Calculating Percentage Composition Suppose we wish to find the percent of carbon by mass in oxalic acid – H 2 C 2 O 4. 1.First we calculate the formula weight for the entire compound = 2 (1.01) + 2 (12.01) + 4 (16) = grams 2.Then we find the total mass of carbon in the compound = 2 (12.01) = g 3.To find the percentage we divide the total mass of carbon in the compound by the formula weight and then multiply by 100 to get the answer in percentage form: / x 100 = % carbon

Determining an Empirical Formula A compound is analyzed and determined to contain grams nitrogen and 2.23 grams oxygen. 1.If the amount of each element is not given in moles, then it must first be converted to moles: 2.Select the lowest number of moles and divide each element by that amount: mol N / mol = 1 mol N and mol O / mol = 2 mol O 3.Use your answers in the previous step to write the empirical formula. In this case the correct formula would be NO 2. Note: if you do not get whole number answers in the previous step, you must find an integer to multiply by in order to get a whole number ratio g N1 mol N = mol N g N 2.23 g O1 mol O = mol O 16 g O

Empirical vs. Molecular Formulas An empirical formula represents the simplest whole number ratio of atoms in a formula. A molecular formula represents the actual formula for a compound. In order to determine the molecular formula, we would need the compounds molar mass. Consider the compound on the previous slide: if it’s molar mass was approximately 92 g/mol, what would it’s molecular formula be? FormulaMass NO 2 ~ 46 g/mol ~ 92 g/mol