Which substance in each of the following pairs would you expect to have larger dispersion forces?

1.
It is expected that octane (C₈H₁₈) has greater dispersion forces than ethane (C₂H₆).  This is because octane has a greater molecular size than ethane, leading to greater dispersion forces between octane molecules.

2.
It is expected that HI has greater dispersion forces than ethane HCl.  This is because HI has a greater molecular size than HCl, leading to greater dispersion forces between HI molecules.

3.
It is expected that H₂Se has greater dispersion forces than ethane H₂O.  This is because H₂Se has a greater molecular size than H₂O, leading to greater dispersion forces between H₂Se molecules.  (However, H₂O has greater intermolecular forces than H₂Se, because H₂O molecules are held together by strong hydrogen bonds.)

Dispersion forces are most directly related to molar mass. So, in each pair, select the one with the greater molar mass.