Part III. Acetic acid is a weak acid and establishes the following equilibrium in water: HC2H3O2(aq) + H2O(l) H3O+(aq) + C2H3O2-(aq). In Part III, the methyl orange indicator is used to monitor the equilibrium shifts of the acetic acid/acetate ion system. The methyl orange equilibrium established with water is Hmo(aq)(orange-red) + H2O(l) H3O+(aq) + mo-(aq)(yellow). You compared the color of the solutions in three test tubes that initially contained 3 mL of 0.1 M acetic acid and a few drops of methyl orange indicator. In the first test tube, you added 1.0 M NaC2H3O2 drop wise. What color change was observed and what did this color change indicate about the shift in the methyl orange equilibrium?

Explanation:

Equilibrium of acetic acid is as follows

CH₃COOH + H₂O ⇆ CH₃COO⁻ + H₃O⁺

Equilibrium of methyl orange ( which is acidic ) is as follows

HMo( orange )  + H₂O ⇄ Mo⁻ ( yellow)+ H₃O⁺

In acidic solution , Due to presence of H⁺ ion , equilibrium shifts towards the left. So solution is orange . So in acetic acid solution , methyl orange will give orange/red  colour.

When we add sodium acetate solution , it breaks into sodium ion which hydrolyses in water to give sodium hydroxide which is basic in nature due to giving OH⁻ ion . It shifts the balance of methyl orange to the right giving

Mo⁻  having yellow colour. Hence colour of solution becomes yellow.

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