Question

The mineral greenockite is a yellow sulfide of cadmium that is 78.0% cadmium and 22.0% sulfur. How do you determine the empirical formula of greenockite?

Answer 1

The empirical formula is `"CdS"`. The method is below.

Explanation:

Start by assuming a mass of `100` grams of the compound. Then, the percentages can be exchanged for masses.

i.e. there are `"78 g"` of cadmium and `"22 g"` of sulfur.

Convert each mass to moles:

`"Cd: " "78 g" / "112.4 g/mol" = "0.694 mol"`

`"S: " "22 g" / "32.06 g/mol" = "0.686 mol"`

To two digits, these values are equal, so the whole number ratio of mol (and therefore atoms) in the compound is `1 : 1`.

The empirical formula is `"CdS"`

Note, if it is not obvious how to get a whole number ratio as above, the best way to proceed is to divide each number of moles by the smallest value. This should make it easier to get the ratio.