pH value does not allow any statement about the strength of an acid?
Hello,
Can someone explain to me the statement that the pH value of a solution does not allow any conclusions to be drawn about the strength of an acid?
I know that an acid is strong when protolysis is complete, and an acid is weak when protolysis is incomplete. But I can't seem to figure out the answer to this statement.
kind regards
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The strength of an acid can be seen much more above the half equivalence point ([A-]=[AH], pKs=pH according to Henderson-Hasselbalch).
Infinitely diluted, each acid is deprotonated. However, in the case of strong acids, a proportion of material is always deprotonated even at a high concentration, while, for example, acetic acid is not present. As is known, a 1 M HCl solution is more etchable than a 1 M ethyl acetate solution (~ 5%). Both have as many protons, in vinegar, however, the majority is not free . One is put in the salad, the other dissolves bones and marble. keyword: steepness of the titration curves is greater in hydrochloric acid. Acetate is a buffer!
Don't forget: many acids are only so "strong" because they are more soluble or oxidizing – hydrofluoric or nitric acid.
At some point, it makes no difference whether 99.9 or 99.999% of the protons are free. Then we need other media than water such as DMSO and other "acid strengths" – such as the pKs value and the Hammetche acidity function. They're super acids.
The pH is only a matter of dilution. A very strong acid can also be diluted in such a way that the pH value is practically at water (=pH value 7).
The acid strength is a question of the pKs value. This tells you how much this acid likes to split off a proton (simplified).
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It depends on the terms that are often misunderstood:
Acid strength is a property of the particle type, namely the tendency to release protons. HCl is a strong acid because it practically protolyses to 100%, acetic acid far less.
The pH value is a property of the substance, of the respective acid solution. He says how high the concentration of H3O+ ions is.
A 0.01 molar hydrochloric acid (strong acid!) has a higher pH (less acid) than a 3 molar acetic acid (weak acid).