Question

A compound that contains only nitrogen and oxygen is 30.4% N by mass; the molar mass of the compound is 92 g/mol. What is the empirical formula and the molecular formula of the compound?

Answer 1

Empirical formula is `NO_2`, and the molecular formula is `N_2O_4`, dinitrogen tetroxide.

Explanation:

In `100* g` of this compounds there are `30.4* g` `N`, and the balance `O`. We divide thru by the atomic masses in order to approach the empirical formula: `(30.4*g)/(14.01*g*mol^-1)` `=` `2.17*mol*N`;

`(69.6*g)/(16.0*g*mol^-1)` `=` `4.35*mol*O`.

After we divide thru by 2.17, we get an empirical formula of `NO_2`. And as I (and many others) have said before, the empirical formula `"is the simplest whole number ratio defining constituent atoms"`
`"in a species."`

Now the molecular formula is always a multiple of the empirical formula; i.e. `MF = (EF)_n`, or `92.4*g*mol^-1` `=` `2xx(14.01 + 2 xx 16.00)(g*mol^-1)`. So `n` `=` `2`, and the molecular formula is `N_2O_4`, i.e. dinitrogen tetroxide. the dimer of `NO_2` radical.