3.2 Determination of formulae Flashcards
What is the molecular formula
The number of atoms of each element in a molecule
Eg. H2, N2, O2, S8
What is the empirical formula
The simplest whole-number ratio of atoms of each element in a compound
Ratio is always the same
Important for substances that do not exist as molecules, eg. carbon, silicon, sodium chloride, which form giant crystalline structures of atoms or ions
what do carbon, silicon and sodium chloride form
Giant crystalline structures of atoms or ions
What is relative molecular mass
Mr, compares the mass of a molecule with the mass of an atom of carbon-12
How to calculate the relative molecular mass
Add together the relative atomic masses of the elements which make up a molecule
What is relative formula mass
Compares the mass of a formula unit with the mass of an atom of carbon-12
Calculated by adding the atomic masses of the elements in the empirical formula
What is the investigation of the chemical composition of a substance called
analysis
Wrongly rounding ratios
If you calculated a ratio as 1:1.67 don’t be tempted to round to 1:2
See if you can convert it into a whole number ratio by multiplying both by the same factor
In this case, by 3, so you get a ratio of 3:5
What is water of crystallisation
Water molecules that are a part of the crystalline structure of hydrated salts
What does anhydrous mean
Lacking water
Experiment to determine the water of crystallisation in hydrated crystals
- Weigh an empty crucible
- Add the hydrated salt into the weighed crucible
- Weigh the crucible and the hydrated salt
- Using a pipe-clay triangle, support the crucible on a tripod, heat it and its contents gently for one minute and then strongly for another three.
- Leave it to cool, then weigh
How accurate is an experimental formula
Some assumptions are made, all the water has been lost, and no further decomposition
