Question #917b3
1 Answer
Explanation:
The idea here is that the cyanide anion,
#"CN"_ ((aq))^(-) + "H"_ 2"O"_ ((l)) rightleftharpoons "HCN"_ ((aq)) + "OH"_ ((aq))^(-)#
So even without doing any calculations, you should expect the
Now, look up the acid dissociation constant for hydrocyanic acid
#K_a = 6.17 * 10^(-10)#
Use the fact that an aqueous solution at room temperature has
#K_a * K_b = K_W = 10^(-14)#
to calculate the base dissociation constant of the cyanide anion.
#K_b = (10^(-14))/(6.17 * 10^(-10)) = 1.62 * 10^(-5)#
Now, notice that every mole of cyanide anions that picks up a proton from water forms
If you take
#["CN"^(-)] = ["CN"^(-)]_ "initial" - x#
This means that in order to have a concentration of
By definition, the base dissociation constant is equal to
#K_b = (["HCN"] * ["OH"^(-)])/(["CN"^(-)])#
In your case, this will be equal to
#K_b = (x * x)/(0.154 - x)#
#1.62 * 10^(-5) = x^2/(0.154 - x)#
Now, notice that the base dissociation constant is considerably smaller than the initial concentration of the cyanide anions, so use the approximation
#0.154 - x ~~ 0.154#
This means that you have
#1.62 * 10^(-5) = x^2/0.154#
Solve for
#x = sqrt(0.154 * 1.62 * 10^(-5)) = 1.58 * 10^(-3)#
Since
#["OH"^(-)] = 1.58 * 10^(-3)# #"M"#
An aqueous solution at room temperature has
#"pH + pOH = 14"#
so you can say that the
#"pH" = 14 - [-log(["OH"^(-)])]#
#"pH" = 14 - [-log(1.58 * 10^(-3))]#
#color(darkgreen)(ul(color(black)("pH" = 11.20)))#
I'll leave the answer rounded to two decimal places, but you can leave the answer rounded to three decimal places
#"pH" = 11.197#
because you have three sig figs for the initial concentration of the acid.
As predicted, the