What is the mass in grams of 3.56*10^25 molecules of benzene

So you need to divide this number by avogadro's number, which tells us how many many moles the substance contains. Then multiply the number of moles you have by the molar mass of benzene, to find the amount of grams.

3.56*10^25 / 6.02x 10^23 = 59.13 moles of the substance. 59.13 moles x 78.11g/mole = 4620 4620 grams of Benzene