42) A chemist wanted to determine the concentration of a solution of lactic acid

Question

42) A chemist wanted to determine the concentration of a solution of lactic acid, HC3H5O3. She found that the pH of the solution was 2.51. What was the concentration of the solution? The Ka of lactic acid is 1.4 10-4. __

Explanation / Answer

When lactic acid dissociates, this happens. CH2OH-CH2-COOH -> [Lactate-]+[H+] Since the [H+] is >> than would be supplied by water, we neglect the water contribution. Then if the initial concentration is C, the equilibrium expression is [Lactate-][H+]/[Acid]= Ka Fromthe reaction, the [Lactate-]=[H+]. Since the pH=2.51, we can compute [H+] as 10^-2.51 or 10^0.17 x 10^-3 or about 1.5x10^-3. Since the the amount of lactic acid dissociated, ......... [Acid]=C-1.5x10^-3. Then the equilibrium expression is 2.25x10^-6/ (C-0.0015) = 1.4x10^-4. and (C-0.0015) = 0.016 mole/L The initial conc is about 0.0175 M.

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