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?

Dec 18, 2016

The empirical formula is $\text{CdS}$. 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 $\text{78 g}$ of cadmium and $\text{22 g}$ of sulfur.

Convert each mass to moles:

$\text{Cd: " "78 g" / "112.4 g/mol" = "0.694 mol}$

$\text{S: " "22 g" / "32.06 g/mol" = "0.686 mol}$

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 $\text{CdS}$

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.