Chemical formulae and equationsIB MYP Sciences: Subtopic test

1. 1
   A teacher in Lagos burns 0.24 g of magnesium ribbon in an open crucible. The shiny grey metal burns with a bright white flame and leaves a white powder, magnesium oxide. When the powder has cooled, she finds that its mass is 0.40 g.

   (a)

   Which is the correct balanced symbol equation for the reaction of magnesium with oxygen?

   [1 mark]

   • AMg + O₂ → MgO
   • BMg + O₂ → MgO₂
   • C2Mg + O₂ → 2MgO
   • D2Mg + 2O → 2MgO

   (b)

   Which row gives the correct state symbols for magnesium, oxygen and magnesium oxide in this reaction?

   [1 mark]

   • AMg(s), O₂(g), MgO(s)
   • BMg(l), O₂(g), MgO(s)
   • CMg(s), O₂(l), MgO(l)
   • DMg(s), O₂(g), MgO(g)

   (c)

   Explain why the mass of the white powder is greater than the mass of the magnesium, even though mass is conserved in chemical reactions.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student in Mumbai places an open flask containing calcium carbonate chips and dilute hydrochloric acid on a balance. The reaction forms calcium chloride solution, water and carbon dioxide gas. Bubbles are seen, and over three minutes the reading on the balance falls from 150.0 g to 148.9 g.

   (a)

   Which is the correct balanced symbol equation for the reaction?

   [1 mark]

   • ACaCO₃ + HCl → CaCl + H₂O + CO₂
   • BCaCO₃ + HCl → CaCl₂ + H₂O + CO₂
   • CCaCO₃ + 2HCl → CaCl₂ + 2H₂O + CO₂
   • DCaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

   (b)

   Why did the reading on the balance fall?

   [1 mark]

   • AAtoms were destroyed during the reaction
   • BCarbon dioxide gas escaped from the open flask
   • CThe acid became lighter as it was used up
   • DMass is always lost when a gas is made, even in a sealed container

   (c)

   Explain why the total mass of reactants and products is the same in this reaction, even though the balance reading fell.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student in Seoul heats 3.18 g of shiny pink-brown copper foil in an open crucible. The copper turns black. When the crucible has cooled, the black solid, copper oxide, has a mass of 3.98 g.

   (a)

   Write a word equation for the reaction, then a balanced symbol equation with state symbols.

   [3 marks]

   (b)

   The student's hypothesis was that the mass of the solid would increase because copper reacts with a gas in the air. (i) Calculate the mass of gas that combined with the copper. (ii) State, with a reason, whether the results support the hypothesis. (iii) Suggest one improvement that would make the results more reliable and explain why it helps.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A city in Japan runs buses powered by hydrogen fuel cells. In a fuel cell, hydrogen reacts with oxygen from the air and the only product is water. Most hydrogen is made from natural gas, but some is made by splitting water using electricity from solar and wind farms.

   (a)

   Describe how the burning of hydrogen can be shown by a word equation and a balanced symbol equation with state symbols, and use the idea of atoms to explain why mass is conserved. Calculate the mass of water formed when 4 g of hydrogen reacts completely with 32 g of oxygen.

   [6 marks]

   (b)

   Discuss and evaluate the use of hydrogen fuel cells to power buses.

   [6 marks]

   Total for question 4: 12 marks

End of questions