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The Mole (Edexcel GCSE Chemistry: Combined Science)
Revision Note
The Mole
- Chemical amounts are measured in moles
- The symbol for the unit mole is mol
- One mole of a substance contains the same number of the stated particles, atoms, molecules, or ions as one mole of any other substance
- The number of atoms, molecules or ions in a mole (1 mol) of a given substance is the Avogadro constant. The value of the Avogadro constant is 6.02 x 1023 per mole
- 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 formula triangle showing the relationship between moles, particles and the Avogadro constant
Worked example
Particles from Moles: How many hydrogen atoms are in 0.010 moles of CH3CHO?
Answer:
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- There are 4 H atoms in 1 molecule of CH3CHO
- So, there are 0.040 moles of H atoms in 0.010 moles of CH3CHO
- The number of H atoms is the amount in moles x L
- This comes to 0.040 x (6.02 x 1023) = 2.4 x 1022 atoms
Worked example
Moles from Particles: How many moles of hydrogen atoms are in 3.612 x 1023 molecules of H2O2?
Answer:
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- In 3.612 x 1023 molecules of H2O2 there are 2 x (3.612 x 1023) atoms of H
- So, there are 7.224 x 1023 atoms of H
- The number of moles of H atoms is the number of particles ÷ L
- This comes to 7.224 x 1023 ÷ (6.02 x 1023) = 1.20 moles of H atoms
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Linking the Mole and Relative Atomic Mass
Linking the Mole and Relative Atomic Mass
- One mole of any element is equal to the relative atomic mass of that element in grams or for a compound the relative formula mass in grams
- This is called the molar mass
- If you had 6.02 x 1023 atoms of carbon in your hand, that number of carbon atoms would have a mass of 12 g (because the Ar of carbon is 12)
- So one mole of helium atoms would have a mass of 4 g (Ar of He is 4), one mole of lithium would have a mass of 7 g (Ar of Li is 7) and so on
- To find the mass of one mole of a compound, we add up the relative atomic masses
- So one mole of water would have a mass of (2 x 1) + 16 = 18 g
- So one carbon atom has the same mass as 12 hydrogen atoms
Moles and Molar Mass
- Although elements and chemicals react with each other in molar ratios, in the laboratory we use digital balances and grams to measure quantities of chemicals as it is impractical to try and measure out moles
- Therefore we have to be able to convert between moles and grams
- We can use the following formula to convert between moles, mass in grams and the molar mass:
Formula triangle for moles, mass and molar mass
Worked example
Mass from Moles: What is the mass of 0.250 moles of zinc?
Answer:
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- From the periodic table the relative atomic mass of Zn is 65.38
- So, the molar mass is 65.38 g mol-1
- The mass is calculated by moles x molar mass
- This comes to 0.250 mol x 65.38 g mol-1 = 16.3 g
Worked example
Moles from Mass: How many moles are in 2.64 g of sucrose, C12H22O11 (Mr = 342.3)?
Answer:
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- The molar mass of sucrose is 342.3 g mol-1
- The number of moles is found by mass ÷ molar mass
- This comes to 2.64 g ÷ 342.3 g mol-1 = 7.71 x 10-3 mol
Examiner Tip
Always show your workings in calculations as its easier to check for errors and you may pick up credit if you get the final answer wrong.
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