Determining a Molecular Formula Mesitylene, a hydrocarbon found in crude oil, has an empirical formula of C3H4 and an experimentally determined molecular weight of 121 amu. What is its molecular formula? Ethylene glycol, used in automobile antifreeze, is 38.7% C, 9.7% H, and 51.6% 0 by mass. Its molar mass is 62.1 g/mol. (a) What is the empirical formula of ethylene glycol? (b) What is its molecular formula?
Answer: Step 1 Mesitylene or 1,3,5-trimethylbenzene is a derivative of benzene with three methyl substituents positioned symmetrically around the ring. The other two isomeric trimethylbenzenes are 1,2,4-trimethylbenzene (pseudocumene) and 1,2,3-trimethylbenzene (hemi melitene). All three compounds have the formula C H (CH 6 3whic3 3commonly abbreviated C H Me . Mesityle6 3 3 is a colourless liquid with sweet aromatic odor. It is a component of coal tar, which is its traditional source. It is a precursor to diverse fine chemicals. The mesityl group (Mes) is a substituent with the formula C H Me and is found in various other compounds. 6 2 3 Step 2 To know the molecular formula of a compound we have to know the empirical formula of a compound. We are given an empirical formula and a molecular weight of a compound and asked to determine its molecular formula. Step 3 The subscripts in a compound’s molecular formula are whole-number multiples of the subscripts in its empirical formula. We find the appropriate multiple by using Equation Whole-number multiple = molecular weight/ empirical formula weight. The formula weight of the empirical formula C3H4 is 3112.0 amu 2+411.0 amu2 = 40.0 amu Whole-number multiple = molecular weight/ empirical formula weight = 121 40.0 = 3.03