56mL of a stock solution of hydrobromic acid, HBr, is added to 85mL of water, yielding a solution having a pH of 1.0. What was the molarity of the stock solution?

Question

I know Molarity = moles of solute/ liters of solution

Answer 1

pH = 1.0; (H^+) = 0.1
So you added 56 mL of ?M to 85 mL H2O and the result was 0.1M.
?M x (56/(56+85) = 0.1
Solve for ?M
This assumes, of course, that the volumes are additive. Technically they are not but is dilutions that is usually considered to be so.
Or if you want to do it by mols, you can.
volume = 85 + 56 = 141 mL = 0.141 L
mols in final solution = M x L
That many mols will be in the 56 mL; therefore, that many mols/0.056 = ?M
You shuld get the same answer either way.