A buffer solution contains 0.20 mole of NH,OH and 0.25 mole of NH₄Cl per litre, then the pH value of the solution is (given that dissociation constant of NH₄OH at room temperature is 1.81 x 10^-5):

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DSSSB PGT Chemistry (Female) Official Paper (Held On: 06 Jul, 2018 Shift 1)

Answer 1

CONCEPT:

Buffer Solution and Henderson-Hasselbalch Equation

A buffer solution consists of a weak base and its conjugate acid (or a weak acid and its conjugate base).
The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation:
pH = pKa + log₁₀ ([Base]/[Acid])
For a weak base like NH₄OH, the relationship between pKa and pKb is:
pKa + pKb = 14

EXPLANATION:

Given:
- Moles of weak base (NH₄OH) = 0.20 mole/L
- Moles of conjugate acid (NH₄Cl) = 0.25 mole/L
- Dissociation constant of NH₄OH, Kb = 1.81 × 10^-5
First, calculate pKb:
pKb = -log₁₀(Kb)

pKb = -log₁₀(1.81 × 10^-5)

pKb = 4.74
Next, calculate pKa using the relationship pKa + pKb = 14:
pKa = 14 - pKb

pKa = 14 - 4.74

pKa = 9.26
Now apply the Henderson-Hasselbalch equation:
pH = pKa + log₁₀ ([Base]/[Acid])

pH = 9.26 + log₁₀ (0.20/0.25)

pH = 9.26 + log₁₀(0.8)

pH = 9.26 + (-0.10)

pH = 9.16

Therefore, the pH value of the solution is 9.16.