The Avogadro Constant & the Mole
Higher Tier
The Mole & Avogadro's Constant
- 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
- 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:
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- 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
- One mole of sodium (Na) contains 6.02 x 1023 atoms of sodium
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- 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
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 Tip
- 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
