How are molecular and empirical formulas determined from a 3) Find the empirical formula of the compound by finding the ratio of the number of moles of elements. percent composition calculation? Review: Empirical formulas are reduced versions of the An empirical formula is a chemical formula with the lowest molecular formula. In a practical sense, this means that we should divide the number of moles of each element by the whole-number ratio of elements in a compound. smallest answer for number of moles. Since the smallest A molecular formula is the actual chemical formula of a number is “7.14 moles”, we’ll divide both values by 7.14. compound. carbon: 7.14 ÷ 7.14 = 1 Empirical formulas are reduced versions of the molecular hydrogen: 14.3 ÷ 7.14 = 2 formula. For example, a compound that has an empirical formula of CH2O may have a molecular formula of C2H4O2, Empirical formula: CH2 C3H6O3, C4H8O4, etc. Handy hint: If the problem gives you the empirical Steps 1-3 for Calculating Empirical Formulas and Steps 4-5 for formula, you can start at the next step! Always given Calculating Molecular Formulas 4) Divide the molar mass of the compound in the problem by Once you have a percent composition of a chemical compound, the molar mass of the empirical formula. it’s possible to figure out the molecular formula. Here’s how, For our compound, the molar mass of the compound is using the example of a compound with a molar mass of 28 28 g/mol grams/mol in which the percent composition of the components For the empirical formula, CH2, the molar mass is are as follows: 14.01 g/mol o C: 85.7% Divide as instructed in step 4 above o H: 14.3% 28 g/mol ÷ 14.01 g/mol ≈ 2 5) Multiply the subscripts in the empirical formula by the 1) Assume you have 100 grams of the chemical. This serves to number you found in step 4 to find the molecular formula. convert the percentages into grams. In our example, we now have CH2 × 2 = C2H4 C = 85.7 g H = 14.3 g NOTE: Sometimes the empirical formula and molecular formula are 2) Figure out how many moles of each element you have in the the same. compound. C = 85.7 ÷ 12.01 = 7.14 mol C H = 14.3 ÷ 1 = 14.3 mol H Empirical Formula Steps Percent to mass Mass to mole Divide by small Multiply ‘til whole Molecular Formula Steps Molar Mass compound ÷ Molar Mass empirical Multiply the subscripts by the result **If the subscripts in a formula will reduce, it is NOT an empirical formula .