In: Chemistry
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.)
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