Using the initial concentration of CH_3 COOH (from Problem 2) and the equilibriu

ID: 964657 • Letter: U

Question

Using the initial concentration of CH_3 COOH (from Problem 2) and the equilibrium concentration of H_3O^+ calculated (from Problem 3), complete the reaction table for vinegar. Then, calculate the acidity constant. Show your work. Reaction Table: CH_3COOH (aq) + H_2O(l) implies CH_3COO^-(aq) + H_3O^+(aq) Initial Change Equilibrium Using the concentrations at equilibrium, the acidity constant (K_a) is calculated as The acidity constant of acetic acid is 1.8 times 10^-5. Calculate the % error of the measured K_a value.

Explanation / Answer

CH3COOH(aq.) + H2O < -------- > CH3COO-(aq.) + H3O+(aq.)

Initial

0.8329

Change

0.8329 – (1.1 x 10-3)

1.1 x 10-3

Equilibrium

0.8318 M

1.1 x 10-3 M

Ka = [CH3COO-][H3O+]/[CH3COOH]

Ka = (1.1 x 10-3)( 1.1 x 10-3) / (0.8318)

Ka = (1.1 x 10-3)2 / (0.8318)

Ka = 1.21 x 10-6 /0.8318

Ka = 1.45 x 10-6.

Ka = 0.145 x 10-5

Hence calculated Ka of acetic acid = 0.145 x 10-5

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Acidity constant of acetic acid is 1.8 x 10-5 (actual)

Then % error of measured Ka can be calculated as,

% error =( Measured Ka / actual Ka) x 100

% error = (0.145 x 10-5/ 1.8 x 10-5 ) x 100

%error = 8.06 %

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