Chapter 3: Stoichiometry — Part 2

01 Definitions of mass

Relative atomic mass, A_r

The average mass of an atom of an element compared with 1/12 of the mass of a carbon-12 atom. A_r has no units (it is a ratio). Found on the periodic table (e.g. Na = 23, O = 16).

Relative molecular mass, M_r

The sum of the relative atomic masses of all atoms in a molecular substance (e.g. H₂O: M_r = 2 + 16 = 18).

Relative formula mass

Used for ionic compounds and any formula unit. Calculated the same way as M_r — add the A_r values for every atom in the formula. Example: NaCl ? 23 + 35.5 = 58.5.

02 Reacting masses in simple proportions

From a balanced equation, the mass ratio of reactants and products can be found using M_r values — without using the mole concept.

Mass ratio from equations

In 2Mg + O₂ ? 2MgO, the reacting mass ratio is 48 g Mg : 32 g O₂ : 80 g MgO (using M_r × coefficients).

Formula triangle (m / M_r / mol)

Links mass (m), relative formula mass (M_r), and amount in moles (n): n = m ÷ M_r. Mnemonic: cover the quantity you want — e.g. cover n to get m = n × M_r.

Mass of reactant from product

Use the M_r ratio from the balanced equation. If 80 g of MgO is formed, the Mg used was (48/80) × 80 = 48 g. Scale up or down using the coefficients and M_r values.