What are the major sources of error in your determination of the molar mass?
1.) Some of the sample may have been lost in between testing that could resulted in slight
error when calculating your molar mass.
2.) Weighing sample incorrectly and having too low/high std. deviation
Without more details this is a shot in the dark but I assume that you have the moles and weight of the compound. You then calculated the molar mass from these numbers. The sources of error with doing this experimentially is that you may have lost some of the sample in transferring, you may have weighed the sample incorrectly, and you could have made mistakes in calculations (all human errors).