Mole Ratio in Formulae

Question 1

what is the empirical formula of an element?

Answer 1

the empirical formula of a substance contains the simplest mole ratio of its elements

Question 2

what is the empirical formula of cyclohexane (C6H12)?

Answer 2

CH2 - cancelled 6:12 to 1:2

Question 3

what is the empirical formula of hydrogen peroxide (H2O2)?

Answer 3

HO - cancelled 2:2 to 1:1

Question 4

what are the steps of calculating the empirical formula from the masses (or percentages) of each element?

Answer 4

1. convert the masses (or percentages) into moles
2. cancel down the mole ratio
3. write the simplest mole ratio into a formula

Question 5

how do you round moles in empirical formula calculations?

Answer 5

• rounding moles in empirical formula calculations is really dangerous and leads to error
• you can get away with rounding moles to 2dp but never round to 1dp or whole numbers

Question 6

what is the molecular formula?

Answer 6

the molecular formula of a substance contains the actual number of atom in a molecule

Question 7

what is the empirical formula of cyclohexane (C6H12)?

Answer 7

CH2

Question 8

what is the empirical formula of hydrogen peroxide (H2O2)?

Answer 8

HO

Question 9

how can molecular formula be calculated?

Answer 9

• molecular formula can be calculated from empirical formula and RFM
• this works by finding the uncancelled multiple of the empirical formula with the correct RFM

Question 10

what are the two ways of working out the empirical formula?

Answer 10

1 - write out every multiple of the empirical formula until you find the correct one
2 - calculate how many times the empirical formula fits into the RFM by division e.g RFM of molecular formula / RFM of empirical formula

Question 11

what is water of crytallisation?

Answer 11

• hydrated salt crystals contain water trapped inside them in a fixed mole ratio
• this water is called water of crystallization and it’s written into a formula like this:

CuSO4 (salt) . (dot) 5H20 (water of crystallisation)

Question 12

what is the mole ratio of this equation: CuSO4.5H20?

Answer 12

1:5 (the mole ratio is always in the form 1: x)

Question 13

what happens when hydrated salt crystals are heated?

Answer 13

they lose their water

Question 14

how is water of crystallisation calculated?

Answer 14

water of crystallisation is calculated from the masses of salt and water produced on heating

Question 15

what are the steps of calculating water of crystallisation?

Answer 15

1. calculate the water lost
2. draw a ‘mass, moles, ratio’ table and stick the numbers in
   - be careful to use the final mass of solid for the salt, not the original mass of crystals