Question

A solution containing 0.2 N NH4OH and 0.2 N NH4Cl has pH= 9. pKb of NH4OH will be
1) 6
2) 7
3) 5
4)9​

Answer 1

Answer: The correct answer is Option 3.

Explanation:

The chemical equation for the reaction of ammonium hydroxide with hydrochloric acid follows:

$NH_4OH+HCl\rightarrow NH_4Cl+H_2O$

To calculate the pOH of basic buffer, we use the equation given by Henderson Hasselbalch:

$pOH=pK_b+\log(\frac{[salt]}{[base]})$

$pOH=pK_b+\log(\frac{[NH_4Cl]}{[NH_4OH]})$

We are given:

$pK_b$ = negative logarithm of base dissociation constant of ammonium hydroxide = ?

$[NH_4Cl]=0.2N$

$[NH_4OH]=0.2N$

pOH = 9

Putting values in above equation, we get:

$9=pK_b+\log(\frac{0.2}{0.2})\\\\pK_b=9$

To calculate pH of the solution, we use the equation:

$pH+pOH=14\\pH=14-9=5$

Hence, the correct answer is Option 3.