The mole concept
The number of moles of a substance is its mass divided by its molar mass: .
Example 1: Find the number of moles in 20 g of calcium carbonate, (molar mass = 100 g/mol).
.
Empirical and molecular formulas
The empirical formula is found from the ratio of moles of each element in a compound; the molecular formula is a whole-number multiple of the empirical formula, found by comparing the empirical formula's mass to the compound's actual molar mass.
Example 2: A compound contains 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. Find its empirical formula (relative atomic masses: C = 12, H = 1, O = 16).
Moles: C = , H = , O = . Dividing by the smallest (3.33): C : H : O = 1 : 2 : 1, giving the empirical formula .
Calculations from equations
A balanced equation's coefficients give the mole ratio between reactants and products, which lets you calculate one substance's mass or volume from another's.
Example 3: Find the mass of carbon dioxide produced when 10 g of calcium carbonate is fully decomposed: (molar mass = 100 g/mol, = 44 g/mol).
Moles of = . The equation shows a 1:1 ratio, so moles of = 0.1 mol, giving a mass of .
Concentration (molarity)
Molarity is moles of solute per litre of solution: , where is in litres.
Example 4: Find the concentration of a solution made by dissolving 0.5 mol of a solute in 250 cm³ of water.
. .