A student plans an investigation to find the molar ratio of the reaction between sodium chloride, NaCl, and a lead compound. The student is provided with solid NaCl and 0.200 mol dm⁻³ aqueous lead compound. The reaction between NaCl(aq) and the aqueous lead compound produces an insoluble compound as a precipitate.

1. (a)The student prepares 0.200 mol dm⁻³ NaCl(aq). Calculate the mass of NaCl(s) needed to make 250.0 cm³ of 0.200 mol dm⁻³ NaCl(aq). mass of NaCl = .............................................. g[1]
2. (b)The student weighs the mass of NaCl(s) calculated in (a) in a weighing boat. The solid mass is then transferred into a small beaker. Describe how the student should accurately weigh by difference so the exact mass of NaCl transferred into the small beaker is known.[1]
3. (c)The student is given a small beaker containing the mass of NaCl calculated in (a). Describe how the student should prepare 250.0 cm³ of 0.200 mol dm⁻³ NaCl(aq). Include the names and capacities of each piece of apparatus used in the preparation of the solution.[2]
4. (d)(i)The student plans the following method using the 0.200 mol dm⁻³ aqueous lead compound and the 0.200 mol dm⁻³ NaCl(aq) prepared in (c). Step 1 Mix the NaCl(aq) and the aqueous lead compound in eight separate beakers in the proportions by volume shown in [Table 1.1]. Step 2 Filter the contents of each beaker to collect the precipitate. Step 3 Dry the precipitate for 3 minutes in an oven and allow to cool. Step 4 Weigh and record the mass of precipitate produced in each beaker. State one extra step that would improve this method. Explain why this step is necessary. extra step: explanation:[2]
5. (d)(ii)The volumes of solutions are measured using a burette. Calculate the percentage error when measuring 10.00 cm³ of solution. Show your working. percentage error = ......................................[1]
6. (d)(iii)Explain how you would ensure that the results of the investigation are reliable.[1]
7. (e)(i)The results of the investigation are shown on the graph in [Fig. 1.1]. Draw two straight lines of best fit through the points. Extrapolate both lines so they intersect.[1]
8. (e)(ii)Using [Fig. 1.1] and [Table 1.1], state the volumes of 0.200 mol dm⁻³ NaCl(aq) and 0.200 mol dm⁻³ aqueous lead compound which produce the maximum mass of precipitate. Calculate the molar ratio in which the NaCl and the lead compound react. volume of 0.200 mol dm⁻³ NaCl(aq) = .............................................. cm³ volume of 0.200 mol dm⁻³ aqueous lead compound = .............................................. cm³ molar ratio of NaCl:lead compound = ..............................................[2]
9. (f)Use the molar ratio in (e)(ii) to deduce the formula of the precipitate. formula = ..............................................[1]
10. (g)A student suggests that a simpler method can be used to find the molar ratio. Different volumes of 0.200 mol dm⁻³ NaCl(aq) and 0.200 mol dm⁻³ aqueous lead compound are mixed in test-tubes. The resulting precipitates are allowed to settle. The height of each precipitate is then measured. A further two investigations are carried out. The volumes used and the results of the two investigations are shown. Investigation 1 Precipitate heights are measured after 1 minute. [Table 1.2] [Fig. 1.2] Investigation 2 Precipitate heights are measured after 5 minutes. [Table 1.3] [Fig. 1.3] Neither investigation produced the expected results. Both investigations, 1 and 2, contain weaknesses in the experimental procedure. State how you would modify the experimental procedure in each case so that the expected results are obtained. modification for investigation 1: modification for investigation 2:[2]

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