Question

The pH of a weak mono-acid base at $80\%$ neutralisation with a strong acid in a diluted solution is 7.40. The ionisation constant of the base is:

• A. $1.6\times {10}^{-7}$
• B. $1.0\times {10}^{-5}$
• C. $1.0\times {10}^{-6}$
• D. $2.0\times {10}^{-7}$

Answer 1

The correct option is C $1.0\times {10}^{-6}$

$80\%$ neutralized solution of a weak mono acid base with a strong acid forms a basic buffer solution.

Given, $pH=7.40$

$\therefore pOH=14-7.40=6.60$

$pOH$ of a basic buffer solution$=p{K}_b+log\frac{\left[conjugateacid\right]}{\left[base\right]}$

$\therefore 6.60=p{K}_b+log\frac{80}{20}$

$\Rightarrow p{K}_b=6$

$\Rightarrow K_b={10}^{-6}$