Which of the following mixture gives a buffer solution with pH = 9.25 ? Given : pKb (NH4OH) = 4.75
- 0.5 M NH4OH (0.2 L) + 0.2 M HCl (0.5 L)
- 0.2 M NH4OH (0.5 L) + 0.1 M HCl (0.5 L)
- 0.2 M NH4OH (0.4 L) + 0.1 M HCl (1 L)
- 0.4 M NH4OH (1 L) + 0.1 M HCl (1 L)
Answer
(2) 0.2 M NH4OH (0.5 L) + 0.1 M HCl (0.5 L)
Solution
pOH = 14 - 9.25 = 4.75 = pKb
For a basic buffer: pOH = pKb + [NH4+][NH4OH]
we need moles of NH4Cl formed = moles of NH4OH left.
NH4OH + HCl NH4Cl + H2O
(A) NH4OH = 0.1 mol, HCl = 0.1 mol only NH4Cl remains (no buffer).
(B) NH4OH = 0.1 mol, HCl = 0.05 mol NH4OH left = 0.05 mol, NH4Cl = 0.05 mol
pOH = 4.75 + 1 = 4.75 pH = 9.25
(C) NH4OH = 0.08 mol, HCl = 0.1 mol HCl in excess (no buffer).
(D) NH4OH = 0.4 mol, HCl = 0.1 mol 0.3 mol base, 0.1 mol salt
pOH = 4.75 + 0.10.3 = 4.27 pH = 9.73
Answer: (2)