Estimate the equilibrium constant for the weak acid HCN, if a 4.68×10-3 M aqueous solution of HCN has a [H+] 0.00000151 M (make an exact calculation assuming that initial concentration is not equal to the equilibrium concentration).

HCN = CN- + H+

1 answer

............HCN ==> H^+ + CN^-
I.........4.68E-3....0.....0
C..........-x........x......x
E........4.69E-4-x...x......x

Ka = (H^+)(CN^-)/(HCN)
Ka = (x)(x)/(4.68E-4-x)
You know x = 1.51E-6. Substitute into the above and solve for Ka.