What is the order increasing boiling point for: #"A, butane; B. octane"# #"; C. heptane; D, propane; and E. pentane?"#

#"Propane",C_3H_8;" Butane", C_4H_10;" Pentane, " C_5H_12; "heptane, "C_7H_16; "Octane, "C_8H_18.#

Just keep using the formula, and they will stick in you mind: methyl; ethyl; propyl; butyl; futile. Of course, while the list is in the order of increasing molecular mass and increasing chain length, the order is likewise in the order of increasing boiling point. The longer (and the straighter) the chain, the greater the opportunity for intermolecular interactions, and thus the greater the boiling point.

You should look up these boilig points! #"n-Pentane"# has a normal boiling point of approx. #36# #""^@C#; #"n-heptane"# boils at #98# #""^@C#. Years ago, during an extended petrol strike, I fuelled my car with laboratory bottles of pentane and hexanes; the car ran rough, but it did run.

You can actually illustrate this hydrocarbon volatility with bits of string cut to various lengths. The shorter lengths can be picked out fairly easily. But the longer lengths tend to become entangled, and are not so easily separated from each other. This does model the intermolecular interaction that is observed in the hydrocarbon boiling points.