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? Cyclohexane, a commonly used organic solvent, is 85.6% C and 14.4% H by mass with a molar mass of 84.2 g/mol. What is its molecular formula? (a) C6H (b) CH2 (c) C5H24 (d) C6H12 (e) C4H8.
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 Step 4 Only whole-number ratios make physical sense because molecules contain whole atoms. The 3.03 in this case could result from a small experimental error in the molecular weight. We therefore multiply each subscript in the empirical formula by 3 to give the molecular formula: C9H12. Check We can have confidence in the result because dividing molecular weight by empirical formula weight yields nearly a whole number. Step 5 Combustion analysis can only determine the empirical formula of a compound; it cannot determine the molecular formula. However, other techniques can determine the molecular weight. Once we know this value, coupled with the empirical formulas, we can easily calculate what the molecular formula is.