Question

A compound that contains 6.44g of boron and 1.80g of hydrogen has a molar mass of approximately 28g/mol. What is its molecular formula?

Answer 1

The molecular formula is `B_2H_6`.

Explanation:

The empirical formula would be under the form of `B_xH_y`. We need to find `x and y`.

Step 1.
From the masses of `B` and `H` we will find the number of moles:
`n_B=(6.44cancel(g))/(10.81(cancel(g)/"mol"))=0.596mol`

`n_H=(1.80cancel(g))/(1.01(cancel(g)/"mol"))=1.78mol`

Step 2.
In order to find the molar ratio between `B` and `H` we divide both number of moles by the smallest one `0.596 mol`.

`B: (0.596cancel(mol))/(0.596cancel(mol))=1`

`H: (1.78cancel(mol))/(0.596cancel(mol))=2.99`

Step 3.
Therefore, the empirical formula is `BH_3`.

Step 4.
The molecular mass is `("empirical formula")_n` where `n` is calculated by:
`n=(MM)/("e.f.m.")`

where `MM` is the molecular formula and `"e.f.m."` is the empirical formula mass:

`n=(28cancel(g/"mol"))/(13.84cancel(g/"mol"))=2`

Therefore, the molecular formula is `B_2H_6`.