Calculate the mass (in mg) of KCN needed to make 500. mL of aqueous KCN solution with a pH of 10.00. The Ka of HCN = 5.0 x 10-10. I got the answer 7.8 mg but it's wrong. Can someone please explain how to do it?

Answer 1

pH = 10; pOH = 4
.......CN^- + HOH ==> HCN + OH^-
E......x.............1E-4...1E-4
Kb for KCN = (Kw/Ka for HCN) = (1E-4)(1E-4)/x
Solve for x = KCN in mols/L.
To make 500 mL is 1/2 that. Then
mols = g/molar mass. You know mols and molar mass, solve for g and convert to mg.

Answer 2

Nevermind! I realized I was multiplying by the CN- molar mass instead of KCN.

Answer 3

i'm so sad