Question

Calculate the pH and the concentrations of all species present in 0.18 M H 2 SO 3 . ( K a1 = 1.5× 10 −2 , K a2 = 6.3× 10 −8 ) Express your answer to three significant figures and include the appropriate units

Answer 1

    H2SO3(aq) <-----> HSO3^-(aq) + H^+(aq)

initial   0.18 M        0 M       0 M

change     x                    x         x

at equil   0.18 - x             x          x

   Ka1 = [HSO3^-][H^+]/[H2SO3]

1.5*10^-2 = x*x/(0.18-x)

   x = 0.045 M

At equilibrium [H2SO3] = 0.18 - 0.045 = 0.135 M

                 [ HSO3^-] = 0.045 M

                 [H+] = 0.045 M

      HSO3^- (aq) <-----> SO3^2-(aq) + H^+(aq)

initial 0.045 M        0 M         0.045 M

change    x                     x            x

at equili 0.045-x               x           0.045+x = 0.045 M

   Ka2 = [SO3^2-][H^+]/[HSO3^-]

6.3*10^-8 = x*(0.045)/(0.045-x)

x = 6.29*10^-8

[SO3^2-] = x = 6.29*10^-8 M

[HSO3^-] = 0.045 - 6.29*10^-8 = 0.045 M

[ H^+] = 0.045 M