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1.5.3Principles of Chemistry: Part 1

Moles, Mass and Rfm — Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Revision Notes & Topical Questions

5 min read 7 parts Lesson 18 of 23

📚 Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Hub / Topical Revision
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Part 1 of 5The mole

Unit of amount
  • Unit of amount The mole (symbol mol) is the unit used to measure the amount of a substance.
Avogadro constant
  • Avogadro constant One mole of any substance contains 6.02 × 1023 particles, whether these are atoms, molecules, ions, electrons or formula units.
  • This is the same number for every substance.
Examples
  • Examples 1 mol of sodium contains 6.02 × 1023 atoms, 1 mol of hydrogen H2 contains 6.02 × 1023 molecules and 1 mol of NaCl contains 6.02 × 1023 formula units.

Part 2 of 5Molar mass

Mass of 1 mole, in g/mol
Elements
  • Elements The molar mass equals the Ar written in grams per mole, so 1 mol of carbon has mass 12 g and 1 mol of helium has mass 4 g.
Compounds
  • Compounds The molar mass equals the Mr in g/mol, so 1 mol of water has a mass of (2 × 1) + 16 = 18 g.
Why moles are useful
  • Why moles are useful Substances react in simple mole ratios, but a balance measures grams, so we convert between the two.

Part 3 of 5The key equation

QuantityEquationUnits
Molesn = mass ÷ molar massmol
Massmass = n × molar massg
Molar massmolar mass = mass ÷ ng/mol

Part 4 of 5Worked method

1
Find the molar mass from the Periodic Table by adding the Ar values for the formula.
2
Choose the form of the equation that has the unknown on its own.
3
Substitute the numbers with units and calculate.
4
Example 1: mass of 0.250 mol of zinc (Ar = 65.4) is 0.250 × 65.4 = 16.4 g.
5
Example 2: moles in 2.64 g of sucrose (Mr = 342) is 2.64 ÷ 342 = 7.72 × 10-3 mol.

Part 5 of 5Particle counts

Number of particles
  • Number of particles Number of particles = moles × 6.02 × 1023.
Same number, different masses
  • Same number, different masses 1 mol of carbon atoms and 1 mol of CO2 molecules contain the same number of particles,
    • although their masses are different.
Relative masses
  • Relative masses One carbon atom is 12 times as heavy as one hydrogen atom, which is why 12 g of C and 1 g of H each contain the same number of atoms.

Key terms

TermMeaning
MoleThe unit of amount of substance, containing 6.02 × 1023 particles.
Avogadro constantThe number of particles in one mole, 6.02 × 1023 per mole.
Molar massThe mass of one mole of a substance, in g/mol.
Relative formula massThe sum of the relative atomic masses in a formula.
Formula unitThe simplest ratio of ions in an ionic compound.

Avoid these mistakes

1
Using the equation upside downmoles are mass divided by molar mass, not multiplied.
2
Confusing Mr (no units) with molar mass (g/mol). They have the same number.
3
Forgetting that H2, O2 and Cl2 are diatomic, so their molar mass is twice the Ar.
4
Rounding too early or leaving out the working. Show every step so you can earn marks even if the final answer is wrong.

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