Aqueous ammonia is a solution of ammonia gas. A student titrates two samples of aqueous ammonia, A and B, with 0.500 mol/dm³ hydrochloric acid. The student does four titrations, two with A and two with B. The student: Step 1. rinses and fills a burette with 0.500 mol/dm³ hydrochloric acid Step 2. uses a volumetric pipette to add 25.0 cm³ of A to a conical flask Step 3. adds five drops of methyl orange indicator to the conical flask Step 4. places the conical flask on a white tile Step 5. adds 0.500 mol/dm³ hydrochloric acid from the burette while swirling the contents of the flask, adding drop by drop near the end-point, until the solution just changes colour Step 6. empties the conical flask and rinses it with distilled water Step 7. repeats steps 2 to 6 Step 8. repeats steps 2 to 7 using aqueous ammonia B instead of aqueous ammonia A.

1. (a)Fig. 2.1 shows the burette readings for the two titrations with A. [Figure 2.1 shows burette readings for Titration 1 and Titration 2 with solution A.] Record the burette readings in Table 2.1. Complete Table 2.1 with the volume used in each titration. Table 2.1 Titration 1 Titration 2 final burette reading/cm³ initial burette reading/cm³ volume used/cm³[3]
2. (b)Fig. 2.2 shows the burette readings for one of the titrations with B. [Figure 2.2 shows burette readings for Titration 4 with solution B.] Record the burette readings for Titration 4 in Table 2.2. Complete Table 2.2 with the volume used in each titration. Table 2.2 Titration 3 Titration 4 final burette reading/cm³ 34.9 initial burette reading/cm³ 0.2 volume used/cm³[1]
3. (c)Explain why the student uses a white tile in these titrations.[1]
4. (d)Calculate the mean volume of 0.500 mol/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]
5. (e)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 25.0 cm³ of A in (d) to calculate the concentration of ammonia in A. Give your answer to an appropriate number of significant figures. concentration ..................................................... mol/dm³[2]
6. (f)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]
7. (g)Use your answers to (d) and (f) to calculate the volume of ammonia gas measured at r.t.p. dissolved in 25.0 cm³ of B. volume ..................................................................... dm³[1]
8. (h)The student rinses the burette with 0.500 mol/dm³ hydrochloric acid immediately before it is filled. Explain why the student should not rinse the burette with distilled water immediately before it is filled with 0.500 mol/dm³ hydrochloric acid.[2]
9. (i)Suggest why universal indicator is not suitable for use in these titrations.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.