Chemical Formulae, Equations and Calculations Part 1
2 marks · EasyA student heats a crucible containing a strip of magnesium in air and finds an increase in mass after the reaction.
Explain why the mass increases, and how this method helps to determine the formula of magnesium oxide.
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Must show: Magnesium combines with oxygen from the air, Gain in mass is due to added oxygen, By knowing mass of Mg at start and mass of MgO after, can calculate masses and moles to determine ratio and formula MgO
Common mistakes: Suggests mass is lost; Ignores oxygen from air
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Marking points
B1 · 1 mark · Correctly explains that the increase in mass is due to magnesium reacting with oxygen from the air.
B2 · 1 mark · Explains that the method allows calculation of the empirical formula by comparing the masses of magnesium and oxygen combined.
Worked solution
The mass increases because magnesium reacts with oxygen from the air to form magnesium oxide, so extra oxygen atoms are added to the magnesium.
By measuring the change in mass, we can find out how much oxygen has combined with the magnesium. Using these masses, we can work out the number of moles of magnesium and oxygen atoms and thus the empirical formula of magnesium oxide, MgO.