Empirical formula

Hi all,

I'm starting Sixth Form in September and I have chosen Chemistry as one of my options. I decided to get a slight head start before September and look at calculating empirical formulae. I've been looking through past papers and came across this question:

Empirical formula.png

Firstly I have divided the amount (grams) of each element by their relative atomic masses to get the amount of moles.

Mg = 0.07488 mol
Si = 0.03738 mol
O = 0.15000 mol

However I am unsure what to do next. The mark scheme says to convert them to percentages but I am unsure how to go about this, or even what I'd have to do after that. So any assistance will be appreciated.

EDIT: the answer is Mg2SiO4

(edited 10 years ago)

Reply 1

kohler1994

12

Original post by Veqz

Hi all,

I'm starting Sixth Form in September and I have chosen Chemistry as one of my options. I decided to get a slight head start before September and look at calculating empirical formulae. I've been looking through past papers and came across this question:

Empirical formula.png

Firstly I have divided the amount (grams) of each element by their relative atomic masses to get the amount of moles.

Mg = 0.07488 mol
Si = 0.03738 mol
O = 0.15000 mol

However I am unsure what to do next. The mark scheme says to convert them to percentages but I am unsure how to go about this, or even what I'd have to do after that. So any assistance will be appreciated.

EDIT: the answer is Mg2SiO4

Work out the ratio of them all to each other. That might help as a slight clue...?

Reply 2

propagation

14

You have to divide by the smallest number of moles.

Reply 3

Veqz

OP

4

Original post by kohler1994

Work out the ratio of them all to each other. That might help as a slight clue...?

I have no idea what that means as I've only being studying it for half an hour at best :s

Reply 4

kohler1994

12

Original post by Veqz

I have no idea what that means as I've only being studying it for half an hour at best :s

Divide each of them by the smallest number. i.e. 0.07488/0.03738... There are twice as many moles of Mg as there are Si. And 4 times as many moles of Oxygen as there are Silicon. Hence:

Mg2SiO4

Since the empirical formula is just ratio of the number of moles.