Estimation of pH - More Detail About pH
As noted on the previous page, the pH of a solution is a measure of the number of hydronium ions present in a solution. The greater the number of hydronium ions, the lower the pH. The fewer the number of hydronium ions, the higher the pH. The number of hydronium ions in a solution is determined by two factors (Before continuing it is absolutely critical that you understand the difference between the concentration of an acid/base and the strength of an acid/base):
(1) the concentrations of the acids or bases present
Consider two beakers. The first contains 1 M hydrochloric acid. The second contains 2 M hydrochloric acid. Each beaker contains the same acid, but the second contains a greater number of moles of acid. Since each hydrochloric acid donates a proton to water to form hydronium ions the second beaker will have more hydronium ions, and consequently, a lower pH.
Given two solutions containing the same acid, the solution which has the greater concentration will have the lower pH.
The opposite can be said for bases.
Given two solutions containing the same base, the solution which has the greater concentration will have the higher pH.
(2) the strengths of the acids or bases present
Consider two different beakers. The first contains 1 M hydrochloric acid. The second contains 1 M acetic acid. Each beaker contains the same number of moles of acid. But since hydrochloric acid is a strong acid, all of the HCl molecules form hydronium ions by donating a proton to water. Acetic acid is a weak acid, so that only some acetic acid molecules donate protons to form hydronium ions. The number of hydronium ions in the beaker of HCl will be higher than in the beaker of acetic acid so the pH of the HCl solution will be lower.
Given two solutions containing the same concentration of different acids, the solution which has the stronger acid will have the lower pH.
The opposite can be said for bases.
Given two solutions containing the same concentration of different bases, the solution which has the stronger base will have the higher pH.