Relative masses and the moleAQA A-Level Chemistry: Subtopic test

1. 1
   Magnesium nitrate, Mg(NO₃)₂, is used in some fireworks as an oxidising agent. A technician needs to convert between masses and amounts of this compound, using relative atomic masses Mg 24.3, N 14.0 and O 16.0.

   (a)

   Which statement correctly defines the relative atomic mass of an element?

   [1 mark]

   • AThe weighted mean mass of an atom of the element compared with 1/12 of the mass of an atom of ¹²C
   • BThe mass of an atom of the element compared with the mass of an atom of hydrogen
   • CThe mass in grams of one mole of atoms of the element
   • DThe mass of the most abundant isotope compared with 1/12 of the mass of an atom of ¹²C

   (b)

   What is the relative formula mass of magnesium nitrate, Mg(NO₃)₂?

   [1 mark]

   • A86.3
   • B100.3
   • C132.3
   • D148.3

   (c)

   Calculate the amount, in mol, of magnesium nitrate in 7.42 g of Mg(NO₃)₂. Give your answer to 3 significant figures.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student has 3.60 g of water, H₂O, in a beaker. Use relative atomic masses H 1.0 and O 16.0, and the Avogadro constant, 6.02 × 10²³ mol⁻¹.

   (a)

   How many molecules of water are there in 3.60 g of water?

   [1 mark]

   • A2.17 × 10²⁴
   • B1.20 × 10²³
   • C3.61 × 10²³
   • D6.02 × 10²³

   (b)

   How many hydrogen atoms are there in 3.60 g of water?

   [1 mark]

   • A1.20 × 10²³
   • B3.61 × 10²³
   • C2.41 × 10²³
   • D4.82 × 10²³

   (c)

   Calculate the mass, in grams, of one molecule of water. Give your answer in standard form to 3 significant figures.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A technician prepares a solution of sodium carbonate, Na₂CO₃, by dissolving 6.36 g of the solid in water and making the volume up to exactly 250 cm³ in a volumetric flask. Use relative atomic masses Na 23.0, C 12.0 and O 16.0, and the Avogadro constant, 6.02 × 10²³ mol⁻¹.

   (a)

   Calculate the concentration of the sodium carbonate solution in mol dm⁻³.

   [3 marks]

   (b)

   A 25.0 cm³ sample is taken from the solution. Calculate the number of sodium ions in this sample. Give your answer in standard form to 3 significant figures.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student makes the statement 'One mole of calcium chloride, CaCl₂, contains 6.02 × 10²³ atoms and has a mass of 111.1 g.' In a second task, the student dissolves 5.55 g of CaCl₂ in water and makes the volume up to 500 cm³. Use relative atomic masses Ca 40.1 and Cl 35.5, and the Avogadro constant, 6.02 × 10²³ mol⁻¹.

   (a)

   Evaluate the student's statement. Use calculations to explain which parts of it are correct and which are incorrect.

   [6 marks]

   (b)

   Calculate the concentration of the calcium chloride solution in mol dm⁻³, the concentration of chloride ions, and the number of chloride ions in a 10.0 cm³ sample of the solution. Give the number of ions in standard form to 3 significant figures.

   [6 marks]

   Total for question 4: 12 marks

End of questions