Question

What is the empirical formula of a oxide of cobalt that contains a `1.216*g` mass of metal, and a `0.495*g` mass of oxygen?

Answer 1

`Co_2O_3`......`"cobaltic oxide"`

Explanation:

We interrogate the molar quantities of metal and oxygen.....

`"Moles of cobalt"=(1.216*g)/(58.9*g*mol^-1)=0.0207*mol.`

`"Moles of oxygen"=(0.495*g)/(16.0*g*mol^-1)=0.0309*mol.`

We divide thru by the smallest molar quantity, that of the metal, to get a trial empirical formula of.....

`Co_((0.0207*mol)/(0.0207*mol))O_((0.0309*mol)/(0.0207*mol))=CoO_(1.49)`....

But by specification, the empirical formula is the simplest whole number ratio defining constituent atoms in a species...and so....

`Co_2O_3`.....

This is not a good question inasmuch as `Co_2O_3` is poorly characterized and possibly unknown. There are `CoO`, and `Co_3O_4`, a mixed cobalt oxide of `CoO`, and `Co_2O_3`....The person who set the question was not an inorganic chemist.

Answer 2

`Co_2O_3`

Explanation:

Assuming complete reaction of both, convert the masses into moles and normalize. I’ll use a singlet oxygen because we don’t know the ratio yet, even though the actual gas would be diatomic.

`(1.216g)/(58.9(g/"mol") Co) = 0.0206` mole `Co` ; `(0.495g)/(16(g/"mol" O)) = 0.031` mole O

`Co_0.0206O_0.031` ; or `"Co"/O = 0.0206/0.031 = 2/3`
`Co_2O_3`