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Linking Moles, Mass & Mr (Cambridge O Level Chemistry)
Revision Note
Linking Moles, Mass & Mr
- 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
- 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 formula mass or relative molecular mass in grams
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 65.4
- So, the molar mass is 65.4 g/mol
- The mass is calculated by moles x molar mass
- This comes to 0.250 mol x 65.4 g/mol = 16.35 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
- The number of moles is found by mass ÷ molar mass
- This comes to 2.64 g ÷ 342.3 g/mol = 7.71 x 10-3 mol
Worked example
In 15.7 g of water (Mr = 18):
- How many molecules are there?
- How many atoms are there?
Answer - part a
- The molar mass of water is 18 g/mol
- The number of moles is found by mass ÷ molar mass
- This comes to 15.7 g ÷ 18 g/mol = 0.872 mol
- There are 6.02 x 1023 molecules of water in 1 mole of water
- In 0.872 moles of water, there are 6.02 x 1023 x 0.872 = 5.25 x 1023 molecules
Answer - part b
- In each molecule of water there are 3 atoms (2 hydrogen atoms, one oxygen atom)
- The number of atoms in 15.7 g = 3 x 5.25 x 1023 = 1.58 x 1024 atoms
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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