General Chemstry 2

Question

What ratio of NaCN to HCN is needed to prepare a pH 10.30 buffer? (Ka of HCN is 4.9 × 10⁻¹⁰)

Answer 1

To start off, it must be noted that NaCN will form the Na⁺ and CN^- ions. This means that we now have the acid, HCN, and its conjugate base, CN^-. To calculate the ratio of conjugate base to acid, the Henderson-Hasselbalch equation can be used.

pH = pK_a + log( [A-]/[HA] )

The pK_a can be determined from the Ka where pK_a = -log (K_a).

pK_a = -log(4.9 x 10_­^-10)

Therefore, the pK_a is 9.31.

Because we are looking for the ratio, we want [A-]/[HA] so rearrange the Henderson-Hasselbalch equation.

[CN^-]/[HCN] = 10^(pH-pKa)

[CN^-]/[HCN] = 10^(10.30-9.31)

[CN^-]/[HCN] =10^0.99

[CN^-]/[HCN] = 9.77