Conservation of mass and relative formula mass

The relative formula mass (Mᵣ) of a compound is the sum of the relative atomic masses (Aᵣ) of all the atoms in its formula. Aᵣ values are in the periodic table. Neither Aᵣ nor Mᵣ has units.

Example: sulfuric acid, H₂SO₄ (H = 1, S = 32, O = 16)

Mᵣ = (2 × 1) + 32 + (4 × 16) = 98

Example with brackets: calcium hydroxide, Ca(OH)₂ (Ca = 40)

The bracket doubles everything inside it: Mᵣ = 40 + 2 × (16 + 1) = 74

The law of conservation of mass says that no atoms are lost or made in a chemical reaction. Atoms are just rearranged, so the total mass of the reactants equals the total mass of the products.

In a balanced equation, the Mᵣ values show this:

2Mg + O₂ → 2MgO

(2 × 24) + (2 × 16) = 2 × (24 + 16)

48 g + 32 g = 80 g

In a closed system, nothing can get in or out, so the measured mass stays the same.

In an open system (an open container), a gas can get in or out, so the mass seems to change. It hasn't really — you just aren't weighing the gas.

• Mass goes up when a reactant is a gas from the air. Burning magnesium gains oxygen, so the solid gets heavier.
• Mass goes down when a product is a gas that escapes.

Example: 10.0 g of calcium carbonate is heated: CaCO₃ → CaO + CO₂. Only 5.6 g of calcium oxide is left, because 4.4 g of carbon dioxide escaped (10.0 − 5.6 = 4.4 g).

Every measurement has some uncertainty. Repeating a measurement shows how much results vary.

uncertainty = range ÷ 2, where the range is the largest result minus the smallest.

Example: three titres are 24.2 cm³, 24.6 cm³ and 24.4 cm³.

• Mean = (24.2 + 24.6 + 24.4) ÷ 3 = 24.4 cm³
• Range = 24.6 − 24.2 = 0.4 cm³
• Uncertainty = 0.4 ÷ 2 = 0.2 cm³

Result: 24.4 ± 0.2 cm³