Which of the following combinations would be best to buffer an aqueous solution at a pH of 4.94? A HNO₂ and NO₂, K₂ = 4.5 x 10-4

Question

Which of the following combinations would be best to buffer an aqueous solution at a pH of 4.94?
A HNO₂ and NO₂, K₂ = 4.5 x 10-4
B H₂PO4 and H₂PO4, Ka1 = 7.5 x 10-³
C H₂PO4 and HPO42-, Ka2 = 6.2 x 10-8
D CH3CO₂H and CH3COO-, K₂ = 1.8 × 10-5

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