Edexcel IGCSE Chemistry

Revision Notes

1.5.3 Moles, Mass & RFM

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

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).

Linking the Mole and the 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

Exam Tip

Remember the key link between moles and mass: one mole of any element is equal to that elements atomic mass in grams.

Moles-Mass Calculations

  • 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, downloadable IB Chemistry revision notes

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, C12H11O22  (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.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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