reacting masses

• Steps to determine moles for each substance in a chemical equation:

  • Always balance the equation first
  • Moles for each substance are represented by the big number at the front
  • Mass = moles * Mr
• Using moles to balance equations:

  1. Calculate the Mr for each substance
  2. Calculate the moles of each substance using the equation
  3. Simplify the mole ratio into simple whole numbers
• Limiting reactant is the reactant that is used up, leaving an excess

  Steps to identify the limiting reactant:

  1. Write out the balanced equation
  2. Calculate the Mr for each substance
  3. Calculate the moles of each substance
  4. If the calculated moles don't fit the ratio from the balanced equation, the smaller number is the limiting reactant
• Example of limiting reactant:

  CaO (3 mols) + CO2 (2 mols) -> CaCO3

  Moles ratio of CaO and CO2 is 1:1

  CaO is in excess, CO2 is the limiting reactant
• Steps to find out the unknown mass of a substance (with given mass of one substance):

  1. Balance the equation
  2. Use the mass to calculate the mole of that substance (with the Mr)
  3. Use the ratio to find out the mole of the unknown substance
  4. Use the mole of the unknown substance (* Mr) to calculate the mass