Which one of the following would have the largest dispersion forces? A) CH3CH2SH
B) CH3NH2
C) CH4
D) CH3CH3

A) CH3CH2SH

Explanation:

Dispersion forces are weak attractions found between non-polar and polar molecules. The attractions here can be attributed to the fact that a  non-polar molecule sometimes become polar because the constant motion of its electrons may lead to an uneven charge distribution at an instant. If this happens, the molecule has a temporary dipole. This dipole can induce the neighbouring molecules to be distorted and form dipoles as well. The attractions between these dipoles constitute the Dispersion Forces.

Therefore; the greater the molar mass of a compound or molecule, the higher the Dispersion Force. This implies that the compound or molecule with the highest molar mass have the largest dispersion forces.

Now; for option (A)

CH3CH2SH

The molar mass is :

= (12 + (1 × 3 ) +12 + (1 ×2) + 32+1)

= (12 + 3+ 12 + 2 + 32 + 1)

= 62 g/mol

For option (B)

CH3NH2

The molar mass is:

= (12 + (1 ×  3 ) +14 + (1 ×  2)

= (12 + 3 + 14 + 2)

= 31 g/mol

For option (C)

CH4

The molar mass is :

= 12 + (1 × 4)

= 12 + 4

= 16 g/mol

For option (D)

CH3CH3

The molar mass is :

= 12 + ( 1 × 3 ) + 12 + ( 1 × 3)

= 12 + 3 + 12 + 3

= 30 g/mol

Thus ; option (A) has the highest molar mass, as such the largest dispersion force is A) CH3CH2SH