Equal volumes of all gases measured at the same temperature and pressure contain the same number of molecules.  (Since flask 1 & flask 2 are the same size, and the same temperature and pressure, then there must be the same number of molecules of Gas A and Gas B.)

If Gas A in flask 1 weights 5.50 grams, and Gas B in flask 2 weighs 2.0 grams, what is the weight of one molecule of Gas A compared to one molecule of Gas B?

So, one molecule of Gas A is 2.75 times as heavy as one molecule of Gas B.  

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