Answer to Question #303807 in Physical Chemistry for Chemistry

Question

Sulphuric acid is a dibasic acid. The second dissociation is an incomplete one as shown below: H2SO4is a diprotic acid, dissociating in an aqueous solution in two steps.

H2SO4+ H2O ==> H3O+ + HSO4 -

HSO4 - + H2O <==> H3O+ + SO4 - -

Calculate the pH of 10-3 M Sulphuric Solution.

Hint:

The first dissociation is complete because pK1 = - 3. The second dissociation is

partial, since pK2 = +2. If the stoichiometric molarity of the solution is C mol/L, [H+] < 2*C.

[H+] = C+x, where C is due to the first dissociation and x due to the second.

[HSO4 -] = C – x , [SO4 - -] = x

K2 = [H+][SO4 - -]/[HSO4 -] = (C+x)*x/C-x

Rearrangement gives the quadratic equation

x2 +(C+K2)x - K2*C = 0

Solve for x and then find pH.

Answer 1

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