Ammonia is a weak base and establishes the following equilibrium in water: NH3(aq) + H2O(l) ---> NH4+ (aq) + OH-(aq). In Part III, the phenolphthalein indicator is used to monitor the equilibrium shifts of the ammonia/ammonium ion system. The phenolphthalein equilibrium established with water is Hph(aq)(colorless) + H2O(l) --> H3O+ (aq) + ph-(aq)(pink or red). You compared the color of the solutions in three test tubes that initially contained 3 mL of 0.1 M ammonium hydroxide and a few drops of phenolphthalein indicator. In a second test tube, you added 6 M NH4OH dropwise. Explain, in terms of the ammonia/ammonium ion equilibrium, why the phenolphthalein equilibrium shifted to the right.

Explanation:

phenolphthalein indicator is an acid - alkaline organic dye that changes color over a particular pH range depending on the medium. As such , asides phenolphthalein, methyl orange, Alzarian yellow are other indicators that are used during avid - base titration experiment.

In this case, phenolphthalein in an acidic medium is colorless while in basic medium is pink. Also phenolphthalein been a weak acid reacting with the hydroxide thereby reducing the concentration, this will have an effect on the equilibrium position by favoring the forward reaction i.e product formation. hence more products will be formed.

In the case of ammonium chloride, the addition of more ammonium ions results in an increase in the concentration of the products and as such, if more ammonium ions are removed, the equilibrium position will shift to favor the backward reaction i.e reactant formation.

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