Conservation of mass, percentage by mass and concentration
Atoms are not made or destroyed in a chemical reaction, only rearranged. That one idea explains balanced equations and lets you work out unknown masses. This page also covers percentage by mass and concentration in grams per dm³.
Part 1 of 3: Learn it
In short
- Total mass of reactants = total mass of products.
- If a gas escapes, the mass in the open container seems to fall; if a gas is taken in, it seems to rise.
- Concentration (g/dm³) = mass of solute (g) ÷ volume of solution (dm³).
Where this is in your specification
Spec points: AQA 4.3.1.1 to 4.3.1.3 and 4.3.2.5 (Combined Trilogy 5.3.1 and 5.3.2.5), Edexcel Topic 1, OCR C3.1
| Board | Topic: Quantitative chemistry and the mole |
|---|---|
| AQA 8462 | 4.3 (8462); 5.3 (8464) |
| Edexcel 1CH0 | Topics 1 and 5 |
| OCR J248 | C3.1 |
| Eduqas C410 | 3 |
| Cambridge IGCSE 0620 | 3.1 to 3.3 |
| National 5 C813 75 | 1(c) Formulae and reacting quantities |
Conservation of mass
In a balanced equation there are the same number of each kind of atom on both sides, so the mass before and after is the same. If you know all the masses but one, subtract to find it.
In an open flask, heating a metal carbonate gives off carbon dioxide, so the reading on the balance falls. Heating magnesium in air makes the mass go up, because oxygen from the air joins the metal. In both cases mass is still conserved: the gas just was not on the balance.
Percentage by mass
For carbon dioxide, Mᵣ = 12 + 2 × 16 = 44, and the oxygen contributes 32, so oxygen is 32 ÷ 44 × 100 = 72.7% of the mass.
Concentration in g/dm³
- 1 dm³ = 1000 cm³, so divide a volume in cm³ by 1000 to get dm³.
- Concentration is mass divided by volume, so the mass of solute is concentration times volume.
- A more concentrated solution has more solute in the same volume.
Why does the mass of an open flask fall when marble chips react with acid?
Show the answer
Carbon dioxide gas is made and escapes into the air.
Part 2 of 3: See it worked
Worked examples
Example 1
10.0 g of calcium carbonate is heated until it has all decomposed: CaCO₃ → CaO + CO₂. 5.6 g of calcium oxide is left. What mass of carbon dioxide was given off?
- Mass of reactants = mass of products
- 10.0 = 5.6 + mass of CO₂
- Mass of CO₂ = 10.0 − 5.6
Answer: 4.4 g.
Example 2
12 g of sodium hydroxide is dissolved to make 250 cm³ of solution. Find its concentration in g/dm³.
- Volume = 250 ÷ 1000 = 0.25 dm³
- Concentration = 12 ÷ 0.25
Answer: 48 g/dm³.
Example 3
4.8 g of magnesium burns completely to make 8.0 g of magnesium oxide. What mass of oxygen reacted?
- Mass of magnesium + mass of oxygen = mass of magnesium oxide
- Oxygen = 8.0 − 4.8
Answer: 3.2 g.
Common mistakes
- Saying mass is lost in a reaction. A gas has escaped; the atoms still exist.
- Using cm³ in the concentration equation without converting to dm³.
- In percentage by mass, forgetting to multiply the Aᵣ by the number of atoms of that element.
- Using the mass of the solution instead of the mass of the solute.
What mass of solute is in 400 cm³ of a 30 g/dm³ solution?
Show the answer
0.4 dm³ × 30 g/dm³ = 12 g.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
Why does the mass of an open flask fall when marble chips react with acid?
Carbon dioxide gas is made and escapes into the air.
What mass of solute is in 400 cm³ of a 30 g/dm³ solution?
0.4 dm³ × 30 g/dm³ = 12 g.
Find the percentage by mass of calcium in CaCO₃. (Aᵣ: Ca = 40, C = 12, O = 16)
Mᵣ = 40 + 12 + 48 = 100, so 40 ÷ 100 × 100 = 40%.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
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