Question

A salt contains `31.84%` potassium, `28.98%` chlorine, and `39.18%` oxygen by mass. What is its empirical formula?

Answer 1

`KClO_3`

Explanation:

As always with these problems, we assume an `100*g` mass of compounds, and then interrogate the molar quantities of the constituent atoms...

`"Moles of potassium"-=(31.84*g)/(39.10*g*mol^-1)=0.814*mol`.

`"Moles of chlorine"-=(28.98*g)/(35.45*g*mol^-1)=0.817*mol`.

`"Moles of oxygen"-=(39.18*g)/(16.00*g*mol^-1)=2.45*mol`.

We divide thru by the lowest molar quantity to give an empirical formula of `KClO_3` (or close enuff to!), i.e. `"potassium chlorate"`.

And the molecular formula is always a multiple of the empirical formula.....`(39.1+35.45+3xx16)_n*g*mol^-1=122,5*g*mol^-1`. Clearly, `n=1`, and the molecular formula is `KClO_3`, `"potassium chlorate"`.