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EMPIRICAL & MOLECULAR FORMULAS Define Empirical Formula – Define Molecular Formula – Problem Solving Tips & Ideas for Finding Empirical & Molecular Formula • Convert mass to moles (divide the given mass by the formula mass of the compound). • Be sure to use at least 3 sigfigs. Fewer digits lead to wrong answers. • Find molar ratios of atoms by dividing the bigger number of moles by the smallest number of moles. • To calculate the molecular formula of a compound, you must be given the molar mass, but you still need to find the empirical formula 1) Succinic acid occurs in fungi and lichens. Its empirical formula is C2H3O2 and its molar mass is 118.1 g/mol. What is the molecular formula? 2) A compound used in analytical chemistry, dimethylglyoxime, has the empirical formula C2H4NO. If its molar mass is 116.1 g/mol, what is the molecular formula of the compound? 3) Acetylene is a colorless gas used as a fuel in welding torches, among other things. It is 92.26% C and 7.74% H. Its molar mass is 26.02 g/mol. Calculate the empirical and molecular formulas of acetylene. 4) A large family of boron-hydrogen compounds, the boron hydrides, have the general formula BxHy. (All react with air and burn or explode.) One member of this family contains 88.5% B; the remainder is hydrogen. Which of the following is its empirical formula: BH2, BH3, B2H5, B5H7, or B5H11. 5) Nitrogen and oxygen form an extensive series of oxides with the general formula NxOy. One of them, a blue solid, contains 36.84% N. What is the empirical formula of this oxide? 6) Cumene is a hydrocarbon, a compound composed only of C and H. It is 89.94% carbon, and the molar mass is 120.2 g/mol. What are the empirical and molecular formulas of cumene? 7) Mandelic acid is an organic acid composed of carbon (63.15%), hydrogen (5.30%), and oxygen (31.55%). Its molar mass is 152.14 g/mol. Determine the empirical and molecular formulas of the acid. 8) Nicotine, a poisonous compound found in tobacco leaves, is 74.0% C, 8.65% H, and 17.35% N. Its molar mass is 162 g/mol. What are the empirical and molecular formulas of nicotine? 9) Cacodyl, a compound containing arsenic, was reported in 1842 by the German chemist Robert Wilhelm Bunsen. It has an almost intolerable garlic-like odor. It’s molar mass is 210 g/mol, and it is 22.88% C, 5.76% H, and 71.36% As. Determine the empirical and molecular formulas. 10) In order, list the steps necessary to find the empirical formula and molecular formula of a compound.
