Question

What is the molecular formula of a substance that decomposes into 133 g of H and 21.3 g of O, and was found to have a molar mass of 34.1 g?

Answer 1

error in your question!

Explanation:

from molar mass the logical answer is H2O2 (likely) ...
molar mass is 34
atomic mass of O is 16
atomic mass of H is 1
the only realistic combination of this is 2 x O + 2 x H, H2O2.

From your data you get a weird formula of H99O1 (unlikely) ....

Answer 2

Surely you mean `1.33*g` of hydrogen..........we eventually get a molecular formula of `H_2O_2`, i.e. `"hydrogen peroxide"`.

Explanation:

Let us suppose that there were `1.33*g` hydrogen, and `21.3*g` oxygen:

`"Moles of hydrogen"=(1.33*g)/(1.00794*g*mol^-1)=1.32*mol`

`"Moles of oxygen"=(21.3*g)/(15.999*g*mol^-1)=1.33*mol`

And thus we gets an empirical formula of `HO`.

But we know that the `"molecular formula"` is a whole number multiple of the `"empirical formula"`:

`{"empirical formula"}xxn="molecular formula"`

And thus `{1.01+16.0}*g*mol^-1xxn=34.1*g*mol^-1`.

Clearly `n=2`, and thus we gets a molecular formula of `2xx{HO}-=H_2O_2`.