Question on pH

When equal volumes of two solutions, one with pH = 2 and the other with pH = 1 are mixed, what will be the pH of the resulting solution?

Answer:

As both the solutions have the pH values as 1 and 2 we can conclude that they are acidic solutions. Let us call the solution with pH = 1 as A and that with pH = 2 as B.

The total H⁺ concentration from solution A is 10^-1 and that from the solution B is 10^-2.

Let 10mL of both the solutions be added (the actual volume does not make any difference in the pH of the final solution).

Millimoles of H⁺ from solution A = 10^-1 * 100

Millimoles of H⁺ from solution B = 10^-2 * 100

Total millimoles of H⁺ in the final solution is (10^-1 * 100) + (10^-2 * 100)

The total concentration of H⁺ in the final solution is ((10^-1 * 100) + (10^-2 * 100))/200 = 11/200 = 5.5 * 10^-2 mol/L 

So, the pH of the final solution is –log (5.5 * 10^-2) = 2 – log5.5 = 1.26