Question

Structures containing the mineral magnetite are found in birds' beaks and may help them navigate. Magnetite is an oxide of iron and in a 100 g sample there are 72.36 g of iron (Fe). What is the empirical formula of this sample of magnetite?

Answer 1

`Fe_3O_4`

Explanation:

I did not know that about birdies' beaks... Anyway, for the empirical formula, we simply interrogate the molar quantities of iron and oxygen in the sample.

`"Moles of Fe"-=(72.36*g)/(55.8*g*mol^-1)=1.30*mol`

`"Moles of O"-=(100*g-72.36*g)/(16.00*g*mol^-1)=1.73*mol`...and so we divide thru by the SMALLEST molar quantity to get a trial empirical formula of...

`Fe_((1.30*mol)/(1.30*mol))O_((1.73*mol)/(1.30*mol))-=FeO_1.33`

But by definition, the empirical formula is the simplest `"WHOLE number ratio"` defining constituent atoms in a species...so we multiple the trial by three....to get...

`3xxFeO_1.33-=Fe_3O_4`

Formally we could represent this as `FeO*Fe_2O_3`..i.e. a mixed oxidation state iron oxide...`1xxFe(+II)+2xxFe(+III)`..

Note that SELDOM will you be quoted an oxygen percentage in an empirical formula problem...you are usually quoted `%C,H,N`, and if this percentage does NOT sum to `100%` the balance is assumed to be due to oxygen. Reasonably, oxygen content is a hard quantity to measure. Why so?