Lactic Acid, HC3H5O3 is a weak acid found in sour milk, where it is produced by

Question

Lactic Acid, HC3H5O3 is a weak acid found in sour milk, where it is produced by the action of lactobacilli on lactose, or milk sugar. The ionization of lactic acid in water is depicted by the following chemical equilibrium reaction (see below). If a 2.5 x 10^ -4 mole of lactic acid is dissolved with 10.0 mL of water to obtain a solution of lactic acid with a pH of 2.75 at equilibrium.

HC3H5O3 (aq) + H2O (l) = H3O+ (aq) + C3H5O3- (aq)

1) calculate the initial concentration of lactic acid solution

2) use the pH of the lactic acid solution to calculate the concentration of [H3O]+ hydronium ions at equilibrium

3) Calculate the concentrations of the other species at equilibrium (use a table to summarize your calculations)

4) Write the expression of the equilibrium constant Ka, and calculate its value

Explanation / Answer

HC3H5O3 (aq) + H2O (l) <===> H3O+ (aq) + C3H5O3- (aq)

1) initial concentration of lactic acid solution =

      = 2.5*10^-4/(10/1000) = 0.025 M

2) [H3O^+] = 10^-pH

   = 10^-2.75

   = 0.00178 M

3) at equilibrium

   [H3O^+] = [C3H5O3-] = 0.00178 M

[HC3H5O3] = 0.025 - 0.00178 = 0.0232 M

4)

ka = [H3O^+] [C3H5O3-]/[HC3H5O3]

   = 0.00178* 0.00178/0.0232

= 1.365*10^-4

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