Question

You are asked to prepare 1.9 L of a HCN/NaCN buffer that has a pH of 9.83 and an osmotic pressure of 1.12 atm at 298 K. What masses of HCN and NaCN should you use to prepare the buffer? (Assume complete dissociation of NaCN.) Express your answers using two significant figures separated by a comma.

Answer 1

osmotic pressure = MRT

or, M = osmotic pressure/RT = 1.12 atm /0.082 L atm /K/mol * 298K = 0.046 M

pH = pKa + log[NaCN/HCN]

or, 9.83    = 9.2 + log [NaCN/HCN]

or, [NaCN/HCN] = 4.26

Calculate decimal fraction of NaCN.

[NaCN] = 4.26/1+4.26 = 0.81

[HCN] = 1/1+4.26 = 0.19

Molarity of NaCN = 0.046 M * 0.81 = 0.0373 M

moles of NaCN = 1.9L * 0.0373 M = 0.071 moles

Mass of NaCN =0.071 molea * 49.01 g/mol = 3.47 gm

Molarity of HCN = 0.19 * 0.046 M = 0.00874

moles of HCN = 1.9L * 0.00874 = 0.017 moles

mass of HCN = 0.017 moles * 27.03g/mol = 0.45 gm