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6.1.1Quantitative Chemistry

Conservation of Mass

5 min read 5 parts Lesson 2 of 12 1 quiz

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Section 1 of 5Word and Symbol Equations

6.1.1 Conservation of Mass

Equations show what reacts and what is produced, and symbol equations add the formulae and state symbols.

Word equation
  • Lists the names of the reactants on the left and the products on the right, separated by an arrow.
Symbol equation
  • Uses formulae in place of names, so the atoms involved in the reaction are shown clearly.
Diatomic molecules
  • Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine are written as molecules, such as Cl₂ and O₂.
State symbols
  • (s) solid, (l) liquid, (g) gas and (aq) aqueous, meaning dissolved in water, are added after each formula.

Section 2 of 5Balancing Equations

Balancing changes only the numbers in front of formulae, never the subscripts inside them.

1
Write the unbalanced equation using the correct formulae for every reactant and product.
2
Work through the elements one at a time, adjusting the numbers in front of formulae to match the atoms on each side.
3
Treat a group such as NO₃⁻ as a single unit if it appears unchanged on both sides of the equation.
4
Balance elements that appear on their own, such as O₂ or Cu, last, then check every element again.

Section 3 of 5Conservation of Mass

Conservation of Mass
Principle
  • No atoms are created or destroyed in a chemical reaction, so the total mass of the products equals the mass of the reactants.
Closed system
  • If nothing can enter or leave the flask, the total mass stays the same throughout the reaction.
Open system
  • If a gas escapes from an open flask, the measured mass appears to fall, but the mass of the gas has not disappeared.
Gas reactant
  • If a gas is taken in from the air during a reaction, the mass appears to increase, for the same reason.

Section 4 of 5Worked Example

Question Write the balanced symbol equation, with state symbols, for the complete combustion of propane, C₃H₈(g), in oxygen to form carbon dioxide and water.

  1. UnbalancedC₃H₈(g) + O₂(g) → CO₂(g) + H₂O(l).
  2. Balance carbonthree CO₂ are needed, so C₃H₈(g) + O₂(g) → 3CO₂(g) + H₂O(l).
  3. Balance hydrogen with 4H₂O, then oxygen: 10 O atoms on the right need 5O₂, giving C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l).

Answer C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l)

Section 5 of 5Exam Checklist & Tips

6.1.1 – Key Points for the Exam

  • Balanced equation An equation in which the number of atoms of each element is the same on both sides.
  • Conservation of mass The principle that the total mass of the reactants equals the total mass of the products.
  • Coefficient The number written in front of a formula to balance an equation.
  • Subscript A number written within a formula showing how many atoms of an element it contains.
  • Diatomic Describes a molecule made of two atoms of the same element, such as O₂.

Common Mistakes

  • Changing the subscripts in a formula, such as writing CO instead of CO₂, to balance the equation.
  • Stating that mass is lost in a reaction when the actual cause is a gas escaping from an open flask.
  • Balancing the oxygen atoms first, before the carbon and hydrogen atoms have been balanced.
  • Writing the formulae of diatomic elements as single atoms, such as O instead of O₂.
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