Question

The Ka's for H2CO3 are K1 = 4.5 x 10^-11. Calculate the pH of the solution when 45.9 ml of 0.25 M KHCO3 is mixed with a)35 mL 0.15 M HBr.     b) 50.0 mL of 0.15 M KOH

Answer 1

H2CO3 Ka1 = 4.3 x 10^-7

Ka2 = 5.6 x 10^-11

millimoles of HCO3- = 45.9 x 0.25 = 11.475

millimoles of HBr = 35 x 0.15 = 5.25

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

11.475       5.25                            0

6.225            0                       5.25

pOH = pKb1 + log [HCO3- / H2CO3]

        = 7.63 + log [6.225 / 5.25]

        = 7.70

pH = 14 - 7.70 = 6.3

pH = 6.30

(b)

millimoles of KOH = 50 x 0.15 = 7.5

HCO3- +   OH-   -------------------> CO32- + H2O

11.475       7.5                                    0            0

3.975           0                                    7.5          7.5

pH = pKa2 + log [CO32- / HCO3-]

     = 10.25 + log [7.5 / 3.975]

     = 10.53

pH = 10.53