1. 1
   A student is investigating compounds of magnesium in the laboratory. She heats a sample of magnesium ribbon in a crucible with a lid, occasionally lifting the lid to allow air in, until the magnesium has fully reacted to form magnesium oxide (MgO). She records the mass of the crucible and its contents before and after heating. Relative atomic masses: Mg = 24, O = 16.

   (a)

   A student calculates the relative formula mass of magnesium oxide, MgO. The relative atomic masses are Mg = 24 and O = 16. What is the relative formula mass of MgO?

   [1 mark]

   • A16
   • B24
   • C40
   • D56

   (b)

   The student then calculates the percentage by mass of magnesium in magnesium oxide, MgO. Which of the following is closest to this percentage?

   [1 mark]

   • A40%
   • B60%
   • C16%
   • D24%

   (c)

   Describe how the student could use this method to determine the empirical formula of magnesium oxide, and explain why the crucible lid must occasionally be lifted during heating.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician reacts 25 g of calcium carbonate chips with an excess of dilute hydrochloric acid, according to the equation CaCO3 + 2HCl $\rightarrow$ CaCl2 + H2O + CO2. Relative atomic masses: Ca = 40, C = 12, O = 16, H = 1, Cl = 35.5.

   (a)

   A technician places a conical flask containing dilute hydrochloric acid on a mass balance and adds calcium carbonate chips. The flask is left open to the air, and carbon dioxide gas is released as the reaction proceeds. What happens to the reading on the balance as the reaction proceeds?

   [1 mark]

   • AThe reading decreases because carbon dioxide gas escapes from the open flask
   • BThe reading increases because new atoms are created during the reaction
   • CThe reading stays exactly the same because mass is always conserved
   • DThe reading decreases only if the reaction is exothermic

   (b)

   The technician repeats the experiment, but this time seals the flask with a stopper so that no gas can escape while the reaction takes place. What would the balance reading show as this sealed reaction proceeds?

   [1 mark]

   • AIt would decrease, but by less than in the open flask
   • BIt would decrease at the same rate as in the open flask
   • CIt would increase, as the gas pressure pushes down on the balance
   • DIt would stay constant, as the system is now closed and no mass can leave

   (c)

   Explain, in terms of the law of conservation of mass, why the balance reading decreases during the reaction in the open flask, but would remain constant during the same reaction in a sealed flask.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The technician collects all of the calcium chloride produced in the reaction described above and dissolves it completely in water to make 500 cm3 (0.500 dm3) of aqueous calcium chloride solution.

   (a)

   Calculate the maximum mass of calcium chloride, CaCl2, that could be produced when 25 g of calcium carbonate reacts completely with excess dilute hydrochloric acid, according to the equation CaCO3 + 2HCl $\rightarrow$ CaCl2 + H2O + CO2. Relative atomic masses: Ca = 40, C = 12, O = 16, Cl = 35.5. Show your working.

   [3 marks]

   (b)

   Using your answer to part (a) (or 27.75 g of calcium chloride if you were unable to complete it), calculate the concentration of the calcium chloride solution in g dm$\text{-3}$, given that it was made up to a total volume of 0.500 dm3. Show your working, including units in your final answer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student is investigating the composition of copper oxide by heating copper powder in a crucible in air until it fully reacts. She wants to compare the accuracy of her method to a case where the experimental technique is changed.

   (a)

   A student heats 3.2 g of copper powder in a crucible in air, occasionally lifting the lid, until no further colour change occurs and a black solid (copper oxide) remains with a final mass of 4.0 g. Using these results, determine the empirical formula of the copper oxide formed. Relative atomic masses: Cu = 64, O = 16. Show your full working.

   [6 marks]

   (b)

   The student repeats the experiment, but this time keeps the crucible lid on throughout, never lifting it to let air in. Explain what effect this would have on her calculated empirical formula for copper oxide, and describe two improvements she could make to her overall method to increase the accuracy of her result.

   [6 marks]

   Total for question 4: 12 marks

End of questions