posted by james .
Calculate the average speed of a water molecule (with a mass of 3.0e-26 kg) on the hot side of Mercury (710 Kelvin).
This is definitely not my area but try some of the following sites to see if anything will shed light on your problem:
model the water molecule as having kinetic energy, or the speed= sqrt (3/2 RT/m)
this is the rms speed, not average.