SCH3U1
The Laws of Chemical Change
Two laws were of great importance in discovering the composition of compounds.
Law of Conservation of Mass (Antoine Lavoisier, 1789)
The Law of Conservation of Mass (LCM) states that the products of a chemical reaction have
the same mass as the starting materials or reactants.
Law of Definite Proportions (Joseph Proust, 1800)
The Law of Definite Proportions (LDP) states that compounds always have the same
percentage composition by mass. Thus, in the formation of a compound, the mass of one
element needed to combine with another element is always the same. This is also known as
the Law of Constant Composition.
These laws can be used to make predictions about the composition of substances and the
quantities involved in chemical reactions.
1) 39 g of potassium reacts with oxygen to yield 47 g of potassium oxide.
How many grams of oxygen gas reacted?
2) Calcium reacts with 1.60 g of oxygen to form 5.61 g of calcium oxide.
a) Calculate the % (by mass) of calcium and oxygen in calcium oxide.
b) How many grams of calcium would react with 12.2 g of oxygen?
3) Aluminum oxide is composed of 53% aluminum and 47% oxygen. How many grams of
aluminum are there in 28g of aluminum oxide?
SCH3U1
The Composition of Substances 1
1. 24.3 g of magnesium reacts with oxygen to produce 40.3 g of magnesium oxide.
a) How much oxygen combined with the magnesium?
b) What is the % of oxygen in magnesium oxide?
c) How much magnesium would be required to make 10,000 g of magnesium oxide?
2. a) If 54 g of aluminum reacts with 48 g of oxygen, how much aluminum oxide would be
produced?
b) What is the % of aluminum in aluminum oxide?
c) How much aluminum would combine with 0.28 g of oxygen?
3. A roll of steel wool with a mass of 1.05 g is ignited, cooled and the mass determined again.
After burning, the residue has a mass of 1.50 g. Does this information contradict the Law of
Conservation of Mass? Explain.
4. 621.6 g of lead combines completely with 64.0 g of oxygen. How many grams of lead would
combine with 27.0 g of oxygen?
5. If 10.00 g of mercury (II) oxide is heated, oxygen and 9.20 g of mercury is formed.
a) What mass of oxygen was in the oxide?
b) Calculate the % of mercury and % oxygen in the compound.
6. It is found that 6.35 g of copper combines with sulfur to form 9.55 g of copper (II) sulfide.
a) What is the mass of sulfur that combined with the copper?
b) What is the % of sulfur in copper (II) sulfide?
c) Calculate the mass of sulfur that would react with 50.0 g of copper.
7. A compound contains 22.5% oxygen and 77.5% manganese. How many grams of
manganese will combine with 10.0 g of oxygen?
8. When heated, 433.22 g of a pure compound yielded 401.22 g of mercury and 32.00 g of
oxygen.
a) Calculate the mass of mercury that will combine with 100.0 g of oxygen.
b) What is the % of mercury in the compound?
9. a) Show that the ratio of iron to oxygen in statements (ii) and (iii) is equivalent to that in
statement (i).
(i) Iron combines with oxygen in the mass ratio 3.49:1.00
(ii) A compound of iron and oxygen contains 77.8% iron by mass.
(iii) A mass of 0.186 g of iron and 0.0533 g of oxygen combine to form a compound.
b) What law does this illustrate?
Answers:
1. a) 16.0 g b) 39.7% c) 6.03 x 103 g
2. a) 102 g b) 53% c) 0.32 g
3. No. 0.45 g of oxygen reacted.
4. 262 g
5. a) 0.80 g b) 92.0 %; 8.0 %
6. a) 3.20 g b) 33.5% c) 25.2 g
7. 34.4 g
8. a) 1254 g b) 92.6 %
9. The ratio of Fe/O approx. 3.49/1 for all statements ; LDP