Question

A chemistry student has a sample of an unknown compound containing carbon, hydrogen, and oxygen. The mass of the sample is 2.50 g. After combustion, the student finds that 1.80 g of carbon dioxide, 0.75 g of water, and 0.20 g of oxygen are produced. Using mass balance calculations, determine the empirical formula of the unknown compound.

Answer 1

To determine the empirical formula of the unknown compound, we first need to find the moles of carbon, hydrogen, and oxygen in the compound.1. Calculate the moles of carbon and hydrogen from the combustion products:1.80 g of CO  molar mass = 44.01 g/mol :moles of C =  1.80 g  /  44.01 g/mol  = 0.0409 mol0.75 g of HO  molar mass = 18.02 g/mol :moles of H = 2 *  0.75 g  /  18.02 g/mol  = 0.0833 mol2. Calculate the moles of oxygen in the compound:The 0.20 g of oxygen produced during combustion was originally part of the compound. To find the moles of oxygen, we use the molar mass of oxygen  16.00 g/mol :moles of O =  0.20 g  /  16.00 g/mol  = 0.0125 mol3. Determine the empirical formula:Divide the moles of each element by the smallest number of moles to find the mole ratio:C: 0.0409 / 0.0125 = 3.27  3H: 0.0833 / 0.0125 = 6.66  7O: 0.0125 / 0.0125 = 1The empirical formula of the unknown compound is CHO.