Why is HCL strong acid

Strong and weak acids

Three acids in comparison

Now let's do a very simple experiment: We prepare 1 molar solutions of hydrochloric acid, phosphoric acid and acetic acid and measure the pH values ​​of the solutions. Here is the result (actual values, not copied from literature):

  • Hydrochloric acid: pH = 0.13
  • Acetic acid: pH = 2.42
  • Phosphoric acid: pH = 1.14

You should think that three different acids with the same concentration also have the same pH value. How can these surprising results be explained?

Let's make it clear once again what is actually meant by the pH value:

The pH value is a measure of the concentration of oxonium ions in an aqueous solution. The lower the pH, the higher the c (H.3O+).

Obviously c (H3O+) quite large in hydrochloric acid, clearly smaller in phosphoric acid, and very small in acetic acid.

If you then consider that the pH value is a logarithmic measure, the results become even more interesting: In hydrochloric acid there are about 10 times as many oxonium ions as in phosphoric acid, and in phosphoric acid there are about 10 times as many oxonium ions as in acetic acid.

  • Hydrochloric acid: c (H3O+) = 10-0,13 mol / l = 0.74 mol / l
  • Phosphoric acid: c (H.3O+) = -1,14 mol / l = 0.07 mol / l
  • Acetic acid: c (H.3O+) = -2,42 mol / l = 0.004 mol / l

When you dissolve an acid in water, the acid dissociates. One or two protons are split off and taken up by water molecules, acid residue ions and oxonium ions are formed.

Example hydrochloric acid

A 1 molar hydrochloric acid normally has a pH value of 0. In the measurement carried out in the student experiment, a pH value of 0.13 was measured, which is not a particularly large deviation. We now set up the following consideration:

  • Concentration c (HCl) in 1 molar hydrochloric acid originally: 1.00 mol / l
  • Concentration c (H3O+) in 1-molar hydrochloric acid for the measurement: 1.00 mol / l (ideally) or 0.74 mol / l for the student experiment.

The hydrogen chloride HCl dissociates almost 100 percent in water!

Example phosphoric acid

The same consideration for the phosphoric acid looks like this:

  • Concentration c (HCl) in 1 molar phosphoric acid originally: 1.00 mol / l
  • Concentration c (H3O+) in 1 molar phosphoric acid for the measurement: 0.07 mol / l for the student experiment, and ideally 0.1 mol / l.

The phosphoric acid H3PO4 dissociates in water to about 10 percent. Of 100 phosphoric acid molecules, only one in ten splits off a proton.

Example acetic acid

If we make similar considerations for acetic acid, we come to that


The acetic acid CH3Approx. 1 percent of COOH dissociates in water. Out of 100 acetic acid molecules, only every hundredth splits off a proton.

The concept of acid strength

Acids such as hydrochloric acid, which dissociate (almost) completely in aqueous solution, are called strong acids. Acids such as acetic acid, which only dissociate 1% or less, are called weak acids.

In the chemical literature, the term acidity is differentiated a little more finely. Then there are very strong, strong, medium, weak and very weak acids.

  • very strong Acids: HCl, H2SO4 i.a.
  • Strength Acids: H3PO4, HCOOH et al.
  • medium strength Acids: CH3COOH, H22CO3 i.a.
  • weakness Acids: HCN, H2O et al.
  • very weak Acids: CH3-CH2-OH, NH3 i.a.

Hydrochloric acid is one of the very strong acids (dissociation of approx. 100%), phosphoric acid of the strong acids (dissociation of approx. 10%), and acetic acid of medium strength acids (dissociation of approx. 1%).

This classification of acids may be sufficient for everyday use, but these five categories are still not precise enough for a chemist. On the next page we will get to know a procedure for determining the exact acid strength.