Question

Calculate the concentration of all species in a 0.160M solution of H2CO3.

Enter your answers numerically separated by commas. Express your answer using two significant figures.

[H2CO3], [HCO?3], [CO2?3], [H3O+], [OH?]

Answer 1

H2CO3 <---------------->    HCO3-   + H+

I            0.160                             0                 0

C            -x                                 +x               +x

E          0.160-x                           +x                +x

Ka1 = [HCO3-][H+] / [H2CO3]

4.3 x 10^-7 = x^2 / (0.160-x)   (x is neglected in 0.160-x = 0.160)

x^2 = 6.88 x 10^-8

x = 2.62 x 10^-4

             HCO3^-    <------------------------>        CO3^2-    +   H+

I          2.62 x 10^-4                                      0                2.62 x 10^-4

C          -x                                                    +x              +x

E       2.62 x 10^-4 - x                                   +x              2.62 x 10^-4 + x

Ka2 = [CO3^2-][H+] / [HCO3^-]

5.6 x 10^-11 = x(2.62x 10^-4 + x) / (2.62 x 10^-4 - x)

x = 5.6 x 10^-11

Thus,

[H2CO3] = 0.160 - (2.62 x 10^-4) = 0.16 M

[HCO3^-] = 2.62 x 10^-4 - ( 5.6 x 10^-11) = 2.6 x 10^-4 M

[CO3^2-] = 5.6 x 10^-11 M

[H3O+] = 2.62 x 10^-4 + 5.6 x 10^-11 = 2.6 x 10^-4 M

[OH-] = 3.8 x 10^-11