Question

Hydroxylamine, HONH2, readily forms salts such as hydroxylamine hydrochloride, which are used as antioxidants in soaps. Hydroxylamine has Kb of 9.1 × 10–9. What is the pH of a 0.025 M HONH2 solution?

Answer 1

Answer : The pH of the solution is, 9.18

Solution :  Given,

Concentration (c) = 0.025 M

Base dissociation constant = $k_b=9.1\times 10^{-9}$

The equilibrium reaction for dissociation of $HONH_2$ (weak base) is,

                          $HONH_2+H_2O\rightleftharpoons HONH_3^++OH^-$

initially conc.         c                              0             0

At eqm.             $c(1-\alpha)$                       $c\alpha$            $c\alpha$

First we have to calculate the concentration of value of dissociation constant $(\alpha)$.

Formula used :

$k_b=\frac{(c\alpha)(c\alpha)}{c(1-\alpha)}$

Now put all the given values in this formula ,we get the value of dissociation constant $(\alpha)$.

$9.1\times 10^{-9}=\frac{(0.025\alpha)(0.025\alpha)}{0.025(1-\alpha)}$

By solving the terms, we get

$\alpha=0.000603$

Now we have to calculate the concentration of hydroxide ion.

$[OH^-]=c\alpha=0.025\times 0.000603=1.5\times 10^{-5}M$

Now we have to calculate the pOH.

$pOH=-\log [OH^-]$

$pOH=-\log (1.5\times 10^{-5})$

$pOH=4.82$

Now we have to calculate the pH.

$pH+pOH=14\\\\pH=14-pOH\\\\pH=14-4.82\\\\pH=9.18$

Therefore, the pH of the solution is, 9.18