How many hydrogen ions does an acid have with  pH2?

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We cannot determine the exact number of hydrogen ions an acid has from the pH value alone, but we can determine the concentration of the hydrogen ions in moles per liter.  Let's look at the mathematical definition of pH.

pH = -log[H+]

The reads as the pH of an acid is equal to the negative log of the hydrogen ion (proton) concentration.  So if we have the pH value for an acid, we can plug this into the equation and solve for [H+].

2 = -log[H+]

-2 = log[H+]

[H+] = 10^-2 = 0.01

So for an acid with a pH of 2, the proton concentration is 0.01 M.

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