The Mole (Cambridge (CIE) IGCSE Chemistry)

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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

Molar volume formula triangle in dm3

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

Molar volume formula triangle in cm3

This shows the relationship between moles of gas, volume in cmand 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

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