The correct decreasing order of densities of the following compounds is :

Solution

Chemistry ma’am · 3 steps

The ring stays the same size, so every heavier halogen you add packs more mass into nearly the same space.

1. What sets density. All four molecules are built on the same benzene ring, so they take up roughly similar volumes. Density is mass per volume, so it rises with the mass of the atoms attached to the ring.
2. Halogens add mass. Swapping a light H (mass 1) for Cl (35.5) or Br (80) adds a lot of mass and only a little volume. So more halogen atoms, and heavier ones, mean a denser liquid.

   Compound	Groups on the ring	Molar mass (g mol⁻¹)	Density (about, g cm⁻³)
   (D) 1-bromo-3-chlorobenzene	Br + Cl	191.5	1.63
   (C) 1,3-dichlorobenzene	Cl + Cl	147	1.29
   (B) chlorobenzene	Cl	112.5	1.11
   (A) benzene	none	78	0.88

3. So. Density falls as the halogen load falls: (D) > (C) > (B) > (A), which is option (a).

The idea to keep. For haloarenes and haloalkanes on the same skeleton, density goes up with the number of halogens and with their atomic mass: I > Br > Cl > F. Benzene with no halogen is lighter than water.

Answer from the NTA final answer key. Solution by PrepWiser.