Buffer solution; pH of NH₄OH-NH₄Cl mixture

Short answer

A buffer resists pH change upon acid/base addition. Moles NH4OH=0.080, NH4Cl=0.045, pKb=4.745, pOH=4.745+log(0.045/0.080)=4.495, so pH=14−4.495=9.51.

What is a buffer solution? Calculate the pH of the resulting buffer by mixing 400 ml of 0.2 M ammonium hydroxide and 150 ml of 0.3 M ammonium chloride solution. [Kb = 1.8×10⁻⁵]

A buffer solution is a solution that resists a change in its pH when a small amount of a strong acid or strong base is added, or when it is diluted. It contains a weak acid with its salt (acidic buffer) or a weak base with its salt (basic buffer).

Calculation

NH₄OH (weak base) and NH₄Cl (its salt) form a basic buffer.

Moles NH₄OH = 0.2 × 400/1000 = 0.080 mol
Moles NH₄Cl = 0.3 × 150/1000 = 0.045 mol
(Both are in the same final volume, so the ratio of moles = ratio of concentrations.)

pKb = −log(1.8×10⁻⁵) = 4.745
pOH = pKb + log([salt]/[base])
    = 4.745 + log(0.045/0.080)
    = 4.745 − 0.250 = 4.495
pH  = 14 − 4.495 = 9.505

Answer: pH ≈ 9.51

✓ Answer checked by Claude Opus 5.5 (high effort)

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Buffer solution; pH of NH₄OH-NH₄Cl mixture

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