You have 12.5 grams. And 5 liters of some random gas.
Step 1: You must find the mol amount of that 12.5g mass.
12.5 / 5 = 2.5 mol
Step 2: Now find the molecular mass of that gas "at STP" (STP- is 22.4 L)
2.5 x 22.4 = 56 g/mol
Good Luck on Chemistry SAT, cause I know exactly where you got this question from.
