Empirical formulas are chemical formulas in lowest terms. Many ionic substances have only empirical formulas. Empirical formulas are needed for combustion problems as well. When a substance is analyzed by combustion, only the lowest ratio of the elements is determined. This Powerpoint presentation explains how to recognize empirical formulas and how to convert molecular formulas to empirical formulas if they are not already. It also explains how to determine the molecular formula from the empirical formula and the formula mass.
Free worksheets for this topic are available on my website at http://evanschemistrycorner.com.