Molecular Formulas Section 11.4 Caffeine Caffeine is known medically as trimethylxanthine, and the chemical formula is C8H10N4O2 It’s important Caffeine is an Among its many operates using mechanisms that cocaine, and to know that addictive drug. actions, it the same amphetamines, heroine use to stimulate the brain. Relatively speaking, caffeine's effects are milder than amphetamines, cocaine and heroin, but it is manipulating the same channels in the brain, and that is one of the things that gives caffeine its addictive qualities. Molecular Formulas The final step in identifying an unknown compound is determining the molecular formula, which shows the actual number of atoms in each molecule of the substance. To determine the molecular formula, the molar mass of the compound must be determined through experimentation. For example, the molar mass of acetylene (C2H2) is 26 g/mol The empirical formula for acetylene is CH and the molar mass of the empirical formula is 13 g/mol Therefore, if the molar mass of the molecular formula for acetylene is divided by the molar mass of the empirical formula for acetylene 26 g/mol ÷ 13 g/mol = 2 The resulting value of “2” shows that the molecular mass must contain twice as the number of carbon atoms and twice the number of hydrogen atoms as the empirical formula. C2H2 How to Find Molecular Formulas 1. calculate the empirical formula 2. use the equation : (empirical formula mass)x = molar mass 3. find value for x: x = molar mass/empirical formula mass 4. multiply each subscript in empirical formula by value for x How to Find Molecular Formulas Example: Given that the empirical formula of a compound is CH and the molar mass is 104 g/mol, calculate the molecular formula. mass of C = 12.0 g/mol mass of H = 1.01 g/mol empirical formula mass = 13.0 g/mol to find the number of CH units in the compound: # CH units = (104 g/mol)(1 mol/13.0 g) = 8.00 Molecular formula = 8(CH) or C H Molecular Formula Practice I 1. Analysis of a chemical used in photographic developing fluid indicates a chemical composition of 65.45% C, 5.45% H, and 29.09% O. The molar mass is found to be 110 g/mol. Determine the molecular formula. 2. A compound was found to contain 49.98 g carbon and 10.47 g hydrogen. The molar mass of the compound is 58.12 g/mol. Determine the molecular mass of the formula. 3. A colorless liquid composed of 46.68% nitrogen and 53.32% oxygen has a molar mass of 60.01 g/mol. What is the molecular formula?