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Linking Moles, Mass & Mr (CIE IGCSE Chemistry)
Revision Note
Linking moles, mass & Mr
Extended tier only
- One mole of any element is equal to the relative atomic mass of that element in grams
- If you had 1 mole of carbon atoms 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 has a mass of 4 g (Ar of He is 4)
- One mole of lithium has a mass of 7 g (Ar of Li is 7) and so on
- One mole of any compound is the relative molecular mass or relative formula mass in grams
- To find the mass of one mole of a compound, add up the relative atomic masses
- For example, carbon dioxide has an Mr of:
(1 x C) + (2 x O)
(1 x 12) + (2 x 16) = 44
- For example, carbon dioxide has an Mr of:
Moles, mass and relative mass
-
The number of moles of any chemical can be calculated using:
Moles =
-
We can use the following formula triangle to convert between moles, mass in grams and the molar mass:
-
Calculating the number of moles of an element uses the same equation, but with relative atomic mass replacing M
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
- So, the molar mass is 65 g / mol
- The mass is calculated by moles x molar mass:
- 0.250 mol x 65 g / mol = 16.25 g
Worked example
How many moles are in 2.64 g of sucrose, C12H22O11 (Mr = 342)?
Answer:
- The molar mass of sucrose is 342 g / mol
- The number of moles is found by mass ÷ molar mass:
- = 7.72 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.
Calculating moles & masses
- Chemical amounts are measured in moles
- The mole, symbol mol, is the SI unit of amount of substance
- One mole of any 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
Worked example
For magnesium chloride, MgCl2, calculate the number of:
- Molecules in 1 mole
- Atoms in 1 mole
- Chloride ions in 1 mole
- Magnesium ions in 2 moles
Answers:
- The formula is MgCl2, so 1 mole of MgCl2 is:
- 1 x 6.02 x 1023 = 6.02 x 1023 molecules
- There are 3 atoms in MgCl2, so 1 mole of MgCl2 contains:
- 3 x 6.02 x 1023 = 18.06 x 1023 atoms
- There are 2 chloride ions in MgCl2, so 1 mole of MgCl2 contains:
- 2 x 6.02 x 1023 = 12.04 x 1023 chloride ions
- There is 1 magnesium ion in MgCl2, so 2 mole of MgCl2 contains:
- 2 x (1 x 6.02 x 1023) = 12.04 x 1023 magnesium ions
Worked example
In 15.7 g of water (Mr = 18):
- How many molecules are there?
- How many atoms are there?
Answers:
- The number of molecules:
- The molar mass of water is 18 g / mol
- The number of moles is found by mass ÷ molar mass
- 15.7 g ÷ 18 g / mol = 0.872 mol
- There are 6.02 x 1023 molecules of water in 1 mole of water
- So, in 0.872 moles of water, there are:
- 6.02 x 1023 x 0.872 = 5.25 x 1023 molecules
- The number of atoms:
- In each molecule of water, there are 3 atoms (2 hydrogen atoms, one oxygen atom)
- So, the number of atoms in 15.7 g = 3 x 5.25 x 1023 = 1.58 x 1024 atoms
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