Cyanide ion, CN - , is important in metal plating industries as well as in leach
ID: 889516 • Letter: C
Question
Cyanide ion, CN-, is important in metal plating industries as well as in leaching matals from ores because it keeps metals dissolved under conditions where they would otherwise form solids (precipitate) and settle out of solution. It is important to maintain pH>10.5 in these solutions to avoid forming toxic hydrogen cyanide gas. If a solution is prepared by dissolving 10-2M NaCN in water, will the pH be in the region where the solution is safe? What is {HCN} in the solution? The pKa of HCN is 9.24.
Explanation / Answer
NaCN ( Salt of weak acid and strong base) will be hydrolyzed in the following way...
NaCN + H2O = HCN + NaOH
For the solution pH =0.5[ pKw + pKa + logC]....... (where C= conc. of salt = 10-2M)
and [HCN] = (KwC/Ka)
Thus we get, pH = 0.5(pKw + pKa + logC)
= 0.5( 14 + 9.24 + log10-2 )
= 0.5 ( 14 + 9.24 -2 )
= 10.62 > 10.5
So, pH of the solution will be in the safe region.
Given, pKa = 9.24 ; We get, Ka = 5.75 × 10-10
According to the second equation, [HCN]
= (KwC/Ka)
= { (10-14 × 10-2 )/ 5.75 × 10-10}
= 4.2 × 10-4 M
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