The mole and stoichiometry (Higher tier)
What is a mole?
A mole is the unit chemists use to count atoms, molecules or ions.
1 mole = 6.02 x 10^23 particles (Avogadro's constant, N_A).
The mass of one mole of any substance equals its relative formula mass (M_r) in grams.
Example: M_r of CO_2 = 12 + 2(16) = 44. So 1 mol CO_2 has mass 44 g.
Number of moles
n = m / M_r
Where n = moles, m = mass in g, M_r = relative formula mass.
Example: how many moles in 11 g of CO_2? n = 11 / 44 = 0.25 mol.
Stoichiometry from balanced equations
The big numbers in front of formulas (coefficients) give the mole ratio.
Example: 2 H_2 + O_2 -> 2 H_2O
This says 2 mol H_2 react with 1 mol O_2 to make 2 mol H_2O.
Worked calculation
How much MgO is made from 6 g of Mg burning in oxygen? Mg = 24, O = 16, so MgO = 40.
Equation: 2 Mg + O_2 -> 2 MgO. Mole ratio Mg:MgO = 1:1.
- Moles Mg = 6 / 24 = 0.25 mol.
- Moles MgO = 0.25 mol.
- Mass MgO = n x M_r = 0.25 x 40 = 10 g.
WJEC exam tip
Always lay calculations out in three lines: (1) moles of the known reactant; (2) use the balanced ratio to find moles of the unknown; (3) convert moles back to mass. Skipping steps loses M1 marks.
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