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Which of the following mixtures constitute a buffer solution?

1. HCOOH and HCOONa
2. Na2CO3 and NaHCO3
3. NaCl and HCl
4. NH4Cl and (NH4)2SO4

Correct answer: HCOOH and HCOONa

Solution

HCOOH (formic acid, weak acid) and HCOONa (sodium formate, its conjugate base) form an acidic buffer. Na2CO3 and NaHCO3 also form a buffer (HCO3⁻ as weak acid, CO3²- as its conjugate base). NaCl+HCl is a strong acid system, not a buffer. NH4Cl and (NH4)2SO4 both provide only NH4⁺ with no NH3 present, so no buffer action.

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