# 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?

May 19, 2017

#### 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) ....

May 19, 2017

Surely you mean $1.33 \cdot g$ of hydrogen..........we eventually get a molecular formula of ${H}_{2} {O}_{2}$, i.e. $\text{hydrogen peroxide}$.

#### Explanation:

Let us suppose that there were $1.33 \cdot g$ hydrogen, and $21.3 \cdot g$ oxygen:

$\text{Moles of hydrogen} = \frac{1.33 \cdot g}{1.00794 \cdot g \cdot m o {l}^{-} 1} = 1.32 \cdot m o l$

$\text{Moles of oxygen} = \frac{21.3 \cdot g}{15.999 \cdot g \cdot m o {l}^{-} 1} = 1.33 \cdot m o l$

And thus we gets an empirical formula of $H O$.

But we know that the $\text{molecular formula}$ is a whole number multiple of the $\text{empirical formula}$:

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

And thus $\left\{1.01 + 16.0\right\} \cdot g \cdot m o {l}^{-} 1 \times n = 34.1 \cdot g \cdot m o {l}^{-} 1$.

Clearly $n = 2$, and thus we gets a molecular formula of $2 \times \left\{H O\right\} \equiv {H}_{2} {O}_{2}$.