Moles, Mass & RFM (Edexcel IGCSE Chemistry (Modular))
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
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
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
Exam Tip
You need to appreciate that the measurement of amounts in moles can apply to atoms, molecules, ions, electrons, formulae and equations. E.g. in one mole of carbon (C) the number of atoms is the same as the number of molecules in one mole of carbon dioxide (CO2).
Calculating moles and masses
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
What is the mass of 0.250 moles of zinc?
Answer:
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
How many moles are in 2.64 g of sucrose, C12H22O11 (Mr = 342.3)?
Answer:
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
Exam 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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