Find the pH of each mixture of acids.

Question

0.020 M HBr and 0.015 M HClO4.

They are both strong acids Si they dissociate completely. Si do I just add 0.020 and 0.015 and take the -log of that concentration to get the pH?

Like thus....
0.020+0.015=0.035
pH=-log(0.035)
pH=1.46

Would this be right?

Answer 1

There is something missing in these problems; namely, how much of each is being mixed. It's obvious that if I take 1 L of 0.020 M HBr and mix it with 1 mL of 0.015M HClO4 that the pH will be different from a mixture of 1L 0.015 M HClO4 + 1 mL 0.020 M HBr. From the previous posts on this same kind of question, I think you have done this right but I still don't like the question. It's ignoring the dilution factor of each acid on each other. And it's giving the student the idea that all we need is the molarity to calculate pH of mixtures. The earlier problems (with HCOOH + CH3OOH as well as with HClO4 + HCOOH) the volume didn't matter as much. In the HClO4/HCOOH mixture the HCOOH contribution was so small it was not added in; however, it still would have diluted the strong acid.