Which will make a basic buffer?

Correct option is 4) 100mL of 0.1M HCl + 200mL of 0.1M NH4OH

1. CH3COOH + NaOH → CH3COONa + H2O
   25mL × 0.1M = 2.5mmol
   50mL × 0.1M = 5mmol
   0 2.5mmol
   2.5mmol
   This is a basic solution due to excess NaOH. This is not a basic buffer.

2. CH3COOH + NaOH → CH3COONa + H2O
   100mL × 0.1M = 10mmol
   100mL × 0.1M = 10mmol
   0 10mmol
   10mmol
   Hydrolysis of salt takes place. This is not a buffer.

3. HCl + NH4OH → NH4Cl + H2O
   100mL × 0.1M = 10mmol
   200mL × 0.1M = 20mmol
   0 10mmol
   10mmol
   This is a basic buffer.

4. HCl + NaOH → NaCl + H2O
   100mL × 0.1M = 10mmol
   100mL × 0.1M = 10mmol
   0 10mmol
   10mmol
   Neutral solution.