A dry-cleaning solvent (molecular weight = 146.99 g/mol) that contains C, H, and

Question

A dry-cleaning solvent (molecular weight = 146.99 g/mol) that contains C, H, and Cl is suspected to be a cancer-causing agent. When a 0.317 g sample was studied by combustion analysis, 0.569 g of CO2 and 0.0771 g of H2O formed. How many moles of each of the following were in the original sample?

Explanation / Answer

This question sounds a lot more difficult than it is in actuality. The trick here is to understand how combustion works (complete combustion) as well as understand the idea of molar ratios. The general chemical equation of a complete combustion reaction is the following: CxHy + [(2x+y)/2]O2 -> x CO2 + [y/2] H2O The ratio of CxHy to CO2 is 1 to x; therefore if you can determine the number of moles of CO2 produced, than you can determine the number of carbon atoms in your hydrocarbon. Similarly the ratio of CxHy to H2O is 1 to (y/2). If you can determine the number of moles of water produced, than you can determine the hydrogen atoms in your hydrocarbon. So let’s attempt this question. 0.198g of unknown (MW of 146.99g/mol) With the mass and molecular weight you can determine the moles of unknown. (0.198g / 146.99 g/mol = 0.001347 mol of unknown) 0.355g of CO2 produced (MW of 43.99g/mol) => 0.008066 mol of CO2 (Same calculation as above) 0.0481g of H2O produced (MW of 18.01g/mol) => 0.002669 mol of H2O Compare the molar ratios Unknown : CO2 : H2O 0.001347 : 0.008066 : 0.002669 Dividing by 0.001347 gives 1 : 6 : 2 (rounding to the nearest one) Since the unknown has a molar ratio of 1 to 6 with CO2, there are 6 carbon atoms in the unknown. The molar ratio of unknown to water is 1 to 2 indicating that there are 4 hydrogen atoms in the unknown. So we have C6H4Clz, the MW of C6H4 is 76.09g/mol. The total MW of the compound is 146.99 g/mol. So, 146.99 – 76.09 = 70.9g/mol. The MW of chlorine is 35.45 g/mol and 70.9 g/mol divided by 2 is 35.45 g/mol. Therefore the chemical formula of the unknown is C6H4Cl2 also known as dichlorobenzene. Now that you have the chemical formula you can determine the moles of C, H and Cl in the original sample. Hope this helps.

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