On complete combustion in air, a sample of an unknown hydrocarbon yielded 176 grams of CO2 and 108 grams of H2O with no additional products. Which one of the following might be the formula of the hydrocarbon? (Relative atomic masses: H=1, C =12, O =16)

a) CH2b) CH3c) CH4d) C2H4 e) C2H8

The equation for the combustion of the hydrocarbon is: CxHy+ (A + 0.5B)O2 --> A.CO2+ B.H2OMoles = mass/ArMoles of CO2= 176/44 = 4 = AMoles of H2O = 108/18 = 6 = BSubstitute the moles into the equation: CxHy+ 7O2 --> 4CO+ 6H2OTo balance the equation, there must be 4 carbon atoms and 12 hydrogen atoms on the left side. The formula of the hydrocarbon must either contain 4 carbon and 12 hydrogen atoms, or contain a number of carbon and hydrogen atoms that are factors of 4 and 12, respectively.Therefore, the options for the formula of the hydrocarbon are: C4H12, C2H6or CH3 Out of those, CH3is the only option given, so the answer is B.

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Answered by Zoe C. BMAT (BioMedical Admissions) tutor

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