A flask contains 500 mL of an acid with a pH of 4. If 500 mL of water is added, what will the pH of the new solution be? I don't want to know the answer please* I'm just wondering if it is as easy as adding the two pH's ie. 7 + 4 or do I have to work out the H+ of each ie. 10-7 and 10-4 and then add, multiply or divide those. And why?