Stoichiometry
Mole concept, balanced equations, molar calculations
The Mole Concept
A mole is a unit of measurement in chemistry. One mole of any substance contains 6.022 × 10²³ particles (Avogadro's number). The molar mass (M) of a substance is the mass of one mole, measured in g·mol⁻¹. For example, the molar mass of water (H₂O) = 2(1) + 16 = 18 g·mol⁻¹.
Balancing Chemical Equations
A balanced equation has the same number of each type of atom on both sides. Steps:
1. Write the unbalanced equation with correct formulae.
2. Count atoms on each side.
3. Add coefficients (not subscripts!) to balance.
4. Check that all atoms balance.
Example: N₂ + 3H₂ → 2NH₃ (nitrogen: 2=2, hydrogen: 6=6 ✓)
Example
Mole Calculations
How many moles in 44 g of CO₂?
M(CO₂) = 12 + 2(16) = 44 g·mol⁻¹
n = m/M = 44/44 = 1 mol
What mass of water is produced when 2 mol of H₂ reacts?
2H₂ + O₂ → 2H₂O
2 mol H₂ produces 2 mol H₂O
m = nM = 2 × 18 = 36 g
Note
Key Formulae
n = m/M (moles = mass ÷ molar mass)
n = N/Nₐ (moles = number of particles ÷ Avogadro's number)
For gases at STP: n = V/22.4 (volume in dm³)
Always balance the equation first before doing calculations.
Key Vocabulary
MoleA unit representing 6.022 × 10²³ particles of a substance
Molar massThe mass of one mole of a substance in g·mol⁻¹
Avogadro's number6.022 × 10²³ — the number of particles in one mole
CoefficientThe number placed before a formula to balance an equation
StoichiometryThe calculation of quantities in chemical reactions
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Mole
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