Question

Determine the pH of a solution that contains 5.3*10^-2 M in HClO4 and 3.0*10^-2 M in HCl?

Answer 1

Add (H^+) from HClO4 and (H^+) from HCl but be careful. You may NOT add molarities most of the time; you can here. If you want to see how it works, we can take 1L of the solution.
Then we have M x L = 0.053 x 1L = 0.053moles HClO4.
And we have M x L = 0.030 x 1L = 0.030 moles HCl.
Total moles = 0.053 + 0.030 = 0.083 moles (you CAN add moles--always), then
M = moles/L = 0.083/1L = 0.083 M.
Then pH = -log(H^+)