chemical calculations

• avogadro constant 


  6.023 x 10²³ - the amount of atoms in a mole
• mass = Mr x mols
• the masses of atoms are measured relatively to carbon-12 atoms
• relative formula mass is calculated by adding all the masses of all elements in it, according to the ratio shown in the formula
• mol = relative formula mass in grams
• balanced symbol equations can be used to calculate the mols in a formula e.g. in H2 + Cl2 → 2HCl there are 2 mols of HCl and 1 mol of H and Cl
• you can work backwards from the mass of a substance to find the balanced symbol equation
• limiting reactant


  the reactant that gets used up first
• excess reactant 


  the reactant that you still have more of
• product is determined by the limiting reactant
• yield = (actual mass / theoretical mass ) x 100
• yield


  how much product is actually made, percentage yield comparing how much could've been
• factors affecting the yield:

  • product residue in apparatus
  • incomplete reversible reactions
  • unexpected products
  • loss from seperating product from reaction mixture
• atom economy is important to maximise efficiency and conserve the earth's resources
• atom economy/utilisation


  how much of the reactants end up as the desired product
• percentage atom economy = (Mr of desired product / Mr of all the reactants) x 100
• calculating the mass of a solute in a solution with known concentration:

  • calculate the mass (g) of solute in 1 dm³ (1000 cm³) of solution
  • calculate the mass of solute in 1 cm³ solution
  • calculate the mass of the solute in the given volume of solution
• more concentrated solutions have more solute than weaker solutions of the same volume
• titration 


  measuring the exact volumes of an acid and alkali needed to react with eachother
• end point 


  when the reaction between an acid and alkali has finished
• the end point can be shown with an acid/base indicator
• titration can be used to find the unknown concentration of a solution - you need to know the concentration of one of the solutions, then once you find the end point volume you can find the moles based on the balanced symbol equation
• concentration is measured in mol/dm³ or g/dm³
• under the same conditions, a certain volume of gas has a constant number of atoms in it
• mols in a gas = volume of gas / 24 dm ³ (or 24000 cm³)