1. 1
   A student burns 2.4 g of magnesium completely in oxygen and finds that 4.0 g of magnesium oxide is formed. The student wants to use this data to find the empirical formula of magnesium oxide. (Ar: Mg = 24, O = 16)

   (a)

   What does the term 'empirical formula' mean?

   [1 mark]

   • AThe simplest whole number ratio of atoms of each element in a compound
   • BThe exact number of atoms in one molecule of a compound
   • CThe relative formula mass of a compound
   • DThe percentage by mass of each element in a compound

   (b)

   What mass of oxygen combined with the magnesium during this reaction?

   [1 mark]

   • A1.6 g
   • B4.0 g
   • C2.4 g
   • D6.4 g

   (c)

   Calculate the empirical formula of magnesium oxide, showing your working.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student analyses a sample of an iron oxide compound and finds it is made from 5.6 g of iron combined with 2.4 g of oxygen. (Ar: Fe = 56, O = 16)

   (a)

   How many moles of iron are present in the sample?

   [1 mark]

   • A0.1 mol
   • B5.6 mol
   • C0.15 mol
   • D1.0 mol

   (b)

   The student calculates that the mole ratio of iron to oxygen is 0.1 : 0.15. Which whole number ratio, and empirical formula, does this simplify to?

   [1 mark]

   • A2 : 3, giving the empirical formula Fe2O3
   • B1 : 1, giving the empirical formula FeO
   • C1 : 2, giving the empirical formula FeO2
   • D3 : 2, giving the empirical formula Fe3O2

   (c)

   Show full working to confirm the empirical formula of this iron oxide compound, starting from the given masses of iron and oxygen.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A chemist analyses an unknown white powder and finds it is made from 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass. The relative formula mass of the compound is later found to be 180.

   (a)

   A chemist analyses a compound and finds it is made from 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass. Calculate the number of moles of each element present in 100 g of the compound, showing your working. (Ar: C = 12, H = 1, O = 16)

   [3 marks]

   (b)

   Use your mole values to find the simplest whole number ratio of atoms, and hence the empirical formula, of the compound. Then, using the relative formula mass of 180, calculate the molecular formula of the compound.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An industrial chemist analyses a sample of a brown gas produced in a nitric acid plant. A 4.6 g sample of the gas is found to contain 1.4 g of nitrogen and 3.2 g of oxygen. The relative formula mass of the actual gas molecule is later measured as 92.

   (a)

   Calculate the empirical formula of this compound, showing full working. (Ar: N = 14, O = 16)

   [6 marks]

   (b)

   Using the empirical formula found and the relative formula mass of 92 given for the actual gas molecule, calculate the molecular formula of this compound, showing full working, and explain what this tells you about the compound compared to its empirical formula.

   [6 marks]

   Total for question 4: 12 marks

End of questions