Question

You are asked to prepare 2.0 L of a HCN/NaCN buffer that has a pH of 9.6 and an osmotic pressure of 1.29 atm at 298 K. a) What mass of HCN should you use to prepare the buffer? b) What mass of NaCN should you use to prepare the buffer? (Assume complete dissociation of NaCN.)

Answer 1

pKa for HCN = 9.21
pH = pKa + log [salt]/[acid]
9.6 - 9.21 = - log [salt]/[acid]
[salt]/[acid] = 0.39

π = iMRT = (1.29)(M)(0.082)(298)
1.29 atm/(2)(0.082)(298) = M
M = 0.04 M
the molarity is 0.04 M and the ratio of NaCN to HCN is 0.17 : 1.

From there, and knowing you need 2 liters, and knowing molar mass of HCN and molar mass of NaCN, you should be able to calculate how to make the buffer. A buffer solution is an aqueous solution of definite pH that only slightly changes pH with the addition of an acid or a base. Buffer solutions can either be acidic or basic. An acidic buffer (pH<7) is commonly made by mixing a weak acid and its conjugate base. A basic buffer (pH>7) is similarly made by mixing a weak base and its conjugate acid.