20 mL of 0.1 M H2SO4 solution is added to 30 mL of 0.2 M NH4OH solution. The pH of the resultant mixture is: [pKb(NH4OH) = 4.7]

20ml 0.1M H2SO4 ⇒ nH+=4 mmol
30ml 0.2M NH4OH ⇒ nNH4OH=6 mmol
NH4OH + H+ ⇌ NH4+ + H2O
⇒ 6 4 0 0
⇒ 2 0 4 4
Solution is a basic buffer
pOH = pKb + log[NH4+]/[NH4OH] = 4.7 + log2 = 4.7 + 0.3 = 5
pH = 14 - pOH = 9