pH calculation for strong monoprotic acids
A strong monoprotic acid that we'll indicate as HA in solution is completely dissociated into H+ and A-.
We can consider the reaction practically quantitative. Let's suppose we have a solution of 0.2 M HCl.
Considering that, as we said, HCl is totally dissociated in water, we will have that the concentration of H+ (or rather and more correctly H3O+) is also 0.2 M. Are we interested only in H+ which comes from the dissociation of the hydrocloric acid or we must take into account the self-ionization balance of water?
We know that in a neutral solution . The concentration of hydrogen ions is therefore extremely low; in addition, that originate from the dissociation of acid repress this balance (they move it to the left) and the concentration of coming from self-ionization gets really ridiculous and we can rightfully neglect it.
Then the overall comes virtually just from the dissociation of the acid. Since we have the concentration of hydrogen ions, to find the pH we just have to calculate the opposite of its logarithm.
In our example, we would have:
pH calculation for a strong monoprotic base
Similarly, we can consider a strong monoprotic base completely dissociated in solution. Let's take for example NaOH; in solution you 'll find it completely dissolved as Na+ and OH-. This means that the concentration of OH- in solution will be equal to Cb, the initial concentration of the base.
As in the previous case, which comes from the equilibrium of water self-ionization it is negligible.
Now can therefore say that all the comes from the dissociation of the base.
we have then:
These formulas are valid for monoprotic strong acids and bases (where monoprotic acid means → H3O+, monoprotic base (can accept jsut one proton)→ OH- ) of commonly used concentrations. If the acid or strong base are very diluted we must take into account the equilibrium of self-ionization of water → pH calculation: very diluted strong acids and bases