why are the melting pt. & boiling pt. of butane & 2-methylpropane different...?

Question

...when the molar mass & density are almost identical?

Answer 1

The construction of the molecule chain apparently has a lot to do with it. Even when just talking butane. Normal butane, (N-Butane) has a boiling point 20 deg F higher that Iso butane (I-Butane). Even though they each have the same number of carbon and hydrogen atoms, and the same molecular mass, the I-Butane molecule instead of being a straight chain as in N-Butane, has a bend in it. This gives it different properties.

Answer 2

The more branched the isomer, the less the attractive intermolecular forces (vdW forces) because the molecules can't pack together so well.