Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations

3.1.1  Conservation of mass and balanced chemical equations

Law of conservation of mass

  • No atoms are lost or made during a chemical reaction
  • So mass of products = mass of reactants
  • Total mass before = 127.6 + 126.86 = 254.46
  • Total mass after = 153.09 + 101.37 = 243.46
  • So mass of products = mass of reactants

  • CO2 is a gas
  • Which escapes during the reaction
  • So the mass at the end of the experiment is less expected as mass has been lost

Explain why an unbalanced chemical equation cannot correctly describe a chemical reaction (2)

  • Must end up with the same no of atoms as at the start
  • Otherwise matter is shown to be lost / gained
  • Won’t show correct amount of each element / compound
3.1.2  Relative formula mass

Relative formula mass (Mr)

  • 207 + 2 × [14 + (3 × 16)] = 331
  • In a balanced chemical equationSum of Mr of reactants in the quantities = sum of Mr of products in the quantities
  • Why is there a change in mass?A reactant or product is a gas & its mass has not been taken into account
  • Percentage composition
  • 3.1.3 Mass changes when a reactant or product is a gas

When a metal reacts with O2

  • Mass of the oxide produced is greater than the mass of the metal
  • In thermal decompositions of metal carbonates
  •          CO2is produced
  •          Which escapes into the atmosphere
  •          Leaving the metal oxide as the only solid product
  • The equation for the reaction is 2HCl(aq) + CaCO3(s) → CaCl2(aq) + H2O(l) + CO2 (g). Explain why there is a loss in mass in this investigation (2)
  • A gas is produced
  • Which escapes from the flask
3.1.4 Chemical measurements

(Do practice questions)