11 min read Mathew Wahome

The Mole Concept and Stoichiometry

Why moles?

Atoms are too small to count or weigh singly, so chemists count them in groups called moles. One mole contains 6.02 x 10^23 particles (Avogadro's constant, L). This is the basis of quantitative chemistry in the KICD Senior School Chemistry syllabus, used in industries such as Magadi soda and sugar processing.

Relative masses

Relative atomic mass (Ar): e.g. H = 1, C = 12, O = 16, Na = 23, Ca = 40.
Relative formula mass: sum of Ar values in a formula.
Molar mass: relative formula mass expressed in g/mol.

Example: Na2CO3 = 2(23) + 12 + 3(16) = 106 g/mol.

Key formulae

Moles = mass / molar mass
Number of particles = moles x 6.02 x 10^23
Gas volume = moles x molar volume (24 dm3 at r.t.p.)
Solution: moles = molarity (mol/dm3) x volume (dm3)

Worked example 1: mass to moles

How many moles are in 5.3 g of sodium carbonate? Moles = 5.3 / 106 = 0.05 mol.

Worked example 2: particles

How many molecules are in 0.5 mol of water? 0.5 x 6.02 x 10^23 = 3.01 x 10^23 molecules.

Empirical formula

Steps: (1) write percentages or masses, (2) divide by Ar, (3) divide by the smallest, (4) round to whole numbers.

A compound contains 40% C, 6.7% H and 53.3% O.

C: 40 / 12 = 3.33; H: 6.7 / 1 = 6.7; O: 53.3 / 16 = 3.33
Divide by 3.33: C 1, H 2, O 1
Empirical formula CH2O.

Stoichiometry (reacting quantities)

A balanced equation gives the mole ratio of reactants and products.

Worked example 3: Magnesium burns: 2Mg + O2 -> 2MgO. What mass of MgO forms from 12 g of Mg? (Mg = 24, O = 16)

Moles Mg = 12 / 24 = 0.5
Ratio Mg : MgO = 1 : 1, so 0.5 mol MgO
Mass = 0.5 x 40 = 20 g

Worked example 4 (gas volume): Zinc reacts with dilute acid: Zn + 2HCl -> ZnCl2 + H2. What volume of hydrogen at r.t.p. forms from 6.5 g of Zn? (Zn = 65)

Moles Zn = 0.1; moles H2 = 0.1
Volume = 0.1 x 24 = 2.4 dm3

Percentage yield

Percentage yield = (actual yield / theoretical yield) x 100.

Common errors

Using an unbalanced equation.
Forgetting to multiply subscripts in molar mass.
Mixing up g and kg or cm3 and dm3 (1 dm3 = 1000 cm3).

Try it

Calculate the molar mass of calcium hydroxide, Ca(OH)2.
Find the number of moles in 90 g of water.
What volume of CO2 at r.t.p. forms when 10 g of CaCO3 reacts with excess acid?
Find the empirical formula of a compound with 2.4 g C and 0.6 g H.