IBChemistryHLRevision NotesModels of the Particulate Nature of MatterCounting Particles by Mass: The MoleThe Mole Unit

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The Mole Unit (HL IB Chemistry)

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The Mole

  • The Avogadro constant (NA or L) is the number of particles equivalent to the relative atomic mass or molecular mass of a substance in grams
    • The Avogadro constant applies to atoms, molecules and ions
    • The value of the Avogadro constant is 6.02 x 1023 mol-1

  • The mass of a substance with this number of particles is called the molar mass
    • One mole of a substance contains the same number of fundamental units as there are atoms in exactly 12.00 g of 12C
    • If you had 6.02 x 1023 atoms of carbon-12 in your hand, you would have a mass of exactly 12.00 g
    • One mole of water would have a mass of (2 x 1.01 + 16.00) = 18.02 g

Worked example

Determine the number of atoms, molecules and the relative mass of 1 mole of:

  1. Na
  2. H2
  3. NaCl

Answer 1:

  • The relative atomic mass of Na is 22.99
  • Therefore, 1 mol of Na has a mass of 22.99 g mol-1
  • 1 mol of Na will contain 6.02 x 1023 atoms of Na (Avogadro’s constant)

Answer 2:

  • The relative atomic mass of H is 1.01
  • Since there are 2 H atoms in H2, the mass of 1 mol of H2 is (2 x 1.01) 2.02 g mol-1
  • 1 mol of H2 will contain 6.02 x 1023 molecules of H2
  • However, since there are 2 H atoms in each molecule of H2, 1 mol of H2 molecules will contain 1.204 x 1024 H atoms

Answer 3:

  • The relative atomic masses of Na and Cl are 22.99 and 35.45 respectively
  • Therefore, 1 mol of NaCl has a mass of (22.99 + 35.45) 58.44 g mol-1
  • 1 mol of NaCl will contain 6.02 x 1023  formula units of NaCl
  • Since there is both an Na and a Cl atom in NaCl, 1 mol of NaCl will contain 1.204 x 1024 atoms in total

Summary:

1 mole of Number of atoms Number of molecules/ formula units Relative mass
Na 6.02 x 1023 - 23.99
H2 1.204 x 1024 6.02 x 1023 2.02
NaCI 1.204 x 1024 6.02 x 1023 58.44

Relative Atomic Mass

Relative atomic mass, Ar

  • The relative atomic mass (Ar) of an element is the weighted average mass of one atom compared to one twelfth the mass of a carbon-12 atom
  • The relative atomic mass is determined by using the weighted average mass of the isotopes of a particular element
  • The Ar has no units as it is a ratio and the units cancel each other out

Table of Relative Molecular Mass Calculations

Substance Atoms present Mr
Hydrogen (H2) 2 x H ( 2 x 1.01) = 2.02
Water (H2O) (2 x H) + (1 x O) (2 x 1.01) + 16.00 = 18.02
Potassium Carbonate (K2CO3) (2 x K) + (1 x C) +(3 x O) (2 x 39.10) + 12.01+ ( 3 x 16.00) = 138.21
Calcium hydroxide (Ca(OH)2) (1 x Ca) + (2 x O) +(2 x H) 40.08 x ( 2 x 16.00) + (2 x 1.01) = 74.10
Ammonium Sulfate (NH4)2SO4) (2 x N) + (8 x H) +(1 x S) + (4 x O) (2 x 14.01) + ( 8 x 1.01) + 32.07 + ( 4 x 16.00) = 132.17

Relative formula mass, Mr 

  • The relative formula mass (Mr) is used for compounds containing ions
  • It is calculated in the same way as relative molecular mass
  • In the table above, the Mr for potassium carbonate, calcium hydroxide and ammonium sulfate are relative formula masses

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    Alexandra

    Author: Alexandra

    Expertise: Chemistry

    Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.