Reacting masses and percentage compositionIB MYP Chemistry: Subtopic test

1. 1
   A flare manufacturer in South Korea burns magnesium ribbon in oxygen to make a bright white light. The equation for the reaction is 2Mg + O₂ → 2MgO. Relative atomic masses: Mg = 24, O = 16.

   (a)

   In the balanced equation, what is the ratio of moles of magnesium used to moles of magnesium oxide formed?

   [1 mark]

   • A1 : 2
   • B2 : 1
   • C1 : 1
   • D1 : 3

   (b)

   What mass of magnesium oxide is made from 12 g of magnesium?

   [1 mark]

   • A12 g
   • B24 g
   • C40 g
   • D20 g

   (c)

   Calculate the percentage by mass of oxygen in magnesium oxide.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A lime works in Morocco heats limestone, calcium carbonate, in a kiln: CaCO₃ → CaO + CO₂. A technician tests a 200 g sample of pure calcium carbonate. Relative atomic masses: Ca = 40, C = 12, O = 16.

   (a)

   What is the percentage by mass of calcium in calcium carbonate?

   [1 mark]

   • A40%
   • B48%
   • C12%
   • D60%

   (b)

   What mass of calcium oxide, CaO, is made when all of the 200 g sample decomposes?

   [1 mark]

   • A56 g
   • B112 g
   • C200 g
   • D224 g

   (c)

   Calculate the mass of carbon dioxide made from the 200 g sample.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student heats different masses of copper powder strongly in an open crucible so that the copper reacts with oxygen from the air to form black copper(II) oxide: 2Cu + O₂ → 2CuO. She weighs the copper before heating and the copper oxide after cooling. The air supply is always more than enough. Mass of copper and mass of copper oxide formed: 0.64 g and 0.80 g; 1.28 g and 1.60 g; 1.92 g and 2.40 g; 2.56 g and 3.00 g. Relative atomic masses: Cu = 64, O = 16.

   (a)

   State a testable hypothesis for this investigation, give a scientific reason for it, and state one control variable.

   [3 marks]

   (b)

   Calculate the mass of copper oxide expected from 2.56 g of copper. Evaluate the results, and suggest one improvement to the method.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemical company in Johannesburg makes iron(II) sulfide, FeS, for laboratories by heating iron filings with sulfur powder: Fe + S → FeS. A trial batch uses 10.0 g of iron and 5.0 g of sulfur. The company wants to reduce wasted raw materials and the environmental impact of its factory. Relative atomic masses: Fe = 56, S = 32.

   (a)

   Calculate which reactant is limiting, the mass of iron(II) sulfide formed, and the mass of the other reactant left over.

   [6 marks]

   (b)

   Discuss whether calculating the exact reacting masses before every batch is the best way for the company to reduce waste and protect the environment.

   [6 marks]

   Total for question 4: 12 marks

End of questions