Which of the following has the highest vapor pressure?

CCl4
CBr4
CH3CH3
CH3CHBr2

Which is the lightest? (that is, which has the lowest molar mass).

To determine which compound has the highest vapor pressure, we need to consider the intermolecular forces present in each compound. Generally, compounds with weaker intermolecular forces tend to have higher vapor pressures.

CCl4 and CBr4 are both nonpolar compounds and exhibit only weak London dispersion forces. Compared to CCl4, CBr4 has stronger London dispersion forces due to the larger size of the Br atom compared to the Cl atom. However, both compounds have relatively low vapor pressures.

CH3CH3 is ethane, a nonpolar compound that only exhibits London dispersion forces. While ethane has weaker London forces compared to the halogens, it still has a higher vapor pressure than CCl4 and CBr4 due to its lower molecular weight.

CH3CHBr2, on the other hand, is 1,2-dibromoethane and has the highest vapor pressure among the given compounds. This is because it is a polar compound, and in addition to London dispersion forces, it also exhibits dipole-dipole interactions. The presence of these stronger intermolecular forces in CH3CHBr2 leads to a higher vapor pressure compared to the other compounds.

Therefore, CH3CHBr2 has the highest vapor pressure among the given compounds.