answer 1

Answer: (A)

$11.78$

First dissociation
$X(OH{)}_3⟶X(OH{)}_2^{+}+O{H}^{-}$
Second dissociation
$X(OH{)}_2^{+}⟶X(OH{)}^{2+}+O{H}^{-}$
Total $\left[O{H}^{-}\right]=4\times {10}^{-3}+2\times {10}^{-3}=6\times {10}^{-3}$
$pOH=3-\log 6=2.22$
$\therefore pH=11.78$