Please solve the following question by explaining also
A large flask fitted with a stop-cock is evacuated and weighed:it's mass is found to be 134.567 gram.It is then filled to a pressure of 735 mm at 31 degree celsius with a gas of unknown molar mass and then reweighed ;it's mass was found to be 137.456g.The flask is then filled with water and weighed again ;it's mass was found to be 1067.9 gram.assuming that the gas is ideal,calculate the molar mass of gas

From the weight of the water, you can determine the volume of the flask. When you know the weight of the gas you now have all the data to determine the molecular weight.

PV = nRT