Question

Equal volume of 3 acids solution with pH 3,4,5 are mixed in a vessel. What will be the hydrogen ion concentration in the mixture ?

Answer 1

Choose any convenient volume such as 100 mL.
Then calculate millimols each.
mmol = mL x M = ?
H^+ for pH 3 = 1E-3
H^+ for pH 4 = 1E-4
H^+ for pH 5 = 1E-5

mmols pH 3 = 100 x 1E-3
mmols pH 4 = 100 x 1E-4
mmols pH 5 = 100 x 1E-5

M when mixed = total mmols/total mL.

It doesn't matter what volume you choose as long as they are equal. The final M is always total mmols/total mL.