EXPERIMENT 14: DETERMINATION OF AMMONIUM ION BY BACK TITRATION Introduction In o

Question

EXPERIMENT 14: DETERMINATION OF AMMONIUM ION BY BACK TITRATION Introduction In one or more previous experiments you learned how to perform titrations. In those titrations, it was necessary for the titration reaction to go to completion in order for the titration to be an effective method of quantitative analysis. Unfortunately not all chemical reactions proceed to completion. In those cases it is often possible to drive the reaction of the analyte (the analyzed substance) to completion by adding an excess of the added reagent. If the amount of the excess of the reagent can be determined, then the amount of the reagent which reacted with the analyte can be calculated by subtracting the amount of the excess from the total amount of added reagent. In this manner it becomes possible to assay those species for which a reaction that goes to completion is not readily available. An analytical method which takes advantage of this approach is back titration. In this laboratory exercise the analyte is an ammonium salt. You probably recall that ammonium ion is the conjugate acid of the weak base ammonia. The analysis is performed in a three step process First the ammonium salt is reacted with an excess of aqueous sodium hydroxide as shown in Reaction (14-1), where the aqueous ammonium ions come from dissolution of the analyte salt. NH4+(aq) + OH-(aq) NH3(g) + H2O(l) (14-1) This reaction normally will not go to completion if only enough hydroxide is added to react with the ammonium ion present. Since the hydroxide is added in excess of the amount required to react with all of the ammonium ion, the reaction is forced to completion. In the second step the dissolved ammonia gas that is formed in Reaction (14-1) is removed from the solution by boiling. The removal of ammonia prior to back titration of excess hydroxide with oxalic acid is necessary to prevent the oxalic acid from also reacting with basic ammonia. If the ammonia is not removed, it can partially react with added acid thereby yielding a back titration end point that is too large and a calculated concentration of ammonium ion that is too small. In the final step of the analysis, the excess hydroxide is titrated with oxalic acid as shown in Reaction (14-2). Since this reaction does go to completion, the amount of excess hydroxide that was added to the ammonium salt can be calculated from the end point. Reaction (14-2) is the reaction for the back titration. 20H (aq)+H C0s(aq) 2H:0)+C0, (aq) (14-2) The pure, solid ammonium salts that are used for the laboratory exercise contain a single ammonium ion for each formula unit of the salt

Explanation / Answer

1. 0.2 M Hydroxide solution means 0.2 moles per litre of solution.

For a 50 ml sample, no. of moles = 0.2 * 50 / 1000 = 0.01 moles Hydroxide ions.

2. 26.8 ml of 0.1 M Oxalic acid is used.

Similar to the previous problem, no. of moles = 0.1 * 26.8 / 1000 = 0.00268 moles oxalic acid.

3. From the reaction between Oxalic acid and hydroxide ions, it is clear that one mole of oxalic acid reacts with 2 moles of hydroxide ions.

Thus for 0.00268 moles of Oxalic acid, no. of moles of Hydroxide ion = 0.00268 * 2 = 0.00536 mole of excess hydroxide ion.

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