Question

You are asked to prepare 4.0 L of a HCN/NaCN buffer that has a pH of 9.48 and an osmotic pressure of 1.44 atm at 298 K. Part A What masses of HCN and NaCN should you use to prepare the buffer? (Assume complete dissociation of NaCN.)

Answer 1

Henderson Hasselbalch equation : pH=pKa + log[salt]/[acid]

Given pH = 9.48

Taking pKa of HCN =9.20 ( varies for different textbooks)

9.48 = 9.20 + log[salt]/[acid]

log[salt]/[acid] = 0.28

[salt]/[acid] = e^0.28 = 1.323

NaCN to HCN ratio is 1.323:1

= iMRT

Given osmotic pressure = 1.44 atm

i= 2 for HCN ; R = 0.082 L atm /mol-K

T = 298K

M = 1.44/2x0.082x298)

Molarity M= 0.0294 M

We need to prepare 4 L of buffer

We know

Molarity = weight / Molar mass x volume in liters

    weight of HCN = 0.0294 x 27 X 4 = 3.175g of HCN

weight of NaCN = 1.323 ( weight of HCN)         ( [NaCN]/ [HCN] = 1.323)

                           = 1.323x3.175 =4.200 g of NaCN