This is the mass of the filled syringe.

The empty syringe has a mass of 42.8570 grams so the mass of the gas is

42.9212 - 42.8570 = 0.0640 grams.

The next step is to make a graph of the mass of the gas vs. molecular weight. This will be your calibration curve. You should measure at least three known gases.

After making your calibration curve, you will measure the mass of your unknown gas and use your curve to determine the molecular weight of the unknown.