how the concentration of weak acids can be measured?

Answer 1

Assume that you know the Ka of the weak acid.

Assume that you can determine the pH of the weak acid.

Ka = (H+)(A-)/(HA) where

(HA) = the concentration of the weak acid
(H+) = the concentration of the H+ ion
(A-) = the concentration of the conjugate base, which is the same as (H+)

If you know the pH, then (H+) = 10^(-pH), which also is equal to (A-)

(HA) = (10^-pH)^2 divided by the Ka value

Here is an example:

Benzoic acid has a Ka of 6.5E-5. If the pH is 2.3, what is the concentration of benzoic acid?

(HA) = (10^-2.3)^2 divided by 6.5E-5

Doing the math, I would get that the concentration is 0.39 M.

Answer 2

you must know the pKa

and use formula pH = (pKa -logc)/2

log c = 2pH -pKa