Scientists use pH to measure how acidic or basic water is. A low pH value means water is acidic and a high value means it's basic (sometimes called alkaline). In acidic water, there is an increased concentration of positively charged hydrogen atoms (H+). This concentration determines the pH value. Specifically, the pH is equal to the base 10 logarithm of the H+ concentration, multiplied by 1. If you know the pH of a water solution, you can use this formula in reverse to calculate the H+ concentration in that solution.
Things You'll Need
 Calculator with antilog function

Enter into the calculator the pH value for which you intend to calculate the H+ concentration. For example, if the pH of your solution is 5, enter 5 into the calculator. pH values will almost always be between 0 and 14, so your number should be within this range.

Multiply the value you just entered by 1. This is the first step toward calculating the concentration of H+ in the solution, based on the equation pH = (1) log [H+], where "log" is short for base 10 logarithm and the square brackets around H+ stand for "concentration." Multiplying the pH by 1 puts this equation in the form log[H+] =  pH. In the example, you would multiply 5 by 1 to get 5.

Take the base 10 antilogarithm (or "antilog") of the value you just calculated. You can take the antilog by using the 10^x key on the calculator. By doing this, you are changing the pH equation into the form antilog (log[H+]) = antilog ( pH). The two reverse operations (antilog and log) on the lefthand side cancel each other out, leaving [H+] = antilog ( pH). So the value you calculate in this step is the concentration of H+ in the solution. The units of this concentration are molarity, or moles H+ per liter of solution. The example with a pH of 5 would therefore have an H+ concentration equal to antilog (5) which equals 0.00001 moles/liter. (properties of antilogs from ref 3)
Tips & Warnings
 Some scientists prefer to use the formula H3O+ instead of H+, to show that the positive hydrogen atom typically combines with a neutral water molecule (H2O) to form H3O+, known as the hydronium ion.
References
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