If 36.0 mL of 7.0e-4 M HClO4 is added to 19.5 mL of 8.2e-4 M LiOH, what is the pH of the solution?

Write the equation.
Calculate mols HCLO4.
Calculate mols LiOH.
Determine how much of which reagent remains after the reaction.

pH = -log(H^+)
post your work if you get stuck.

found the mols of both HClO4 and LiOH
what do you mean by reagent remaining after reaction?

Write the equation. One is an acid, the other a base, they will react to form a salt + H2O. One will be used up and some of the other will remain.

HClO4 + LiOH --> HOH + LiClO4

how do you figure which is used up and which remains?

Look at the problem below (one of your posts) on Ba(OH)2 and HCl. You asked the same question there and I worked the problem in detail. This one is done the same way. Get back to me if you don't understand.