- It is easiest to think of 100 mass units of this compound. In this case, the
100 mass units are composed of 60. amu of Mg and 40. amu of O. Because
you know that 1 unit of Mg is 24 amu (from its atomic mass) and, likewise, 1
unit of O is 16, you can divide 60 by 24 to find the number of units of Mg in
the compound and divide 40. by 16 to find the number of units of O in the
compound. - Now, because we know formulas are made up of whole-number units of
the elements, which are expressed as subscripts, we must manipulate these
numbers to get whole numbers. This is usually accomplished by dividing
these numbers by the smallest quotient. In this case they are equal, so we
divide by 2.5. - So the empirical formula is MgO.
Another Example
Given: Ba = 58.81%, S = 13.73%, and O = 27.46%.
Find the empirical formula.
- Divide each percent by the amu of the element.
- Manipulate numbers to get small whole numbers. Try dividing them all by
the smallest first. In this case, divide each result by 0.43, as shown below. - The formula is BaSO 4.
In some cases you may be given the true formula mass of the compound. To
check if your empirical formula is correct, add up the formula mass of the
empirical formula and compare it with the given formula mass. If it is not the
same, multiply the empirical formula by the small whole number that gives you