Empirical Formulas Estimation

An empirical formula tells us the relative ratios of different atoms in a compound. The ratios hold true on the molar level as well. Thus, H₂O is composed of two atoms of hydrogen and 1 atom of oxygen. Likewise, 1.0 mole of H₂O is composed of 2.0 moles of hydrogen and 1.0 mole of oxygen. We can also work backwards from molar ratios since if we know the molar amounts of each element in a compound we can determine the empirical formula.

Mercury forms a compound with chlorine that is 73.9% mercury and 26.1% chlorine by mass. What is the empirical formula?

Solution

Let's say we had a 100 gram sample of this compound. The sample would therefore contain 73.9 grams of mercury and 26.1 grams of chlorine. How many moles of each atom do the individual masses represent?

For Mercury:

For Chlorine:

What is the molar ratio between the two elements?

Thus, we have twice as many moles (i.e. atoms) of Cl as Hg . The empirical formula would thus be (remember to list cation first, anion last) : HgCl2