Formulae, Equations and Amount of Substance
Empirical and Molecular Formulae
Pearson Edexcel International A Level Chemistry
Empirical formula from composition by mass
- Find any element whose share is not given by difference: 100 − the percentages given, or the sample mass minus the masses given. A hydrocarbon given only its carbon percentage has the rest as hydrogen.
- Divide the percentage or mass of each element by its Ar, for every element in turn: % divided by Ar.
- Divide by the lowest number of moles.
- Multiply the whole ratio by the number that clears the fraction, so that every value is a whole number.
- Write those whole numbers as the subscripts.
Turning the ratio into whole numbers
A value a few hundredths away from a whole number is that whole number. A value left on a quarter, a third, a half or two thirds is never rounded: multiply every value in the ratio by the same number.
| Left over on a value | Multiply every value by |
|---|---|
| a half | 2 |
| a third or two thirds | 3 |
| a quarter or three quarters | 4 |
| a fifth, or any multiple of a fifth | 5 |
The molecular formula
| Quantity | Symbol | Unit |
|---|---|---|
| relative molecular mass | Mr | no unit |
| empirical formula mass, the sum of the Ar values in the empirical formula | no unit | |
| number of empirical formula units in one molecule | n | no unit |
| moles of water per mole of salt | x | no unit |
Multiply every subscript in the empirical formula by n. Never write a number written in front of the empirical formula.
Percentage by mass of an element
Percentage by mass of an element = number of atoms of that element × Ar ÷ Mr of the compound × 100.
Water of crystallisation
A hydrated salt is written X·xH2O, where x is the number of moles of water per mole of salt.
x is a whole number.
- Weigh the empty crucible, then the crucible and hydrated salt.
- Heat, cool and reweigh, and repeat: heat to constant mass.
- Mass of anhydrous salt = mass after heating − mass of the empty crucible.
- Mass of water = mass before heating − mass after heating.
- Amount of each = mass ÷ Mr, taking 18.0 for the water.
- Divide the amount of water by the amount of anhydrous salt to give x, a whole number.
| x comes out | Because |
|---|---|
| too low | not enough water has been removed: the solid has not been heated long enough |
| too high | too much water has been removed: solid has been lost from the crucible |
Give water left in the solid or solid lost from the crucible as the reason. Never write measurement errors as the reason x is wrong.
Formula of a metal oxide by reduction
- Weigh the oxide, reduce it to the metal, and weigh the metal. The mass of oxygen = mass of the oxide − mass of the metal.
- Moles of metal and moles of oxygen, each mass ÷ Ar, give the ratio of the formula.
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