Question

A compound contains 69.94 percent iron and 30.06 percent oxygen. What is its molecular formula if the molar mass of the compound is 199.55 per mole?

Answer 1

`Fe_2O_3`

Explanation:

As with all these problems, we ASSUME, that there is a `100*g` mass of unknown compound.

We break this quantity down into atoms.

And, there is `(30.06*g)/(15.999*g*mol^-1)` `=` `1.88*mol` `O`.

And `(69.94*g)/(55.85*g*mol^-1)` `=` `1.25*mol` `Fe`.

If we divide thru by the lowest molar amount, we get an empirical formula of `FeO_(1.5)`, but because the empirical formula is the simplest whole number ratio defining constituent atoms in a species, we double this result to get WHOLE numbers, i.e. `Fe_2O_3," ferric oxide."`

Note that an examiner would be quite justified at A level to say that `"an oxide of iron has 70% iron content"` without quoting the oxygen percentage, and expect you to realize that the balance is oxygen.