The Mole (Cambridge (CIE) IGCSE Chemistry)
Revision Note
Written by: Caroline Carroll
Reviewed by: Stewart Hird
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The mole & the Avogadro Constant
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Chemical amounts are measured in moles
The mole, symbol mol, is the SI unit of amount of substance
One mole of a substance contains the same number of the stated particles
This can be atoms, molecules or ions
One mole contains 6.02 x 1023 particles; this number is known as the Avogadro Constant
For example:
One mole of sodium (Na) contains 6.02 x 1023 atoms of sodium
One mole of hydrogen (H2) contains 6.02 x 1023 molecules of hydrogen
One mole of sodium chloride (NaCl) contains 6.02 x 1023 formula units of sodium chloride
The mass of 1 mole of a substance is known as the molar mass
For an element, it is the same as the relative atomic mass written in grams
For a compound, it is the same as the relative molecular or formula mass in grams
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The mole & volume of gas
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Molar volumes of gas
Avogadro’s Law states that at the same temperature and pressure, equal amounts of gases occupy the same volume of space
e.g. 1 mole of hydrogen gas occupies the same volume as 1 mole of methane gas
At room temperature and pressure, the volume occupied by one mole of any gas was found to be 24 dm3 or 24,000 cm3
This is known as the molar gas volume at RTP
RTP stands for “room temperature and pressure” and the conditions are 20 ºC and 1 atmosphere (atm)
From the molar gas volume, the following formula triangles can be derived:
Molar gas volume (dm3) formula triangle
This shows the relationship between moles of gas, volume in dm3 and the molar volume
If the volume is given in cm3 instead of dm3, then divide by 24,000 instead of 24:
Molar gas volume (cm3) formula triangle
This shows the relationship between moles of gas, volume in cm3 and the molar volume
The formula can be used to calculate the number of moles of gases from a given volume or vice versa
Simply cover the one you want and the triangle tells you what to do
For example, to find the volume of a gas:
Volume = Moles x Molar Volume
Examples of Converting Moles to Volume Table
Gas | Amount (moles) | Volume |
|---|---|---|
Hydrogen | 3 | (3 x 24) = 72 dm3 (3 x 24000) = 72000 cm3 |
Carbon dioxide | 0.25 | (0.25 x 24) = 6 dm3 (0.25 x 24000) = 6000 cm3 |
Oxygen | 5.4 | (5.4 x 24) = 129.6 dm3 (5.4 x 24000) = 129600 cm3 |
Ammonia | 0.02 | (0.02 x 24) = 0.48 dm3 (0.02 x 24000) = 480 cm3 |
For example, to find the number of moles of a gas:
Moles = Volume ÷ Molar Volume
Examples of Converting Volume to Moles Table
Gas | Volume | Moles |
|---|---|---|
Methane | 225.6 dm3 | (225.6 ÷ 24) = 9.4 mol |
Carbon monoxide | 7.2 dm3 | (7.2 ÷ 24) = 0.3 mol |
Sulfur dioxide | 960 dm3 | (960 ÷ 24) = 40 mol |
Oxygen | 1200 cm3 | (1200 ÷ 24000) = 0.05 mol |
Examiner Tips and Tricks
You are not expected to know the value of Avogadro's constant
But, you do need to know the equation as well as how to use and re-arrange it
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