1. Reacting Masses Targets
Calculate reacting masses from balanced equations.
Do Now:
Write FULL IONIC equations for the following ...
HCl + NaOH →
H2SO4 + NaOH →

2. What is the “Law of Conservation of Mass”?
Think back to GCSE
What is the “Law of Conservation of Mass”?
Mass is never lost or gained in chemical reactions. We say that mass is alwaysconserved. In other words, the total mass of products at the end of the reaction is equal to the total mass of the reactants at the beginning.

3. Example 1
What mass of iron is produced when 25g of iron(III) oxide is reduced by carbon? Step 1: Write a balanced equation Fe2O3 + 3C → 2Fe + 3CO Mr= Ar= 55.8 Mass = 25g Mass=? Moles=0.157 mol Step 2: Underline the 2 substances mentioned in the question. Step 3: Write down under the underlined substances what you know about them. Step 4: You will now know 2 things about 1 of the underlined substances. Use this information to work out the number of moles. Step 5: Use the balanced equation to work out the number of moles of the other underlined substance. Step 6: Use the answer to step 5 to work out the mass of the other underlined substance.
Mass = 0.313x55.8 = 17.5g
Moles=2x0.157=0.313 mol

4. Example 2
What mass of iron(III) oxide is produced by the oxidation of 83.7g of iron? Step 1: Write a balanced equation 4Fe + 3O2 → 2 Fe2O3 Ar= 55.8 Mr= Mass = 83.7g Mass=? Moles=1.50 mol Step 2: Underline the 2 substances mentioned in the question. Step 3: Write down under the underlined substances what you know about them. Step 4: You will now know 2 things about 1 of the underlined substances. Use this information to work out the number of moles. Step 5: Use the balanced equation to work out the number of moles of the other underlined substance. Step 6: Use the answer to step 5 to work out the mass of the other underlined substance.
Mass = 0.75x159.6= 119.7g
Moles=0.5x1.50=0.75 mol

5. Units! Kilograms and tonnes
1 kg = 1 x 103 g
1 tonne = 1 x 106 g

6. Check Your Understanding
1. Calcium carbonate is the raw material used in the extraction of iron from iron ore. It decomposes to form calcium oxide and carbon dioxide when heated:
CaCO3(s) → CaO (s) + CO2 (g)
What mass of calcium oxide is obtained by the thermal decomposition of 20 g of calcium carbonate?
What mass of calcium carbonate is needed to produce 2 tonnes of calcium oxide?
2. Sodium reacts vigorously with water to produce sodium hydroxide and hydrogen:
2Na(s) + 2H2O (l) → 2NaOH (aq) + H2 (g)
What mass of sodium hydroxide is produced when 1g of sodium reacts completely with water?
What mass of hydrogen is produced when 50g of sodium reacts completely with water?

7. Check Your Understanding
1. Calcium carbonate is the raw material used in the extraction of iron from iron ore. It decomposes to form calcium oxide and carbon dioxide when heated:
CaCO3(s) → CaO (s) + CO2 (g)
What mass of calcium oxide is obtained by the thermal decomposition of 20 g of calcium carbonate? 11.2g
What mass of calcium carbonate is needed to produce 2 tonnes of calcium oxide? tonnes
2. Sodium reacts vigorously with water to produce sodium hydroxide and hydrogen:
2Na(s) + 2H2O (l) → 2NaOH (aq) + H2 (g)
What mass of sodium hydroxide is produced when 1g of sodium reacts completely with water? g
What mass of hydrogen is produced when 50g of sodium reacts completely with water? g

8. Working out equations from reacting masses
Question: A 16.7g sample of a hydrate of sodium carbonate (Na2CO3.10H2O) is heated at a constant temperature for a specified time until a reaction is complete. A mass of 3.15g of water is obtained. What is the equation for the reaction occurring?

9. Working out equations from reacting masses
Question: Copper forms two oxides. Both of them can be converted to copper by heating with hydrogen. An oxide of copper is heated in a stream of hydrogen to a constant mass. The masses of copper and water formed are Cu = 17.6g and H2O = 2.56g. What is the equation for the reaction occurring?

10. Quick Check
A fertiliser manufacturer makes a batch of 20kg of ammonium nitrate. What mass of ammonia does he need?
A sample of an oxide of iron was reduced to iron by heating with hydrogen. The mass of iron obtained was 4.35g and the mass of water was 1.86g. Deduce the equation for the reaction that occurs.

11. Task
Chemsheets Tasks 10