Deduce Alkaline Buffer on Mixing 2 Solutions

A buffer is a solution that maintains pH when small amounts of H⁺ or OH^- is added to it.

It is essentially a mixture of conjugate acid-base pair:
a. acidic buffer = weak acid + conjugate base
b. alkaline buffer = weak base + conjugate acid

The acid will remove OH^- added while the base partner will remove H⁺ added, thereby maintaining pH.

For this question we need to determine the resultant solution when 2 solutions are mixed.

A useful way to determine resultant solution is using the ICE table when there is an acid-base reaction.

A. 10 cm³ of 0.1 mol dm^-3 NaOH and 20 cm³ of 0.2 mol dm^-3 NH₄⁺Cl^-

NaOH is a strong base while NH₄⁺ is conjugate acid of weak base NH₃.

Resultant solution is 0.001 mol of weak base NH₃ + 0.003 mol conjugate acid NH₄⁺.

This is an alkaline buffer and can be our answer but let's go through the other options first.

B. 25 cm³ of 0.1 mol dm^-3 NaOH and 50 cm³ of 0.1 mol dm^-3 CH₃CO₂^-Na⁺

NaOH is a strong base while CH₃CO₂^- is conjugate base of weak acid CH₃COOH.

Resultant solution is a mixture of 2 bases which cannot function as a buffer.

C. 10 cm³ of 1 mol dm^-3 H₂SO₄ and 20 cm³ of 1 mol dm^-3 CH₃CH₂NH₂

H₂SO₄ is a strong acid while CH₃CH₂NH₂ is a weak base.

Resultant solution is 0.02 mol of conjugate acid CH₃CH₂NH₃⁺ which is not a buffer.

D. 50 cm³ of 0.05 mol dm^-3 NaOH to 50 cm³ of 0.10 mol dm^-3 CH₃NH₃⁺Cl^-

NaOH is a strong base while CH₃NH₃⁺ is conjugate acid of weak base CH₃NH₂.

Resultant solution is 0.0025 mol of weak base CH₃NH₂ + 0.0025 mol of conjugate acid CH₃NH₃⁺.

We have another alkaline buffer and the moles of weak base and conjugate acid are exactly the same.

This buffer is at its maximum buffering capacity so pH will be a constant and can best resist changes to pH.

Therefore answer to this question is option D.

Topic: Buffer Solution, Physical Chemistry, A Level Chemistry, Singapore

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