How do you calculate the concentration based on the pH value?

Answer 1

The pH is a measure of the concentration of protons (H+) dissociated in an acid solution.

To clarify, hydrochloric acid is generally denoted by HCl, however, this is actually hydrogen chloride as a pure chemical. Only when dissolved in water does this become hydrochloric acid as the compound dissociated in water as H+ and Cl- ions by the following dissociation reaction:

HCl <=> H+ + Cl-

The solution of water and acid then contains a concentration of H+ and Cl- ions rather than a concentration of HCl molecules. The concentration of H+ is also known as the pH of an acid which is an abbreviation of the word potenz (german for strength or potency) and H for the proton.

The mathematical relationship relating pH to the concentration of H+ is pH = -log([H+]) where [H+] is the concentration of protons. If we want to know the concentration of protons we rearrange to get the relation [H+] = 10^(-pH) mol/L

So, say it is known the acid is hydrochloric acid with a pH of 1 then the concentration of protons in the solution is [H+] = 10^(-1) = 0.1 mol/L. Which is a very strong acidic solution compared to H2O (pH=7, [H+]=1e-7 mol/L).

Similarly to pH, also pOH exists which is a measure of the basicity of a compound such as NaOH. The mathematical relationship is exactly the same as for pH except the value for pOH is used to find the concentration of OH- molecules.

Answer 2

pH is negative of log of Hydrogen ion concentration. Sum of pH + pOH is 14. So if you know the hydroxide ion concentration, work out its logarithm, and subtract the negative of the answer from 14. eg if OH concentration is 10^(-9.5) then the pH is equal to 14 minus +9.5 = 4.5 (and pOH is +9.5 if anybody uses pOH).