1.3.2 Reacting Masses

• 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 M_r

   mass = 0.25 mol x 40 g mol^-1

   mass = 10 g

   Therefore, the 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)