Question

The ionization constant of formic acid is 1.8 x 10^-4 . Calculate the ratio of sodium formats and formic acid in a buffer of pH 4.25.

Answer 1

Ka = 1.8 × 10⁻¹⁴

pH = 4.25 

For acidic buffer, 

pH = PK_a + log\(\frac{[Salt]}{[acid]}\) 

or log \(\frac{[Salt]}{[acid]}\)= pH − PK_a 

PK_a = −log[K_a ] 

= −log[1.8 × 10⁻⁴ ] 

= − log 1.8 × 4 log10 

= −0.2552 + 4 

= 3.7448 = 3.74 

From eq. (i) 

log \(\frac{[Salt]}{[acid]}\) = 4.25 − 3.74 

= 0.51 

or   \(\frac{[Salt]}{[acid]}\)= antilog 0.51 

= 3.24