Topic 3 Quantitative Chemistry

• Relative formula mass


  All of the relative atomic masses in a compound added together.
• Percentage mass

  Percentage mass of an element in a compound = relative atomic mass x number of atoms of that element divided by total relative formula mass of the compound.
• Calculating moles

  Moles = Mass÷Mr (Relative formula mass)
• Using moles to balance equations

  • Find moles for each product and reactant
  • Divide by smallest number of moles to find the ratio
  • Add in the equation as big numbers
• Avogadro's constant


  6.02 x 10^23
• Calculating number of atoms

  Atoms = Avogadro constant x number of elements (per mole)
• Calculating number of molecules with Avogadro

  Moles x Avogadros
• Limiting reactants

  • Calculate moles of known substance
  • Equate the moles to the ratio of unknown substance
  • Calculate the mass of the unknown substance by Moles = Mass/Mr
• Limiting reactants


  If one reactant gets used up before the rest, the reaction will stop, that reactant is limiting.
• Concentrations of solutions (g/dm^3)

  Concentration (g/dm^3) = mass (g) ÷ volume (dm^3)
• Concentration

  Tells us the mass of a solute in a given volume of solution
• Changing Concentration

  • Increasing mass of solute while volume of solvent remains the same concentration increases
  • Increasing volume of solvent and keeping mass the same decreases concentration
• Concentration (mol/dm^3)


  Concentration = number of moles ÷ volume (dm^3)
• Converting mol/dm^3 to g/dm^3

  1. Start by finding concentration in mol/dm^3
  2. Convert by using moles = mass÷mr.
• Gas volumes

  1 mole of any gas occupies a volume of 24dm^3 at room temperature and pressure

  • Room temp 20 degrees
  • Room pressure 1 atmosphere
• Gas volume


  Volume = Moles x 24
• Percentage yield

  • Some product may be lost when separated from the reaction mixture
  • Reversible reactions cannot go to completion (eg the Haber Process)
  • There may be side reactions (Reactants may react different to how you expect)
• Percentage yield

  Percentage yield = Mass of products made ÷ maximum theoretical mass of product x 100
• Percentage Yields

  Always less than 100%
• Atom Economy


  Percentage of reactants forming useful products
• Atom economy

  Atom economy = relative formula mass of desired products ÷ relative formula mass of all reactants x 100
• Atom economy

  The higher the atom economy the better for profits and the environment.
• Low atom economy

  • Produces lots of waste products
  • Uses up resources quickly
  • Unsustainable
  • Can find a use for the waste products
  • The more products, the more likely atom economy is to be lower.