Chemistry Quarter 2 - Emperical Formula

Empirical Formula

The simplest ratio of elements in a compound is called the empirical formula of that compound. You can think of an empirical formula as an abbreviated chemical formula. Empirical formulas describe the type of atoms and their ratios but usually do not describe exactly how many atoms of each element are in that compound. Empirical formulas are easier to comprehend if you consider how they can be used to describe everyday situations around you. Consider the number of male and female students in a classroom. Imagine there are 5 males, abbreviated M, and 15 females, abbreviated F. The complete chemical formula for this class would be M5F15, whereas the empirical formula for this class would be MF3, which indicates a ratio of three female students to every one male student. So, while the empirical formula, MF3, reflects a constant ratio of three female students to every male student, it does not tell us the exact number of male and female students in the class. Let's look at some examples. A molecule of glucose (C6H12O6) is made up of carbon, hydrogen, and oxygen. You can see that these three elements are found in the proportion of 6:12:6. The simplest ratio of the elements of glucose is 1:2:1. Thus, the empirical formula of glucose is CH2O. Ethylene glycol (C2H6O2) is commonly used as antifreeze in automobiles. This compound is made up of carbon, hydrogen, and oxygen in actual ratio of 2:6:2. This ratio can be simplified as 1:3:1, so the empirical formula of ethylene glycol is CH3O. Interestingly, some compounds may have the same empirical formula despite being very different from each other. For instance, ethylene (C2H4), propylene (C3H6), and butylene (C4H8) all have the empirical formula CH2. In all of these compounds, carbon and hydrogen are found in the ratio of 1:2, yet the compounds' properties are very different from one another. You can determine the empirical formula of a compound if you know the percent composition of that compound. You already know that percent composition is the relative mass of each element in a compound compared to the entire mass of a compound. Using the percent composition, we can find the number of moles of each element, which will lead us to the empirical formula. Let's find the empirical formula of a compound containing 72.18 percent magnesium and 27.82 percent nitrogen. The first step is to convert the percentage values into grams. Since we are working with percent composition, we can assume that we have 100 grams of the compound: percent composition indicates the amount of substance found in 100 grams of compound. Thus, in this case, 100 grams of the compound contains 72.18 grams of magnesium and 27.82 grams of nitrogen. The next step is to find the number of moles of each element. We divide the amount of the compound in grams by the molar mass of the element. So, we have and . Thus, the mole ratio of magnesium to nitrogen is 2.969:1.985. But, an empirical formula is a whole-number ratio. Let's see how to calculate the empirical formula of the compound from the mole ratios of the elements.