Amount of Substance
Empirical Formula
The simplest ratio of atoms of an element in a molecule
Molecular Formula
The actual number of atoms of an element in a molecule
Moles
Measurement for the amount of atoms
Moles = Mass
Ar or Mr |
Moles = concentration (M or mol/dm^3) x volume (dm^3)
|
Concentration is also referred to as molarity
Water has density of 1 g/cm^3
This means 1g of water is 1 cm^3
Water has density of 1 g/cm^3
This means 1g of water is 1 cm^3
Finding Empirical Formula
Divide the mass by the atomic mass to give the amount of moles of each substance. Then find the ratio of each element
A simple method is to use a table:
Mass = Ar = Moles = Ratio = |
Element
|
If given as a percentage assume it is in grams
To find the molecular formula the molecular mass is needed and the empirical formula. By comparing the two you can discover the molecular formula
To find the molecular formula the molecular mass is needed and the empirical formula. By comparing the two you can discover the molecular formula
Avogadro's Constant
This is the number of atoms in 12g of Carbon-12
Stoichiometry
Discovering mass of other reactants or products in a reaction
b
If molar ratio is the same then the number of moles is the same
If different then it's:
Known No. moles x unknown ratio = unknown no. moles
known ratio
b
If molar ratio is the same then the number of moles is the same
If different then it's:
Known No. moles x unknown ratio = unknown no. moles
known ratio
E.g.
CuO + H2 → Cu + H2O
Balanced ratio of moles between CuO and H2O so here are the same number of moles
2Fe + 3O2 → Fe2O3
Unbalanced ratio between Fe and Fe2O3 meaning the above calculation is required to work out the number of moles of Fe2O3
Molar Ratio does not impact the Mr in any other calculation
CuO + H2 → Cu + H2O
Balanced ratio of moles between CuO and H2O so here are the same number of moles
2Fe + 3O2 → Fe2O3
Unbalanced ratio between Fe and Fe2O3 meaning the above calculation is required to work out the number of moles of Fe2O3
Molar Ratio does not impact the Mr in any other calculation
Percentage Purity & Atom Economy
When working out how much of a molecule is pure then use:
mass pure x 100 = Percentage Purity
mass impure
mass pure x 100 = Percentage Purity
mass impure
When working out how much of the reactants is the reactant required use:
mass of desired product x 100
mass of reactants
mass of desired product x 100
mass of reactants
Ideal Gas Equation
pV = nRT
p = Pressure (Pa)
V = Volume (m^3)
n = No. moles
R = Universal Gas Constant → 8.31
T = Temperature (K)
You must be able to get to these units from the units given like from decimetres cubed to metres cubed
V = Volume (m^3)
n = No. moles
R = Universal Gas Constant → 8.31
T = Temperature (K)
You must be able to get to these units from the units given like from decimetres cubed to metres cubed