pH refers to the relative acidity or alkalinity of a substance. A 14-point scale measures pH, where 0 means complete acidity, 14 means complete alkalinity, and 7 means complete neutrality. Pure water is the only substance with a 7.0 pH. When you add water to a substance, you make it either less acidic or less alkaline. The neutral pH in the water dilutes the substance. Determining how much dilution is taking place requires some mathematical calculations.

Convert your solution’s pH into molarity. Since pH = - log(H), where H refers to the molarity of the hydrogen ions, it is also true that H = 10^(-pH). So, if you’re using an acidic solution with a pH of 3, the hydrogen ion molarity is 10^(-3) M.

Note the volume of your solution, and then add a specific amount of water to dilute it. For the purposes of this example, assume that you have 100 mL of solution and are diluting it with 400 mL of water.

Divide your initial volume by your total volume. Your initial volume was 100 mL, and you added 400 mL, resulting in a total volume of 500 mL. Dividing 100 by 500 gives you 0.2.

Multiply the answer from Step 3 by your molarity from Step 1. In this example, multiplying 0.2 by 10^(- 3) yields 2 x 10^(-4) M in scientific notation, or 0.0002 M.