Calculate the relative formula mass of magnesium nitrate, Mg(NO3)2. (Ar: Mg 24, N 14, O 16)
- Count the atoms: 1 Mg, 2 N and 2 × 3 = 6 O.
- Multiply by each Ar: 1 × 24 = 24, 2 × 14 = 28, 6 × 16 = 96.
- Add: 24 + 28 + 96 = 148.
Answer: Mr = 148
Conservation of mass and relative formula mass
Chemistry · Quantitative chemistry. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
The law of conservation of mass says that no atoms are lost or made in a chemical reaction. So the total mass of the products equals the total mass of the reactants. This is why a symbol equation must balance.
The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of all the atoms in its formula.
In an open container the mass can seem to change. If a gas from the air joins a solid, the mass of the solid goes up. If a gas is made and escapes, the mass goes down. Every measurement also has some uncertainty, which can be estimated from the range of repeated results.
Rules to remember
- total mass of reactants = total mass of products
- Mr = sum of the Ar values of all the atoms in the formula
- A number outside a bracket multiplies everything inside it: Ca(OH)2 has 1 Ca, 2 O and 2 H.
- Metal heated in air: mass of solid increases (oxygen is added).
- Metal carbonate heated or added to acid: mass decreases (carbon dioxide escapes).
- In a sealed container the total mass does not change.
- uncertainty = range ÷ 2, written as mean ± uncertainty
Common mistake
Students forget to multiply every atom inside a bracket by the number outside it. Count the atoms first, then add the masses.
Worked example
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