Dumas Method for the molecular mass of a volatile liquid?

Question

A student uses the method of this experiment to determine the gram molecular mass of an unknown volatile liquid. Tell whether each of the following procedural errors would cause the expreimentally determined gram molecular mass to be too large, too small, or unchanged relative to the theoretical value. Explain reasoning in each case.

A.) The student uses tap water in the water bath, which leaves a mineral deposit on the outside of the sample flask.

B.) The student seals that flask as soon as he removes it from the water bath, so that air is not allowed to re-enter the flask before weighing.

Answer 1

A.) The student uses tap water in the water bath, which leaves a mineral deposit on the outside of the sample flask.

assuming that the student left the mineral deposit on the flask, the final weight is too large,

subtract the empty weight & your weight of vapor is too large.

use it to get molar mass & ....

Your answer : the molar mass is too large

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B.) The student seals that flask as soon as he removes it from the water bath, so that air is not allowed to re-enter the flask before weighing.

I am assumiong that you are useing the Dumas method where we are supposed to let the air back in after the vapor re-condenses:  

the final weight is too small,

subtract empty with air & the mass calculated for vapor is too small,

use it to calculate molar mass, & the molar mass will be too small