678-expt9_vinegar - Determination of Acetic Acid in Vinegar...

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Determination of Acetic Acid in Vinegar The principal component of vinegar besides water is the weak acid acetic acid, HC 2 H 3 O 2 (K a = 1.78 x 10 -5 ). In this experiment you will determine the concentration of acetic acid in a vinegar sample by titrating the acetic acid with the strong base sodium hydroxide. The stoichiometry of the neutralization reaction is as follows: Molecular: HC 2 H 3 O 2(aq) + NaOH (aq) NaC 2 H 3 O 2(aq) + H 2 O (1) Net Ionic: HC 2 H 3 O 2(aq) + OH¯ (aq) C 2 H 3 O 2 ¯ (aq) + H 2 O (2) The equivalence point of the titration is the condition when the initial number of moles of H + from the acid has been exactly neutralized by an equal number of moles of OH¯ ion from the sodium hydroxide: # of mols H + = # of mols OH¯ (3) Thus, the only species present at the equivalence point are the products sodium acetate and water. However, salts of weak acids contain the conjugate base of the acid and undergo hydrolysis, or reaction with water (Zumdahl, sections 14.8 and 15.4): C 2 H 3 O 2 ¯ (aq) + H 2 O HC 2 H 3 O 2(aq) + OH¯ (aq) (4) Although this equilibrium lies largely to the left side, a small amount of hydroxide ion is formed. Therefore, at the equivalence point of a weak acid-strong base titration the solution is basic. This is in contrast to strong acid-strong base titrations, which produce a neutral solution at the
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678-expt9_vinegar - Determination of Acetic Acid in Vinegar...

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