the pH value of some acid 急20點

your question should be put in this way:
Ascorbic acid, Citric acid, vinegar (Ethanoic acid);
- which chemical is the strongest acid?
- at certain concentration, what are the pH value of their solutions?

acidity (strength of an acid) is almost NOT related to concentration. only pH is related to concentration.

pKa value is a measure of acidity of molecules. smaller value in pKa indicates higher acidity, i.e. stronger acid. pKa can be zero or negative, meaning that the molecule is Really acidic.
the pKa value of chemicals can be commonly seen in Wikipedia, so you can look it up and compare.

hydrochloric acid: -7
water: 7
ascorbic acid: 4.10
ethanoic acid: 4.76
citric acid: 3.09

so among three, citric acid is most acidic, ascorbic acid less, and ethanoic acid least.
please note that pKa is not related to concentration. no matter the concentration, the pKa will NOT change; pH, however, will.


by some calculation, pH of solutions at certain concentration can be obtained.
there're two definitions of "concentration", one is mass-per-volume, another is number of molecules-per-volume. the later is used in calculating pH.

concentration of citric acid in lemon is about 0.3mole/L, so let's assume the concentration of solutions be 0.3mole/L....

pH of ascorbic acid solution = 2.31
pH of ethanoic acid solution = 2.64
pH of citric acid solution = 1.82
pH of hydrochloric acid = 0.52
pH of pure water = 7.00

citric acid solution is has lowest pH among three.

notice that pH is related to concentration.
for example, if concentration is 0.1mole/L instead..
ascorbic acid = 2.56
ethanoic acid = 2.88
citric acid = 2.06
hydrochloric acid = 1.00
pure water = 7.00

you can see trend is the same, but values are different.