Question

Which of the following solutions has $pOH=p{K}_b\left(N{H}_3\right)$?

• A. 10 mL of 0.1 M $HCl$ + 10 mL of 0.2 M $N{H}_3$
• B. 10 mL of 0.1 M $HCl$ + 20 mL of 0.1 M $N{H}_3$
• C. 50 mL of 0.2 M $HCl$ + 20 mL of 1 M $N{H}_3$
• D. 20 mL of 0.1 M $HCl$ + 10 mL of 0.2 M $N{H}_3$

Answer 1

Answer: (A), (B), (C)

The correct options are
A 10 mL of 0.1 M $HCl$ + 10 mL of 0.2 M $N{H}_3$
B 10 mL of 0.1 M $HCl$ + 20 mL of 0.1 M $N{H}_3$
C 50 mL of 0.2 M $HCl$ + 20 mL of 1 M $N{H}_3$

As we know
$pOH=p{K}_b+log\frac{\left[salt\right]}{\left[base\right]}$
Whenever there will be the formation of a buffer with [salt]=[base], $pOH=p{K}_b$
In (a), (b) and (c) a buffer solution is formed with [salt] = [base]
In other words, the moles of the base should be double that of the acid so that half the moles of the base present neutralise all the acid. This way equal concentrations of the base and the salt would coexist and satisfy the given condition.