Question

If 4.04 g of `N` combine with 11.46 g `O` to produce a compound with a molar mass of 108.0 g/mol, what is the molecular formula of this compound?

Answer 1

Dinitrogen pentoxide, `N_2O_5`.

Explanation:

`"Moles of nitrogen"` `=` `(4.04*g)/(14.01*g*mol^-1)` `=` `0.289*mol`

`"Moles of oxygen"` `=` `(11.46*g)/(15.999*g*mol^-1)` `=` `0.716*mol`

We divide thru by the lowest molar quantity to give an empirical formula of `NO_(2.5)` or `N_2O_5`, because the empirical formula should be integral.

Now the molecular formula is always a multiple of the the empirical formula:

So using the quoted molecular mass:

`108*g*mol^-1` `=` `nxx(2xx14.01+5xx15.999)*g*mol^-1`

Clearly, `n=1`, and the molecular formula is `N_2O_5`.