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?

1 Answer
Dec 18, 2016

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


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.