Question

A strong acid solution with H+=4 is diluted to twice its original volume. Will the pH go up, down, or stay the same?

Answer 1

Calculate it to see.

pH = 4 so (H^+) = 1 x 10^-4 M (use pH = -log(H^+) to calculate that).
Then use mL x M = mL x M to calculate the new pH.