[plu]

I was under the impression that increasing molecular mass increases boling/melting point because molecules with greter molecular masses have greater dispersion forces.  For example, my inorganic text (Rayner-Canham and Overton, Descriptive Organic Chemistry) explains the boiling point trend of the Group 14 halides by talking about dispersion forces.

This is true.  In addition to increasing dispersion forces though, increasing molecular mass also means increasing the energy needed by molecules in a substance to achieve the gas state.  This is given by Graham's law of effusion / the root-mean-square molecular speed equation which both state that the average molecular speed of the molecules in a substance is directly proportional to the squareroot of the substance's molar mass  

[Borek]

I was under the impression that increasing molecular mass increases boling/melting point because molecules with greter molecular masses have greater dispersion forces.  For example, my inorganic text (Rayner-Canham and Overton, Descriptive Organic Chemistry) explains the boiling point trend of the Group 14 halides by talking about dispersion forces.

I am under impression that your impression is correct I must think it over.

[Yggdrasil]

I'm not sure I buy that argument.  Yes, it is true that to be a gas, the molecule must have a high enough velocity to be able to escape the attractive forces of neighboring gas molecules.  But, since we're talking about forces and energy, it's not velocity that we need to consider, but kinetic energy.  So smaller molecules will need to travel at a higher velocity in order to escape neighboring molecules attractive forces, while larger molecules can travel at slower speeds because their larger mass gives them more kinetic energy.