In this example, you would multiply both numbers by 2, giving 2 and 3 (instead of rounding 1.5 up to 2). For example, the empirical formula of a hydrocarbon is CH 2 and its M r is 42. To determine an empirical formula using weight percentages, start by converting the percentage to grams. For example, if your empirical formula contains 29.3 percent sodium, convert it to 29.3 grams. Next, convert the grams to moles by dividing 29.3 grams by the atomic weight of sodium, which is 22.99 grams, to get 1.274. The empirical formula of a compound represents the simplest whole-number ratio between the elements that make up the compound. This 10-question practice test deals with finding empirical formulas of chemical compounds. A periodic table will be required to complete this practice test. Answers for the test appear after the final question: