• The number of moles of a substance can be found by using the following equation:

• It is important to be clear about the type of particle you are referring to when dealing with moles
  • E.g. 1 mole of CaF2?contains one mole of CaF2?formula units, but one mole of Ca2+?and two moles of F-?ions

   

Reacting masses

• The masses of reactants are useful to determine how much of the reactants?exactly?react with each other to prevent waste
• To calculate the reacting masses, the chemical equation is required
• This equation shows the ratio of moles of all the reactants and products, also called the?stoichiometry,?of the equation
• To find the mass of products formed in a reaction the following pieces of information are needed:
  • The mass of the reactants
  • The molar mass of the reactants
  • The balanced equation

   

Answer

Step 1:?The symbol equation is:

2Mg (s)????+ ????O2?(g)??????→??????2MgO (s)

Step 2:?The relative formula masses are:

Magnesium : 24? ? ? ? ?Oxygen : 32?????????Magnesium Oxide : 40

Step 3:?Calculate the moles of magnesium used in reaction

Step 4:?Find the ratio of magnesium to magnesium oxide using the balanced chemical equation

Therefore, 0.25 mol of MgO is formed

Step 5:?Find the mass of magnesium oxide

mass =?mol?x?Mr

mass?= 0.25?mol?x 40?g mol-1mass?= 10 g

Therefore,?mass of magnesium oxide produced is 10 g

Stoichiometric relationships

• The stoichiometry of a reaction can be found if the?exact?amounts?of reactants and products formed are known
• The?amounts?can be found by using the following equation:

• The gas volumes can be used to deduce the?stoichiometry?of a reaction
  • E.g. in the?combustion?of 50 cm3?of propane reacting with 250 cm3?of oxygen, 150 cm3??of carbon dioxide is formed suggesting that the ratio of propane:oxygen:carbon dioxide is 1:5:3

   

C3H8?(g) + 5O2?(g) → 3CO2?(g) + 4H2O (l)