finding out an known gas by determining its molecular weight through an experiment


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> A large evacuated flask initially has a mass of 134.567g . When the flask is filled with the gas of Unknown molar mass to 735 torr at 304K , it’s mass is 137.456g . When the flask is evacuated again and then filled with water at 304K , it’s mass is 1067.9g . Density of water at this temperature is given to be 0.997g/mL . 
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I am trying to find out what is this gas through finding the molecular mass of this gas derived from the ideal gas law-


Molecular weight = $\frac{Density \times R \times T}{P} $ 


To determine density of the gas, use mass of the gas divide by volume of the gas.


Mass of the gas = $137.456 - 134.567 $


Finding volume of the gas is where I started to struggle. I know it’s just $1067.9 - 137.456$ because total mass when gas added into the flask minus weight of the flask, but I don’t understand the concept behind it. 


Is there any diagram to understand the question statement of “ When the flask is evacuated again and then filled with water at 304K , it’s mass is 1067.9g .” 




When the flask is evacuated and filled up with water again, its mass increases by $(1067.9 - 134.576)$g. or $933.324$ g. not $(1067.9 - 137.456)$g. because the gas was not there when it was being filled. It had the weight of an empty flask.   

So the volume of the flask is $933.324/0.997$ mL. or $936.132$mL. which will be the volume of the gas when the flask is only filled up by the gas .


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