Question

`XCl_3` contains 67.2% chlorine by mass. Calculate the molar mass of X and identity the element. ?

Answer 1

Element `X` has a molar mass of approximately `52` `gmol^-1` and is chromium (`Cr`).

Explanation:

We can proceed as though we have one mole of `XCl_3`.

The molar mass of chlorine is `35.5` `gmol^-1`, and 3 mol of `Cl` will therefore have a mass of `3xx35.5=106.5` `g`.

To find the mass of one mole of `XCl_3`, when this makes up `67.2`%, we multiply by `100/67.2`:

`106.5xx(100/67.2)=158.48`

This is the molar mass of one mole of `XCl_3`. To find the molar mass of `X` we subtract the mass of the chlorine:

`158.48-106.5=51.98`

We need the element whose molar mass is closest to `52`. Looking at a periodic table, we find that that is chromium (`Cr`).