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3.2 Use of Amount of Substance in Relation to Masses of Pure Substance -…
3.2 Use of Amount of Substance in Relation to Masses of Pure Substance
Moles
Moles = Mass (g) / Mr
The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant
Avogadro’s constant: 6.02 x 10^23
Equations
Use moles to balance equations
Calculate masses of reactants and products from balanced equations
E.g. 2Li + F2 → 2LiF
2 moles of lithium reacts with one mole of fluorine to produce 2 moles of lithium fluoride
Method
Calculate moles of known reactant/product
Determine ratio of known reactant/product to unknown reactant/product
Find mass of unknown reactant/product
Equal number of atoms of each element on each side of the equation
Concentration
Describes the mass of a substance in a certain volume of solution
Can be measured in mass per unit volume
Concentration (g/dm3) = Mass (g) ÷ volume (dm3)
Limiting reactants
Reactant which gets used up, causing the reaction to stop
Any other reactants are in excess
Determines the amount of product
Calculate the mass of product formed using the mass of the limiting reactant and the balanced equation
Method
Calculate moles of limiting reactant
Determine ratio of limiting reactant to product
Find mass of product
Amount of product formed is directly proportional to amount of limiting reactant