Empirical and molecular formulas
Introduced| English | Meaning |
|---|---|
| empirical formula/emˈpɪrɪkl ˈfɔːmjʊlə/ | A formula showing the simplest whole-number ratio of atoms of each element in a compound. |
| molecular formula/məˈlekjʊlə ˈfɔːmjʊlə/ | A formula showing the actual numbers of atoms of each element in one molecule. |
| water of crystallisation/ˈwɔːtə ɒv ˌkrɪstəlaɪˈzeɪʃn/ | Water molecules present in a fixed proportion within a compound's crystal structure. |
| hydrated/haɪˈdreɪtɪd/ | Combined with water molecules, as in a hydrated salt such as CuSO4·5H2O. |
| anhydrous/ænˈhaɪdrəs/ | Containing no water, especially no water of crystallisation. |
From ratio to real formula
- The empirical formula 实验式 is the simplest whole-number ratio of atoms.
- The molecular formula 分子式 is the actual number of atoms in a molecule.
- Some crystals also trap water — water of crystallisation 结晶水.
Empirical formula route
Follow data from masses to simplest whole-number ratio.
The simplest whole-number ratio of atoms in a compound is the ______ formula.
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The molecular formula is a whole-number multiple of the empirical formula.
Ethane has empirical formula CH₃ and molecular formula C₂H₆. This shows:
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Empirical = simplest whole-number ratio; molecular = the real number of atoms per molecule.
Finding the empirical formula
From masses (or % by mass):
- divide each element's mass by its $A_r$ → moles.
- divide all the mole values by the smallest.
- round to whole numbers — that ratio is the empirical formula.
Example: ethane has empirical formula $\text{CH}_3$ but molecular formula $\text{C}_2\text{H}_6$.
The first step in finding an empirical formula from masses is to:
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Convert masses to moles (mass ÷ Ar), then divide by the smallest and round to whole numbers.
A compound contains 0.2 mol carbon and 0.8 mol hydrogen. Dividing by the smaller amount, what is the H subscript in the empirical formula (C₁H?)?
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Divide both by 0.2: C = 1, H = 4. The empirical formula is CH₄.
Put the steps for finding a molecular formula in order.
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Masses → moles → simplest ratio (empirical) → scale by Mr (molecular).
From empirical to molecular
- You also need $M_r$.
- Find how many times the empirical formula mass fits into $M_r$.
- Multiply the empirical formula by that number.
To get the molecular formula from the empirical formula, you also need:
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Divide Mr by the empirical formula mass, then multiply the empirical formula by that whole number.
Hydrated 水合的 and anhydrous 无水的
- Water of crystallisation is water held inside a crystal.
- A solid that contains it is hydrated; with the water removed it is anhydrous.
- E.g. hydrated copper(II) sulfate $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$ → heat → anhydrous $\text{CuSO}_4$.
An anhydrous salt is one that:
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Hydrated salts hold water of crystallisation; heating drives it off to give the anhydrous form.
You've got it
- empirical = simplest ratio; molecular = actual atoms (ethane $\text{CH}_3$ vs $\text{C}_2\text{H}_6$)
- empirical: mass ÷ $A_r$ → divide by smallest → round
- molecular: multiply empirical by ($M_r$ ÷ empirical-formula mass)
- hydrated holds water of crystallisation; anhydrous has it removed