Quantitative Chemistry
Conservation of mass, moles, concentrations, and yields.
In this topic
Conservation of Mass and Balancing Equations
In a chemical reaction, mass is always conserved. The total mass of reactants equals the total mass of products. No atoms are created or destroyed — they are rearranged.
Balancing equations: the number of atoms of each element must be the same on both sides.
Example: Mg + O₂ → MgO (unbalanced) Balanced: 2Mg + O₂ → 2MgO
State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water)
Key Points
- Never change the formulae — only change the numbers in front
- Balance the most complex molecule first
- Check each element has the same count on both sides
- If mass appears to change, a gas may have escaped or been absorbed
Example Questions
2Balance this equation: Fe₂O₃ + C → Fe + CO₂
2Fe₂O₃ + 3C → 4Fe + 3CO₂. Check: Fe: 4=4 ✓, O: 6=6 ✓, C: 3=3 ✓
[2 marks]
2When calcium carbonate is heated, the mass of the solid decreases. Explain why, given that mass is conserved.
CaCO₃ → CaO + CO₂. Carbon dioxide gas is produced and escapes into the atmosphere. The total mass is conserved, but the mass of the solid residue decreases because CO₂ has been lost as a gas.
[2 marks]
Moles and Calculations
The mole is a measure of the amount of substance. One mole contains 6.02 × 10²³ particles (Avogadro's constant).
Key formulae: - Moles = mass ÷ relative formula mass (Mr) - Mass = moles × Mr - Mr = sum of all relative atomic masses in the formula
Example: Mr of H₂O = (2 × 1) + 16 = 18 If you have 36g of water: moles = 36 ÷ 18 = 2 moles
Concentration (mol/dm³) = moles ÷ volume (dm³) Remember: 1 dm³ = 1000 cm³
Key Points
- Use the periodic table for relative atomic masses
- Convert cm³ to dm³ by dividing by 1000
- In titration calculations, use the balanced equation ratio
- Excess reactant is the one left over after the reaction
Example Questions
2Calculate the number of moles in 11g of carbon dioxide (CO₂).
Mr of CO₂ = 12 + (2 × 16) = 44. Moles = 11 ÷ 44 = 0.25 moles
[2 marks]
325.0 cm³ of 0.1 mol/dm³ NaOH reacts with HCl: NaOH + HCl → NaCl + H₂O. Calculate the concentration of HCl if 20.0 cm³ was needed.
Moles NaOH = 0.1 × (25/1000) = 0.0025 mol. Ratio 1:1, so moles HCl = 0.0025. Concentration = 0.0025 ÷ (20/1000) = 0.125 mol/dm³
[3 marks]