Question

Why do densities of haloalkanes decrease when the size of alkyl group increases?

Answer 1

Answer:

Here's what I get.

Explanation:

Your argument makes sense. For example, the density of C₅H₁₂ is about 0.63 g/mL, and it increases gradually to a limit of about 0.83 g/mL.  

With the lower alkyl halides, the halogen atom is the major contributor to the mass of the molecule. The density of chloromethane is 2.2 g/mL.

As you increase the number of C atoms, the Cl atom becomes a smaller part of the molecule. The alkyl halide becomes more like an alkane.

By the time you have reached four C atoms, the density has reached a limiting value of about 0.89 g/mL.