Question

You are asked to prepare 1.4 L of a HCN/NaCN buffer that has a pH of 9.78 and an osmotic pressure of 1.11 atm at 298 K.

What masses of HCN and NaCN should you use to prepare the buffer? (Assume complete dissociation of NaCN.)

Answer 1

Molarity = π/iRT = 1.11/(2*0.082*298)

= 0.0227 M

This is conc of CN- = 0.0227 M

We know that

pH = pKa + log (CN-/ HCN)

9.78 = 9.21 + log(0.0227-x/x)

0.0227-x / x = 3.715

x= 0.004814 moles

Mass of HCN = 0.004814*27 g/mol = 0.13 g

Mass of NaCN = (0.0227-0.004814)*49 = 0.876 g