Moles, Mass & RFM (Edexcel IGCSE Chemistry: Double Science)

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The mole

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
  • 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

Examiner 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).

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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:

The Moles & Mass Formula Triangle

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

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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Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.