Question

You are asked to prepare 3.6L of a HCN/NaCNbuffer that has a pH of 9.57 and an osmotic pressure of 1.29atm at 298 K.

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

mHCN, mNaCN = ? g

Answer 1

pKa for HCN = 9.21

pH = pKa + log [salt]/[acid]

pH = pKa + log [NaCN]/[HCN]

9.57 = 9.21 + log [NaCN]/[HCN]

[NaCN]/[HCN] = 2.29         ............................. (1)

and

? = iMRT ...... where i is van't Hoff factor, in this case i = 3 because NaCN gives Na+ and CN-, and HCN give H+ and CN-. So you have H+, Na+ and CN- and i = 3

? = (3)(M)(0.082)(298)
1.29 atm/(3)(0.082)(298) = M

M = 0.0176 M

and 0.0176*3.6 L = 0.063 moles ......... total moles

and from (1)

0.063 = 3.29*[HCN]

moles of HCN = 0.019

mass of HCN = 27*0.019 = 0.513 gm

moles of NaCN = 0.063-0.019 = 0.044

mass of NaCN = 49*0.044 = 2.156 gm