Question

A solution is labeled "0.670 M HCN." What are the following in the solution? Express your answers to three significant figures even when only two are valid. Acid-base table [HCN]- [CN -] pH-

Answer 1

ans)

from acid base table

K_a for HCN =  4.0e^-10

         HCN    +    H₂O       H₃O⁺     +     CN^-
i       .670 0                 0
Δ        -x                               +x                +x
f       .670-x                           x                  x

4.0e^-10 = x² / (0.670-x)

2.68e^-10 - 4.0e^-10x = x²

1.64e^-5 - 2.0e^-5x = x

1.64e^-5 = 1.00002x

1.64e^-5 M = x

x = [H₃O⁺] = [CN^-] =1.64e^-5M
pH = - log 1.64e^-5 = 4.78

*[HCN] =0.670 M