"Published in Vacaville, California, USA"
A compound containing only carbon and hydrogen is subjected to combustion analysis. From a sample of the compound, 67.5 g of CO2 and 13.8 g of H2O are obtained. What is the empirical formula of the compound?
Solution:
Since the products from the combustion of a hydrocarbon are given, then we can identify the unknown hydrocarbon by getting the empirical formula.
Moles of each components:
Moles of carbon is
Moles of hydrogen is
From the number of moles of each component, we need to divide all of them by their least number of moles which is hydrogen in order to get the number of atoms in a sample.
Number of carbon in a sample is
Number of hydrogen in a sample is
Since the empirical formula of the hydrocarbon which is CH is not considered as a molecule, then we need to multiply all the atoms by 2 so that the empirical formula is considered as a molecule. There's an excess of 3 valence electrons for carbon and there's no way that carbon can make 8 valence electrons. If we multiply all the atoms by 2, then carbon atoms can make 8 valence electrons by covalent bonding. There's a triple bond between the two carbons and one bond between carbon and hydrogen. Each carbons have now 8 valence electrons. Therefore, the empirical formula for the hydrocarbon which is acetylene is
