Problem 1

An amount of 23 g of a gas (density

\rho = 2.05

g dm

^{-3}

at STP) when burned gives 44 g of carbon dioxide and 27 g of water. What is the structural formula of the gas (compound)?

Step 3 of 3: Empirical = molecular formula

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n(\text{C}) : n(\text{H}) : n(\text{O}) = 1 : 3 : 0.5 = 2 : 6 : 1 \Rightarrow \ce{C2H6O}

Analysis

The empirical formula C2H6O has M = 46 g mol $^{-1}$ , matching the measured molar mass, so it is also the molecular formula. Its two isomers are ethanol (liquid at STP) and dimethyl ether (gas); the unknown gas is $\ce{CH3-O-CH3}$ , dimethyl ether.

Common pitfall. Do not forget the mass balance: C + H account for only 15 of the 23 g, so oxygen must be present.