CC9 Calculations

• The relative atomic mass (Ar) is the average mass of an atom compared to the mass of a carbon-12 atom, which is taken as 12.

  Example: Hydrogen has an Ar of 1 and is 12 times lighter that carbon.
• The relative formula mass (Mr) of a substance is the sum of the relative atomic masses (Ar) of all the atoms or ions in its formula.
• calculating empirical formula - 1) Divide both masses by RAM 2) Ratio and simplify 3) Write formula
• This is used to determine the empirical formula for magnesium oxide.

  • Weigh the magnesium. 
  • Heat the magnesium so that it reacts with oxygen. 
  • Weigh the magnesium oxide produced.
  • mass of magnesium oxide – mass of magnesium = mass of oxygen
  • Using these quantities, calculate the empirical formula for magnesium oxide.
• molecular formula from empirical =  Work out the relative mass of the empirical formula, Divide the Mr by the relative empirical formula mass
• When a solute is dissolved in a solvent a solution is formed. 

  The amount of solute dissolved in a stated volume is the concentration.
• Law of conservation of mass states that:

  The mass of the solution is equal to the mass of the solvent plus the mass of the solute.
• Precipitation reactions happen when soluble substances react together to form an insoluble product, called the precipitate
• closed system = mass of reactants and products are equal as no substances have escaped
• non enclosed system = mass of products can increase or decrease as substances can enter and leave the reaction
• During a chemical reaction, no atoms are lost or made, they are just rearranged to make new substances.
• new/old x odd = calculating masses
• avogadros constant = 6.02 × 1023
• • 1 mol of a substance contains the Avogadro constant number of particles (6.02 × 1023)
• number of moles = mass / Mr
• A reaction stops when all the particles of one of the                           reactants are used up.
• • The limiting reactant is the one that is all used up at the end of the reaction. The amount of product formed is limited by this reactant since the reaction cannot continue without it.
  • The reactant in excess is still there at the end of the reaction (although in a smaller amount than at the start)
• Stoichiometry = balanced equation