Aqueous ammonia is a solution of ammonia gas. You are provided with two samples of aqueous ammonia labelled A and B. You are going to investigate the reaction between these samples and 0.500mol/dm³ hydrochloric acid. Read all the instructions carefully before starting the experiments. Instructions You are going to do four titration experiments. Rinse and fill a burette with 0.500mol/dm³ hydrochloric acid.

1. (a)Experiment 1 • Use a volumetric pipette to add 25.0cm³ of A to a conical flask. • Add five drops of methyl orange indicator to the conical flask. • Record the initial burette reading in Table 1.1. • Add 0.500mol/dm³ hydrochloric acid from the burette while swirling the flask, adding drop by drop near the end-point, until the solution just changes colour. • Record the final burette reading in Table 1.1. Experiment 2 • Empty the conical flask and rinse it with distilled water. • Refill the burette if necessary. • Repeat experiment 1. Complete Table 1.1 with the volume used in each experiment. Table 1.1 | | Experiment 1 | Experiment 2 | |---|---|---| | final burette reading/cm³ | | | | initial burette reading/cm³ | | | | volume used/cm³ | | |[4]
2. (b)Experiments 3 and 4 • Empty the conical flask and rinse it with distilled water. • Refill the burette if necessary. • Repeat Experiments 1 and 2 using B instead of A. • Record the initial and final burette readings for experiments 3 and 4 in Table 1.2. • Complete Table 1.2 with the volumes used in experiments 3 and 4. Table 1.2 | | Experiment 3 | Experiment 4 | |---|---|---| | final burette reading/cm³ | | | | initial burette reading/cm³ | | | | volume used/cm³ | | |[2]
3. (c)State the colour change observed in the flask in each experiment. from ..................................................... to .....................................................[1]
4. (d)Suggest why you are provided with a white tile for use in these titrations.[1]
5. (e)Calculate the mean volume of 0.500mol/dm³ hydrochloric acid needed to neutralise 25.0 cm³ of A and of B. mean volume of acid needed for solution A ..................................................... cm³ mean volume of acid needed for solution B ..................................................... cm³[1]
6. (f)The equation for the reaction between hydrochloric acid and aqueous ammonia is shown. HCl + NH3 → NH4Cl Use the mean volume of acid needed to neutralise 25.0cm³ of A from (e) to calculate the concentration of ammonia in A. Give your answer to an appropriate number of significant figures. concentration ............................................... mol/dm³[2]
7. (g)Calculate the volume of ammonia gas measured at room temperature and pressure, r.t.p., dissolved in 25.0 cm³ of A. The volume of one mole of any gas at r.t.p. is 24 dm³. volume ..................................................... dm³[2]
8. (h)Use your answers to (e) and (g) to calculate the volume of ammonia gas measured at r.t.p. dissolved in 25.0 cm³ of B. volume ..................................................... dm³[1]
9. (i)The burette is rinsed with 0.500mol/dm³ hydrochloric acid immediately before it is filled. Explain why the burette should not be rinsed with distilled water immediately before it is filled with 0.500mol/dm³ hydrochloric acid.[2]
10. (j)Suggest why universal indicator is not suitable for use in these titrations.[1]