C4 - Chemical Calculations

• Conservation of mass

  • Atoms cannot be created or destroyed in a chemical reaction
  • All the atoms present in the reactants must be present in the products
• If the mass of products seems to be less than the reactants, it normally because one of the products is a gas lost to the surroundings
• If the mass of products seems to be greater than one of the reactants, it is normally because one of the reactants is a gas
• (s)


  solid
• (l)

  liquid
• (g)


  gas
• (aq)


  aqueous (dissolved in water)
• Mr


  the sum of the relative atomic masses of each atom in the formula
• Avogadro's constant


  • 6.02 x 10^23
  • The number of atoms in one mole of a substance
• moles = mass/Mr
• A balanced symbol equation shows the ratios of the reactants and products in a chemical reaction
• How to calculate mass of a product from a given mass of reactant:

  • balance the symbol equation
  • calculate the moles of the given mass (moles = mass/Mr)
  • using the balanced equation, work out the number of moles of the unknown subatnce (ratio)
  • calculate the mass of the unknown substance (mass = moles x Mr)
• How to balance an equation:

  • work out the Mr of all the substances
  • calculate the number of moles of each substance in the reaction (moles = mass/Mr)
  • convert to a whole number ratio
  • balance the symbol equation
• • One of the reactants in a reaction will run out before the others
  • The one that runs out is the limiting reactant
  • The reactants left over are in excess
• How to work out which reactants are in excess and which is the limiting reactant:

  • write the balanced symbol equation for the reaction
  • pick one of the reactants and its quantity as given in the equation
  • use the ratio of the reactants in the balanced equation to see how much of the other reactant is needed
  • compare this to the quantity given in the question
  • determine which reactant is in excss and which is limiting
• Concentration


  • the amount of solute in a given volume of solvent
  • g/dm^3
• Concentration (g/dm^3) = mass (g)/volume (dm^3)
• volume (dm^3) = volume (cm^3)/1000
• • lots of solute in little solution = high concentration
  • little solute in lots of concentration = low concentration
• Theoretical yield - the maximum mass of a product expected to be produced
• The products of a reaction may be less than the theoretical yield because:

  • some of the product is lost when it is separated from the reaction mixture
  • side reactions took place between reactants that produced different products
  • the reaction is reversible
• The theoretical yield can be worked out by balancing the equation and working out the mass of the products (moles x Mr)
• Yield - the amount of product actually produced in a chemical reaction
• Percentage yield:

  • The actual yield as a proportion of the theoretical yield
  • (actual yield/theoretical yield) x 100
• Atom economy

  • the proportion of aoms that you started with that are part of useful products
  • higher atom economies are more sustainable, since fewer atoms are being wasted in atoms products that are not useful
  • (Mr of useful products/Mr of all products) x 100
• • At any given temperature and pressure, the same number of moles of a gas will occupy the same volume
  • At room temperature and pressure, one mole of any gas will occupy 24dm^3
• To calculate the number of moles of a gas:

  • volume (dm^3)/24
  • volume (cm^3)/24,000
• concentration (mol/dm^3) = moles in the solute/volume of solution (dm^3)
• Titration - an experimental technique to work out the concentration of an unknown solution in the reaction between an acid and an alkali
• Titration method:

  • use a pipette to extract a known volume of the solution with the unknown concentration
  • a pipette measures a fixed volume only
  • add the solution to a conical flask, and place on a white tile
  • add a few drops of indicator to the flask
  • add the other solution with a known concentration to the burette
  • add the solution in the burette to the flask drop by drop until the end point is reached
  • the end point is when the indicator just changes colour
  • record the volume of the end point
  • repeat until you record at least two concordant titres, and calculate the mean
• Concordant titres


  • Titres - the volume of solution added from the burette
  • Concordant - titres within 0.1 cm^3 of each other
• Calculating concentration from titration

  • Write a balanced symbol equation for the reaction
  • Calculate the moles used from the known solution (concentration x volume)
  • Use the ratio from the balanced symbol equation to find the moles present in the unknown solution
  • Calculate the concentration of the unknown solution (moles/volume)