Without consulting tables, arrange these compounds in order of ...

[robinn137]

Without consulting tables, arrange these compounds in order of decreasing (less negative) heat of
  combustion: hexane, 2-methylpentane, and 2,2-dimethylbutane.

Question 2

What generalization can you make about the densities of alkanes relative to the density of water?

[kusterl]

Answer to Question 1

Branching increases stability of an alkane. Therefore, the more highly branched the isomer, the smaller (less
negative) the heat of combustion. The molecules listed in order of decreasing heat of combustion turn out to be
exactly as they were listed in the question: Hexane, 2-Methylpentane, 2,2-Dimethylbutane

Answer to Question 2

All alkanes that are liquid at room temperature are less dense than water (The O atom of H2O has a larger
atomic mass than the C atom of alkanes). This is why alkanes such as those in gasoline and petroleum float on
water.