In 1804 the chemist John Dalton put forward the following idea. It is sometimes called 'Dalton's Law'. 'When two elements combine with each other to form more than one compound, the ratios of the masses of one element that combine with a fixed mass of the other element are simple whole numbers.' A student used the apparatus shown to find out if Dalton's Law is true for three oxides of lead. Methane gas reduced the heated lead oxides to lead. [Figure of apparatus showing porcelain boats with lead oxides A, B, C, heated by methane gas, with excess methane burning] Lead and oxides of lead are harmful by inhalation and if swallowed. They are very toxic to aquatic organisms and may cause long-term damage in the aquatic environment.

1. (a)State two hazards associated with experimenting with lead oxides. For each hazard, state a precaution (other than eye protection) that the student could take to make sure that the experiment is carried out safely. hazard 1 precaution hazard 2 precaution[2]
2. (b)Complete the table. Record the mass of lead that was combined with 1.0g of oxygen in the lead oxide to one decimal place. Use the space below for any necessary calculations. [Table with columns: lead oxide, mass of porcelain boat/g, mass of boat + lead oxide/g, mass of boat + lead after heating/g, mass of lead/g, mass of oxygen/g, mass of lead that was combined with 1.0g oxygen in the lead oxide/g, and rows for A, B, C with some data filled.] lead oxide | mass of porcelain boat/g | mass of boat + lead oxide/g | mass of boat + lead after heating/g | mass of lead/g | mass of oxygen/g | mass of lead that was combined with 1.0g oxygen in the lead oxide/g ---|---|---|---|---|---|--- A | 5.26 | 9.31 | 9.04 | B | 5.12 | 8.96 | 8.48 | C | 5.23 | 10.52 | 10.06 |[2]
3. (c)(i)Use the values of mass of lead that was combined with 1.0g oxygen in the lead oxide to calculate the ratio of mass of lead in each compound. mass of lead in A : mass of lead in B : mass of lead in C : :[1]
4. (c)(ii)If Dalton's Law is true there should be a simple whole number ratio of the masses of lead in each compound combined with a fixed mass of oxygen. Use your answer to (i) to state and explain whether the student's experimental results support Dalton's Law.[1]
5. (d)In this experiment, identify the independent variable, the dependent variable.[2]
6. (e)(i)Another sample of lead oxide B was found to contain 3.067 g lead and 0.474g oxygen. Calculate the empirical formula of B. Show your working. [A: Pb, 207.2; O, 16.0] empirical formula of B =[1]
7. (e)(ii)What additional piece of information is required to calculate the molecular formula of B?[1]
8. (f)Before the porcelain boats containing the lead were weighed, they were allowed to cool completely with the methane gas still passing over them. Apart from the hazards associated with handling hot apparatus, explain why this procedure is essential to ensure that the results are reliable.[1]
9. (g)The student thought that not all of the lead oxide C had been reduced. What should the student do to make sure all the lead oxide C had been reduced?[1]

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