4CH1

Chemical Formulae, Equations and Calculations

Principles of Chemistry · 6 question types

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 17% of your exam marks.

stable
Very High
Stable17%

Highest-frequency topic: moles, percentage yield and titration calculations appear on nearly every paper.

What a mole is

  • A (symbol mol) is the chemist's unit for counting chemical particles
  • One mole of any substance always contains exactly the same number of particles — the Avogadro constant:

NA=6.02×1023N_\text{A} = 6.02 \times 10^{23}

  • The "particles" can be atoms, molecules, ions, electrons or formula units — whichever the substance is made of
    • 1 mole of helium (He) → 6.02 × 10²³ atoms
    • 1 mole of water (H₂O) → 6.02 × 10²³ molecules
    • 1 mole of sodium chloride (NaCl) → 6.02 × 10²³ formula units

Molar mass

  • The molar mass is the mass, in grams, of one mole of a substance
  • Numerically:
    • For an element, molar mass = Ar in grams
    • For a compound, molar mass = Mr in grams
  • Examples:
    • 1 mol of carbon (Ar = 12) weighs 12 g
    • 1 mol of water (Mr = 18) weighs 18 g
    • 1 mol of sodium chloride (Mr = 58.5) weighs 58.5 g

Converting between mass and moles

  • The key relationship is:

n=mMrn = \frac{m}{M_r}

  • where n = number of moles, m = mass in grams, and Mr = molar mass in g/mol
  • Rearranged:
    • mass = moles × Mr
    • Mr = mass ÷ moles
Formula triangle linking mass, moles and molar mass: mass sits on top, with moles and M below, so moles = mass ÷ M and mass = moles × M
Source: Calculating moles and mass by Save My Exams

Example. How many moles are in 5.0 g of NaOH?

  • Mr(NaOH) = 23 + 16 + 1 = 40
  • n = 5.0 / 40 = 0.125 mol

Example. What is the mass of 2.5 mol of glucose, C₆H₁₂O₆?

  • Mr(C₆H₁₂O₆) = (6 × 12) + (12 × 1) + (6 × 16) = 180
  • m = 2.5 × 180 = 450 g