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Flashcards in Determination Of Formulae Deck (12)
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Molecular formulae

No of atoms of each element in a molecule


Empirical formulae

Simplest whole number ratio of atoms in each element in a compound
Impossible to base formula on the actual number of atoms or ions


Relative molecular mass

Compares mass of a molecule with mass of an atom of carbon 12
Calculate by adding all RAM of elements in a molecule


Relative formula mass

Compares mass of formula unit with mass of carbon 12 atom
Calculated by adding together the relative atomic masses of the elements in the empirical formula


How to find the empirical formula from mass

Find moles of all elements in molecule
Divide by smallest whole number


How to find the molecular formula from the empirical

Find RFM
Divide by RMM
Multiply EM by answer


Hydrated salts

Coloured crystals, have water of crystallisation
Very difficult to remove last traces of water


Water of crystallisation

Water molecules that are part of crystalline structure


How to calculate the formula of a hydrated salt

Find the mass of anhydrous salt and water
Find the mole ratio between the 2


What assumptions can you make from calculated experimental formula?

All water has been lost
No further decomposition
Gives a perfect formula


Why can you not tell if all water has been lost?

Only surface colour seen, water could be left inside
Hard to tell if hydrous and anhydrous colours are similar
Heat to a constant mass to see if all water has been removed


How does further decomposition affect accuracy?

Many salts decompose further when heated
Hard to judge if there is no colour change