Three Flasks of Burning Magnesium
In a class experiment on the combustion of magnesium, students burn pieces of magnesium ribbon inside sealed glass flasks. Earlier careful measurements established that 3.0 g of magnesium combines exactly with 2.0 g of oxygen, with nothing left over when those amounts match. Student A puts 1.2 g of magnesium into a flask containing 1.6 g of oxygen. Student B puts 1.2 g of magnesium into a different flask containing only 0.60 g of oxygen. Student C burns 1.2 g of magnesium in an open crucible, collects the white powder that lands in the crucible, and obtains only 1.8 g instead of the 2.0 g the class expected. All flasks are sealed except C's crucible.
1. For students A and B, state the mass of white product formed and which substance, if either, is left over and in what mass. Show the reasoning.
Read the worked answer
Student A: 1.2 g of magnesium needs 1.2 × (2.0/3.0) = 0.80 g of oxygen. The flask holds 1.6 g, so oxygen is in excess by 1.6 − 0.80 = 0.80 g. All the magnesium reacts, giving 1.2 + 0.80 = 2.0 g of product.
Student B: 0.60 g of oxygen can combine with 0.60 × (3.0/2.0) = 0.90 g of magnesium. Only 1.2 g is present, so magnesium is in excess by 1.2 − 0.90 = 0.30 g. Product mass = 0.90 + 0.60 = 1.50 g.
The fixed 3 : 2 combining ratio decides which reactant runs out first.
2. Student C obtained 1.8 g instead of 2.0 g. Does this show that mass is not conserved in burning? Explain what actually happened and how the experiment could be corrected.
Read the worked answer
Mass is conserved; the missing 0.20 g has simply left the crucible. Burning magnesium throws off fine white smoke - tiny solid particles of the oxide - which drifts away and is not collected on the balance. Some magnesium may also react with nitrogen in the air rather than oxygen.
To test conservation properly, burn the ribbon in a closed vessel (or cover the crucible with a lid) and find the mass of the whole vessel before and after. The total mass of the sealed system should stay the same, because the oxygen that joined the magnesium was already inside and was being weighed.
3. Student D uses 1.2 g of magnesium and 1.0 g of oxygen. Which substance is in excess, by what mass, and what mass of product forms?
Read the worked answer
Check against the 3 : 2 ratio.
- 1.2 g of magnesium would need 1.2 × (2/3) = 0.80 g of oxygen - available 1.0 g, so oxygen is more than enough.
- 1.0 g of oxygen would need 1.0 × (3/2) = 1.5 g of magnesium - only 1.2 g is available.
So magnesium is the limiting substance and oxygen is in excess by 1.0 − 0.80 = 0.20 g. Product mass = 1.2 + 0.80 = 2.0 g. The same product mass as student A, because the same 1.2 g of magnesium reacts in both cases.