Question

A buffer solution contains 0.392 M ammonium chloride and 0.498 M ammonia. If 0.0206 moles of perchloric acid are added to 150 mL of this buffer, what is the pH of the resulting solution ? (Assume that the volume does not change upon adding perchloric acid)

Answer 1

First calculate moles of ammonia & ammonium chloride.

We have relation , No. of moles = Molarity volume of solution in L

No. of moles of NH ₃ = 0.498 mol / L 0.150 L = 0.0747 mol

No. of moles of NH ₄ Cl = 0.392 mol / L 0.150 L = 0.0588 mol

Consider reaction of perchloric acid with buffer solution.

HClO ₄ reacts with ammonia and forms ammonium ions. Hence, concentration of ammonia decreases and that of ammonium ion increases.

NH ₃ + HClO ₄$\rightarrow$ NH₄⁺ + ClO ₄^-

Let's use ICE table.

Concentration ( moles )	NH 3	HClO 4	NH4+
I	0.0747	0.0206	0.0588
C	- 0.0206	- 0.0206	+ 0.0206
E	0.0541	0.0000	0.0794

pH of basic buffer solution is calculated by using Henderson's equation.

pH = pKa of conjugate acid +log [ Base ] / [ Conjugate acid ]

$\therefore$ pH = pKa of  NH₄⁺ + log [NH ₃] / [  NH₄⁺ ]

$\therefore$ pH = 9.245 + log ( 0.0541 mol / 0.150 L ) / ( 0.0794 mol / 0.150 L )

$\therefore$ pH = 9.245 - 0.166

pH = 9.08

ANSWER : pH of ammonia buffer solution after addition of perchloric acid is 9.08