1.6 Calculations Involving Masses
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Rule of conservation of mass
If you do a reaction in a closed system the total mass of the system before and after doesn’t change
Why is mass conserved in a chemical reaction
No atoms are created or destroyed during a reaction
What does it mean if mass increases in an unsealed reaction vessel
One of the reactants is a gas, as it is floating in the air and not in the vessel you can’t measure its mass. When the gas reacts it will become contained inside the reaction vessel.
What does it mean if mass decreases in an unsealed reaction vessel
The reactants will be contained in the reaction vessel as they will be solid, liquid or aqueous and will count for mass. Gas will be a product and will escape from the vessel so you can’t measure its mass.
How to calculate relative formula mass
The relative atomic masses of each of the atoms added together
Mr
Relative formula mass
1 mole of a compound
The relative formula mass of a compound in grams
Empirical formula
The smallest whole number ratio of atoms in the compound
Avagrado’s constant
The number of atoms, molecules or ions in 1 mole
What is the value of avagrado’s constant
6.02 x10^23
Number of moles formula
Number of moles = mass in g/relative formula mass
Concentration formula
Concentration = mass of solute/volume of solution
dm3 to cm3
x1000
cm3 to dm3
/1000
How to calculate empirical formula
1- Calculate the number of moles of each element 2- Divide everything by the smallest number of moles
Describe an experiment to calculate empirical formula of magnesium oxide
1- Heat a crucible until it’s red hot, then leave it to cool, then weigh the crucible and its lid. Add some magnesium ribbon to the crucible and reweigh. 2- Heat the crucible containing magnesium with the lid on with a small air gap 3- Heat strongly until all the magnesium ribbon has turned white, then reweigh the crucible, lid and its contents 4- The mass of the magnesium was the initial mass of the crucible, lid and magnesium- the initial mass of the crucible and lid and the mass of magnesium oxide is the final mass of the crucible, lid and magnesium oxide - the initial mass of the crucible and lid
Why should the crucible be heated before the empirical formula experiment
To make sure it is clean
Why should the lid be mostly on the crucible in the empirical formula experiment
So that solid bits cannot escape
Why is there an air gap for the lid in the empirical formula experiment
So that oxygen can enter the crucible
What does it mean if a reactant is in excess
When the reaction stops it has not all been used up
Limiting reactant
The reactant which is fully used up in a reaction and so limits the amount of product formed
What is the amount of limiting reactant proportional to
It is directly proportional to the amount of product formed