We have learned that the acidity of a solution is owing to the presence of hydrogen ions in a solution.
We need to distinguish between the concentration of acid and the concentration of hydrogen ions of an acid.
In most cases, the concentration of hydrogen ions has greater significant than the concentration of the acid.
The concentration of hydrogen ion in acid depends on the

molarity (concentration) of the acid
The higher the molarity of an acid, the higher the concentration (or molarity) of the hydrogen ions in the acid
strength of the acid
Strong acid has higher concentration of hydrogen ions compare with weak acid of same concentration and same basicity
basicity of the acid
The concentration of hydrogen ions of a diprotic acid is higher than the monoprotic acid and lower than the triprotic acid

Example:

Calculate the number of mol of hydrogen ions that contain in 200cm³ of sulphuric acid 0.4 mol dm^-3.

Answer:

Number of mole of sulphuric acid
\[\begin{gathered}
n = \frac{{MV}}{{1000}} \hfill \\
n = \frac{{(0.4)(200)}}{{1000}} \hfill \\
n = 0.08mol \hfill \\
\end{gathered} \]

Sulphuric acid is a diprotic acid. Each molecule of sulphuric acid will produce 2 hydrogen ions when dissolve in water.

Number of mole of hydrogen ions
= 2 x Number of mole of sulphuric acid
= 2 x 0.08 = 0.16 mol

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