Chemistry Labs

Problem 5

Only one product was obtained by the reaction of bromine with an unknown hydrocarbon. Its density was 5.207 times as great as that of the air. Determine the structural formula of the unknown hydrocarbon. Relative atomic masses: Ar(H)=1A_r(\text{H}) = 1; Ar(C)=12A_r(\text{C}) = 12; Ar(Br)=80A_r(\text{Br}) = 80.
Step 2 of 3: Deduce the hydrocarbon
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M(RH)=151−80+1=72⇒CX5HX12M(\text{RH}) = 151 - 80 + 1 = 72 \Rightarrow \ce{C5H12}
Analysis

A dibromide would exceed 160 g mol−1^{-1}, so the product is a monobromide: the parent hydrocarbon has M=151−80+1=72M = 151 - 80 + 1 = 72 g mol−1^{-1}, which matches CX5HX12\ce{C5H12}.