Steps to Follow: 1) Assume 100 g if you have a percentage (change % to g) 2) Convert to moles, divide grams by molar mass 3) Divide each mole answer in step 2 by the smallest answer from step 2. 4) Round to whole numbers if the answer is close -> If the answer is not close to round you must multiply all the answers by a number --> (_.25 * 4) (_.33 * 3) (_.50 * 2) (_.75 * 4)
Below are two problems where you have to find the empirical formula. Each problem has a video showing how to solve the problem and also the steps are written out below the videos.
Example #1: Find the empirical formula of a compound made up of 46.6% Al and 53.4% P.
1) Assume 100 grams 46.6 g Al 53.4 g P 2)Divide by molar mass 46.6 g Al / 26.9815 g/mol = 1.727 mol Al 53.4 g P / 30.97376 g/mol = 1.724 mol P 3) Divide by smallest answer from #2. 1.727 mol Al / 1.724 = 1 mol Al 1.724 mol P / 1.724 = 1 mol P So... AlP
Example #2: Find the empirical formula of a compound made up of 68.4 g Cr and 31.6 g O.
1) We do not need to assume anything because the units are already grams so skip this step! 2) Divide by molar mass 68.4 g Cr / 51.9961 g/mol = 1.316 mol Cr 31.6 g O / 15.9994 g/mol = 1.975 mol O 3. Divide by the smallest answer from #2. 1.316 mol Cr / 1.316 = 1 mol Cr 1.975 mol O / 1.316 = 1.5 mol O 4.) 1.5 mol O doesn't round but if we multiply by 2 we get to a whole number so you have to multiply both Cr and O by two so the final answer is Cr2O3
YOU TRY!!! 1.) The percent composition of a compound was found to be 63.5% Ag, 8.2% N, and 28.3% O. What is the empirical formula?
Answer: 1.) AgNO3
Molecular Formulas
To find the molecular formula you must know the empirical formula. From there simply divide the molecular mass by the empirical formula mass and multiple the empirical formula by the answer.
Example #1: What is the molecular formula of the compound in example #1 above if it has a molar mass of 58 g/mol?
Answer: Divide the given mass by the mass of AlP so 58/58 = 1. Which means the empirical formula and the molecular formula are the same thing, AlP.
Example #2: What is the molecular formula of the compound in example #2 above if it has a molar mass of 304 g/mol.
Answer: molecular/empirical -> 304/152 = 2 so you must multiply the formula by 2.
(Cr2O3) x 2 = Cr4O6
YOU TRY!!! What is the molecular formula of the question in the YOU TRY above if it has a mass of 339.7 g/mol?