To answer the question, we assume that the given compound is an ideal gas that at STP, one mole of the substance will occupy 22.4 L. From the given volume, we determine the number of moles of substance.
7.8 L / (22.4 L /mole) = 0.3482 moles of cfa
Then, we multiply this number of moles by the molar weight of cfa which is equal to 88 g/mol.
Multiplying,
weight = (0.3482 moles of cfa) x (88 g/mol) = 30.64 grams