• 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
  • Eg. 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 reaction
• 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 atomic masses are:

magnesium : 24.31? ? ? ? ?oxygen : 16.00

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

mass?= 0.25?mol?x 40.31?g mol-1

mass?= 10.08 g

Therefore,?mass of magnesium oxide produced is 10 g?(2 sig figs)

Excess & limiting reactants

• Sometimes, there is an?excess?of one or more of the reactants (excess reactant)
• The reactant which is not in excess is called the?limiting reactant
• To determine which reactant is limiting:
  • The number of moles of the reactants should be calculated
  • The ratio of the reactants shown in the equation should be taken into account eg:

C? + 2H₂????? →???? CH4

What is limiting when 10 mol of carbon are reacted with 3 mol of hydrogen?

• Hydrogen is the?limiting reactant?and since the ratio of C : H₂?is 1:2 only 1.5 mol of C will react with 3 mol of H2

Answer:

Step 1:?Calculate the moles of each reactant

Step 2:?Write the balanced equation and determine the coefficients

2Na + S → Na₂S

Step 3:?Divide the moles by the coefficient and determine the limiting reagent

• • divide 0.40 moles of Na by 2, giving 0.20- lowest
  • divide 0.25 moles of S by 1, giving 0.25

Therefore,?sodium is limiting?and?sulfur is in excess