# AQA AS Chemistry Amount of Substance

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• Created on: 23-10-12 10:55

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Amount of Substance
Relative Atomic Mass and Relative Molecular Mass
Ar is the mean mass of an atom of an element compared to 1/12 of the mass of a 12C atom.
Mr is the total of the mean masses of an atom in each element of a compound compared to 1/12 of the mass
of a 12C atom.
Relative Formula Mass is the term used for ionic compounds.
The Mole and the Avogadro Constant (L)
One mole is the term given to the number of atoms in 12g of 12C, this is basically 6.022x1023
This is used by scientists in formulas and calculations. For example the large number in front of the
compound/element in a formula tells us the number of moles.
Moles, Mr and Mass can be worked out by using the following `magic triangle' and the values that you have
available to you.
The Ideal Gas Equation
pV = nRT
P is the measure of pressure- Pa
V is the measure of volume- m3
N is the measure of moles- mol
R is the gas constant- 8.31JK-1mol-1
T is the measure of temperature- K
Pressure
If given in KPa x1000
If given in MPa x1 000 000
Volume
If given in dm3 ÷1000
If given in cm3 ÷ 1 000 000
Moles
Sometimes you will be given the mass, in which case you need to use the `magic triangle' and do n=
mass/Mr to find out the number of moles.
Temperature
You may be given the temperature in oC. To convert into Kelvin you just need to add 273. So 10oC is
actually 283K.
You can re-arrange the ideal gas equation to find the values that you are interested in. Hence:
n= pV/RT
p= nRT/ V
V= nRT/p
Empirical and Molecular Formulae
E.F. is the simplest whole number ratio of atoms of each element in a molecule.
M.F. is the actual number of atoms of each element in a molecule.
o When given % composition of elements in a molecule you can assume that the %=g. So if you had 100% this
would be equivalent to 100g.

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### Page 2

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Normally you're just given the mass in grams of each element and from this there are easy steps to follow to
work our empirical formulae:
1. Work out the moles of each element by using the `magic triangle'.
2. Do a mole ratio of the elements.
3. Divide by the smallest number of moles for each element.
4. You should now have a whole number ratio.
5. Write out the empirical formula by using the moles from the ratio as subscript.…read more

### Page 3

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Write down everything you know under an equation.
o Work out the number of moles of one of the products or reactants.
o Do a mole ratio to find out the moles of your gas.
o Multiply this by the molar volume which is 22.7dm3. This will give you the volume of gas.
In addition to this you may just be given the volume of a gas and asked to work out how much of something else is

### Page 4

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Product may have been left or broken off
A limiting reactant is one in which there is not enough of. The other product is said to be in excess. This can
affect the % yield.…read more