Using Moles in Equations [Higher Tier]

Amounts of substances in equations

• The big numbers in a balanced equation are the mole ratio: $\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$ means one mole of magnesium reacts with two moles of hydrochloric acid to make one mole of magnesium chloride and one mole of hydrogen
• Reacting mass calculations always follow the same three steps:
  • Moles of the substance you know: mass $\div \, M_r$
  • Ratio from the equation to get the moles of the substance you want
  • Mass of the substance you want: moles $\times \, M_r$
• Worked example: mass of magnesium oxide made from 6 g of magnesium. $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$
  • Moles of Mg $= \frac{6}{24} = 0.25$ mol
  • Ratio Mg : MgO is 2 : 2, so 0.25 mol of MgO
  • Mass of MgO $= 0.25 \times 40 = 10$ g