Posted by Katie on .
What is the pH of the following solutions?
1)a solution that is 5.4×10−2 M in HClO4 and 3.0×10−2 M in HCl
2)a solution that is 1.80% HCl by mass (Assume a density of 1.01g/mL for the solution.)
Please help! I am so confused!

College Chemistry 
DrBob222,
HClO4 is a strong acid. HCl (solution) is a strong acid. Calculate moles L H^+ in each, add them together for total moles H^+.
How much does a L weigh?
1000 mL x 1.01 g/mL = 1010 grams.
How much HCl does that contain? 1.8%; therefore,
1010 x 0.018 = ?? grams HCl.
How many moles is that?
moles = grams/molar mass = xx moles. That is in 1 L therefore, that is the molarity. It's a strong acid so that must be the H^+ in moles/L. Calculate pH from that. 
College Chemistry 
Katie,
Thanks for your help! I understand number 2, but I still don't get number 1. Is there any other way to explain it?

College Chemistry 
DrBob222,
For #1, let's make things simpler by using different molarities and different acids. I think the small numbers make the waters murky. Suppose we had 1 L of a solution that was 1 molar in HCl and 1 M in HCl. (I know that's the same acid but play along with me.) So the total H^+ must be 2 M. It's 1 mole/L from the first HCl and it's 1 mole/L from the second HCl, right. So 1 mole/L from the first acid + 1 moles/L from the second acid makes 2 moles/L from both of them. Now we change that to 1 M in HCl and 1 M in HClO4. The H^+ is a H^+. All of the H^+ look alike.They don't know they came from different sources. So the H^+ is 1 M from HCl and it is 1 M from HClO4 (that's a strong acid, too, and ionizes completely). So the total H^+ is 1 + 1 = 2 M. The Cl^ is only 1 M and the ClO4^ is only 1 M but the H^+ is 2 M. Is the total + charge = total  charge. Yes. 2M + ions and 2M negative ions.

College Chemistry 
Katie,
THANK YOU!