Question

Cyanide ion, CN–, is important in metal plating industries because it can keepmetals dissolved under conditions where they would otherwise form solids(precipitate) and settle out of solution. The details of how this occurs arepresented in Chapter 11. It is important to maintain pH>10.5 in these solutionsto minimize release of toxic hydrogen cyanide gas. If a solution is preparedby dissolving 10-2MNaCN in water, will the pH be in the region where thesolution is safe? What is the HCN concentration in the solution?

Answer 1

NaCN + H2O ---------------------------> HCN + NaOH

0.01                                                     0            0

0.01-x                                                  x            x

Kb = x^2 / 0.02 -x

1.61 x 10^-5 = x^2 / 0.02 -x

x^2 + 1.61 x 10^-5 x - 3.23 x 10^-7 = 0

x = 5.60 x 10^-4

[OH-]= 5.60 x 10^-4 M

pOH = -log [OH-]

pOH = -log (5.60 x 10^-4 )

pOH = 3.25

pH + pOH = 14

pH = 10.7

the pH we got is more than 10.5 . so the pH is safe

concentration of HCN   = 5.60 x 10^-4 M