Question

if water contaminated the sample after it was weighed out, what effect would this have on the molar mass

Answer 1

If you are saying that after the dry sample was weighed and put aside, this sample was accidentally contaminated by water, if you are using the sample to mix into an aqueous solution, there is no change in the molar mass of the compound, because the no. of moles of the compound wouldn't change.

However the concentration of the aqueous solution you are preparing may go for a toss because of the excess water in the water contaminated sample.

From a discipline and accuracy stand point, it is advised to discard the sample and go for another weighing.