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  4. What is the pH of millimolar solution of ammonium hydroxide which is 20 % dissociated ?

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Q. What is the $pH$ of millimolar solution of ammonium hydroxide which is $20\%$ dissociated ?

MHT CETMHT CET 2014

Solution:

Concentration of $NH _{4} OH$ solution $=0.001 M$

Degree of dissociation$=20 \%=\frac{20}{100}=0.2$

Concentration of $OH ^{-}$ i.e., $\left[ OH ^{-}\right]=$

Concentration of solution $\times$ degree of dissociation

$=0.001 \times 0.02=2 \times 10^{-4}$

$\because \left[ H ^{+}\right]\left[ OH ^{-}\right] =1 \times 10^{-14}$

$\therefore \left[ H ^{+}\right]=\frac{1 \times 10^{-14}}{2 \times 10^{-4}}$

$=\frac{1}{2} \times 10^{-10}$

Again, $\because pH =-\log \left[ H ^{+}\right]$

$\therefore pH =-\log \left(\frac{1}{2} \times 10^{-10}\right)$

$=-\log (-10-0.3010)=10.3010$