# AQA AS Chemistry Amount of Substance

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- Created by: Nuha
- Created on: 23-10-12 10:55

First 384 words of the document:

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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