How do you find the pH of a strong acid solution?

NOTE: This is NOT the same as a WEAK acid solution. To find out how to find the pH of a weak acid, see the Related Questions to the left of this answer (and also to find out if you have a strong or weak acid).

A strong acid is defined as one that dissociates completely in water (see the Related Questions link to the left for a list of the strong acids). That means for every mole of acid added, one mole of free H⁺ (or equivalently, H₃O⁺) is present in the solution. The pH of solution is defined this way:

pH = -log₁₀ [H₃O⁺]

or in English, the pH is equal to the negative logarithm (in base 10) of the concentration of H₃O⁺ in the solution (the concentration must be in units of Molarity (M), which is moles per liter).

So to find the pH of a strong acid solution all you need to know is the concentration of the solution. Let me give a couple of examples.

• If you have a 0.01 M solution of hydrochloric acid, HCl, what is the pH?

• If you have a 0.007 M solution of sulfuric acid, H₂SO₄, what is the pH?

(Note that only one of the two H's in H₂SO₄ is released completely -- the second H is not considered "strong"!)

• If you have a 1.0 M solution of nitric acid, HNO₃, what is the pH?

pH = -log (1.0) = 0

Yes, the pH can be zero. In fact it can be negative!

• If you have a 5.0 M solution of HCl, what is the pH?

First answer by JEK. Last edit by JEK. Contributor trust: 1827 [recommend contributor]. Question popularity: 57 [recommend question]