STUDENT SPACE · TOPIC 5
Empirical and molecular formulae
Deduce empirical formulae from composition and molecular formulae from relative molecular mass.
The essentials
- The empirical formula is the simplest whole-number ratio of atoms or ions in a compound.
- The molecular formula gives the actual numbers of atoms of each element in one molecule.
- When magnesium reacts with oxygen, the mass increase gives the mass of oxygen combined.
- 2.4 g Mg + 1.6 g O: moles = 0.10 : 0.10, so empirical formula = MgO.
Key vocabulary
| English | Chinese |
|---|---|
| empirical formula | 最简式 |
| molecular formula | 分子式 |
| whole number ratio | 整数比 |
Quick practice
Can empirical formula CH₂O alone determine the molecular formula? Explain.
Need a hint?
Use the relationships above and check particle ratios and units.
Check your reasoning after trying
No. The relative molecular mass (or equivalent information) is also needed.
Use Lesson 5 in your worksheet for written practice. Your teacher will discuss those answers in class.