The Empirical Formula The lowest whole number ratio of elements in a compound. The molecular formula is the actual ratio of elements in a compound. The two can be the same. CH 2 empirical formula C 2 H 4 molecular formula C 3 H 6 molecular formula H 2 O both
Finding Empirical Formulas Just find the lowest whole number ratio. C 6 H 12 O 6, CH 4 N 2 It is not just the ratio of atoms, it is also the ratio of moles of atoms. Calculating Empirical Formulas We can get ratio from percent composition. Assume you have 100 g. The percentages become grams. Convert grams to moles. Find lowest whole number ratio by dividing everything by the smallest moles.
Example Calculate the empirical formula of a compound composed of 38.67 % C, 16.22 % H, and 45.11 %N. Assume 100 g so: = 3.22 mole C = 16.1 mole H = 3.22 mole N
Example Continued: The smallest number of moles is 3.22 mol so divide your answers by 3.22 to get the mole ratio for the formula. The ratio is: So the formula is: C1H5N1C1H5N1
Additional Examples: A compound is 73.9 % Hg and 26.1 % Cl. What is the empirical formula? Caffeine is 49.48% C, 5.15% H, 28.87% N and 16.49% O. What is its empirical formula? HgCl 2 C4H5N2OC4H5N2O
Empirical to molecular Since the empirical formula is the lowest ratio the actual molecule would weigh the same or more. This difference in weight would be by a whole number multiple. To find this multiple: divide the actual molar mass by the the mass of one mole of the empirical formula. You will get a whole number. Multiply the subscripts of the empirical formula by this number.
Example A compound has an empirical formula of ClCH 2 and a molar mass of 98.96 g/mol. What is its molecular formula? A compound has an empirical formula of CH 2 O and a molar mass of 180.0 g/mol. What is its molecular formula? Cl 2 C 2 H 4 C 6 H 12 O 6