4.4.3 - mole ratios in formulae

Question 1

what does the empirical formula of a substance contain?

Answer 1

the simplest mole ratio of its elements

Question 2

cyclohexane formula = C6H12

e.g. whats the empirical formula of cyclohexane

Answer 2

CH2

Question 3

how can empirical formula be calculated from the masses of each elements?

Answer 3

1. convert the masses into moles
2. cancel down the mole ratio
3. write the simplest mole ration into a formula

Question 4

what does the molecular formula of a substance contain?

Answer 4

the actual numbers of atoms in a molecule

Question 5

cyclohexane empirical formula = CH2

e.g. whats the molecular formula of cyclohexane

Answer 5

C6H12

Question 6

how do you work out the molecular formula from the empirical formula and RFM

Answer 6

calculate how many times the empirical formula ‘fits’ into the RFM by division

Question 7

e.g. calculate the molecular formula if the empirical formula is CH2 and the RFM is 70

Answer 7

RFM of CH2 = 14
multiple = 70/14 = 5
molecular formula = 5 x CH2 = C5H10

Question 8

what is water of crystallisation?

Answer 8

hydrated salt crystals containing water trapped inside them in a fixed mole ratio

Question 9

what form is the mole ratio of water of crystallisation always in?

Answer 9

1 : x

Question 10

how do hydrated salt crystals lose their water

Answer 10

when they are heated

Question 11

e.g. calculate x if 16.7g of MgCl2.xH2O crystals decreased in mass to 9.5g on heating

Answer 11

1. calculate mass of water lost
   mass of water = 16.7 - 9.5 = 7.2
2. draw a ‘mass, moles, ratio’ table and stick the numbers in