UPSC Combined Geo-Scientist, Mains · 2019 · Mains Paper III

Q1(c). Buffer capacity and dilution

Question

Compare buffers made from equal 50 mL portions of (i) 0.001 M NH₄Cl and 0.003 M NH₃, and (ii) 0.1 M NH₄Cl and 0.3 M NH₃. Will both work?

Both have approximately the same alkaline pH; mixture (ii) has much greater buffer capacity.

Both have [NH₃]/[NH₄⁺]=3. Henderson–Hasselbalch gives pH≈ pKₐ(NH₄⁺)+log 3≈9.25+0.477=9.73.
Mixing equal volumes halves each concentration but leaves their ratio unchanged.
Mixture (ii) contains 100 times as many buffer moles in the same final volume, so it can neutralise much more added acid or base for…
Mixture (i) is a real but weak-capacity buffer, more sensitive to contamination and dilution.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

CoversBuffer capacitydilutionAcid–base equilibrium