Revision NotesExam QuestionsPast Papers
IGCSEChemistry: Co-ordinated Sciences (Double Award)CIERevision NotesStoichiometryThe Mole & the Avogadro ConstantLinking Moles, Mass & Mr

Linking Moles, Mass & Mr (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

Revision Note

Caroline

Author

Caroline

Last updated

Did this video help you?

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

Moles, mass and relative mass

  • The number of moles of any chemical can be calculated using:

Moles = begin mathsize 14px style bold mass over bold M subscript bold r end style

  • We can use the following formula triangle to convert between moles, mass in grams and the molar mass:

Formula triangle for mass (g), moles and relative formula massFormula triangle for moles, mass and 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:
    • fraction numerator 2.64 over denominator 342 end fraction = 7.72 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.

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:

  1. Molecules in 1 mole
  2. Atoms in 1 mole
  3. Chloride ions in 1 mole
  4. Magnesium ions in 2 moles

Answers:

  1. The formula is MgCl2, so 1 mole of MgCl2 is:
    • 1 x 6.02 x 1023 = 6.02 x 1023 molecules 
  2. There are 3 atoms in MgCl2, so 1 mole of MgCl2 contains:
    • 3 x 6.02 x 1023 = 18.06 x 1023 atoms
  3. 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
  4. 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):

  1. How many molecules are there? 
  2. How many atoms are there?

Answers:

  1. 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
  2. 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

Units of concentration

  • A solute is a solid substance that dissolves into a liquid 
    • The amount of solute can be expressed in grams (g) or moles (mol)
  • A solvent is the liquid that a solute dissolves in 
    • The amount / volume of a solvent is measured in cm3 or dm3
  • Most chemical reactions occur between solutes which are dissolved in solvents, such as water or an organic solvent
  • A solution is the mixture formed when a solute dissolves in a solvent 
    • The amount / volume of a solution measured in cm3 or dm3 
  • Concentration refers to the amount of solute there is in a specific volume of the solvent
    • The greater the amount of solute in a given volume, the greater the concentration
    • Concentration is sometimes commonly referred to as strength
      • For example, dissolving more coffee in hot water results in a stronger coffee
  • Typically, concentration is expressed in terms of the amount of substance per dm3  
    • Therefore, the units of concentration are:
      • g / dm3 
      • mol / dm3

You've read 0 of your 5 free revision notes this week

Sign up now. It’s free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

    Test yourself

    Did this page help you?

    Previous:The MoleNext:Reacting Masses
    Caroline

    Author: Caroline

    Expertise: Physics Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.