The mole, concentration and empirical formulaeEdexcel International A Level Chemistry: Subtopic test

1. 1
   A technician weighs 5.85 g of sodium chloride, NaCl (molar mass 58.5 g mol⁻¹), dissolves it in water and makes the solution up to 250 cm³. The Avogadro constant, L, is 6.02 × 10²³ mol⁻¹.

   (a)

   What amount, in mol, of sodium chloride did the technician weigh out?

   [1 mark]

   • A0.0100 mol
   • B0.342 mol
   • C0.100 mol
   • D10.0 mol

   (b)

   How many formula units of sodium chloride are in the 5.85 g sample?

   [1 mark]

   • A6.02 × 10²²
   • B6.02 × 10²³
   • C1.20 × 10²³
   • D3.52 × 10²⁴

   (c)

   Calculate the concentration of the solution in mol dm⁻³ and in g dm⁻³.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A laboratory stock bottle holds 500 cm³ of sulfuric acid of concentration 0.250 mol dm⁻³. The molar mass of sulfuric acid, H₂SO₄, is 98.1 g mol⁻¹.

   (a)

   What amount, in mol, of sulfuric acid is in the stock bottle?

   [1 mark]

   • A0.0625 mol
   • B0.500 mol
   • C125 mol
   • D0.125 mol

   (b)

   What is the concentration of the acid in g dm⁻³?

   [1 mark]

   • A0.00255 g dm⁻³
   • B24.5 g dm⁻³
   • C12.3 g dm⁻³
   • D392 g dm⁻³

   (c)

   Calculate the mass of sulfuric acid needed to make 2.50 dm³ of solution of the same concentration.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student analyses two compounds. Compound P, an oxide of iron, contains 2.10 g of iron and 0.90 g of oxygen. Compound Q is a hydrocarbon with relative molecular mass 70.0 that contains 85.7% carbon by mass. Relative atomic masses: H = 1.0, C = 12.0, O = 16.0, Fe = 55.8.

   (a)

   Use the data to calculate the empirical formula of compound P.

   [3 marks]

   (b)

   Calculate the empirical formula and the molecular formula of compound Q.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A water-treatment chemist makes a standard solution by dissolving 3.55 g of anhydrous sodium sulfate, Na₂SO₄, in water and making it up to 500 cm³ in a volumetric flask. In a separate task she identifies compound R, which contains only carbon, hydrogen and oxygen. Complete combustion of 0.920 g of R gives 1.76 g of carbon dioxide and 1.08 g of water, and the relative molecular mass of R is 46.0. Use: H = 1.0, C = 12.0, O = 16.0, Na = 23.0, S = 32.1, CO₂ = 44.0, H₂O = 18.0, L = 6.02 × 10²³ mol⁻¹.

   (a)

   Calculate the amount of sodium sulfate dissolved, the concentration of the solution in mol dm⁻³ and in g dm⁻³, and the number of sodium ions in the flask. Give each answer to 3 significant figures.

   [6 marks]

   (b)

   Use the combustion data to determine the empirical formula and the molecular formula of compound R.

   [6 marks]

   Total for question 4: 12 marks

End of questions