Lactic acid C_3H_6O_3 is found in sour milk where it is produced by the action o

Question

Lactic acid C_3H_6O_3 is found in sour milk where it is produced by the action of lactobacilli in lactose or the sugar in milk. A 0.025 M solution of lactic acid has a pH of 2.75. Calculate the equilibrium constant for this acid Had you not been given the pH of the acid and you have to measure it yourself, how would the method of this experiment be applied to the determination of Ka? Would you expect an improvement in the accuracy of your result with the application of the method of this experiment? Explain why or why not.

Explanation / Answer

a)

lactic acid = HA

pH = 2.75

[H+] = 10^-2.75 = 1.78 x 10^-3 M = 0.00178 M

HA --------------------> H+     + A-

0.025                          0               0   ------------------> initial

-0.00178                0.00178                 0.00178-----------> change

0.0232                    0.00178         0.00178

Ka = [H+][A-]/[HA]

Ka = (0.00178)(0.00178) / (0.0232)

Ka = 1.37 x 10^-4

equilibrium constant = 1.37 x 10^-4

b)

another way to determine Ka value of any weak acid . weak acid is titrating with strong base. when moles of acid is equal to moles of salt which is formed form acid +base , this is called half equivalence point . at this point we have to measure the pH of solution by using pH meter.

here : pH = pKa . so pKa known and Ka = 10^-pKa

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