Determination Of Formulae

Question 1

Molecular formulae

Answer 1

No of atoms of each element in a molecule

Question 2

Empirical formulae

Answer 2

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

Question 3

Relative molecular mass

Answer 3

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

Question 4

Relative formula mass

Answer 4

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

Question 5

How to find the empirical formula from mass

Answer 5

Find moles of all elements in molecule

Divide by smallest whole number

Question 6

How to find the molecular formula from the empirical

Answer 6

Find RFM
Divide by RMM
Multiply EM by answer

Question 7

Hydrated salts

Answer 7

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

Question 8

Water of crystallisation

Answer 8

Water molecules that are part of crystalline structure

Question 9

How to calculate the formula of a hydrated salt

Answer 9

Find the mass of anhydrous salt and water

Find the mole ratio between the 2

Question 10

What assumptions can you make from calculated experimental formula?

Answer 10

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

Question 11

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

Answer 11

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

Question 12

How does further decomposition affect accuracy?

Answer 12

Many salts decompose further when heated

Hard to judge if there is no colour change