Weak acid and Strong acid both will contribute H+ ion.
For the first approximation we can neglect the H+ ions coming from the weak acid solution and calculate the pH of the solution from the concentration of the strong acid only.
To calculate exact pH, we have to take the effect of presence of strong acid on the dissociation equilibrium of the weak acid.
If [SA] = C1 and [WA] = C2, then [H+ ] from SA = C1 the weak acid will dissociate as follows.
(The weak acids dissociation will be further suppressed because of presence of strong acid, common ion effect)
Ka = (C2α + C1)α
Total H+ ion concentration = C1 + C2α
If the total [H+ ] from the acid is more than 10–6 M, then contribution from the water can be neglected, if not then we have to take [H+ ] from the water also.
