You have enough information to find the volume of the air, and you know its pressure, but you also need to know its temperature. (Perhaps you are to assume room temperature?)
n = PV/(RT)
n = number of moles
R = universal gas constant (look it up in your physics book, in appropriate units)
You know P and V, but you need T, in the kelvin scale (or assume room temperature if you have nothing else to go by). Plug in the numbers, get the number of moles (n), and then multiply the number of moles by the molecular weight of air in grams per mole (which you can look up in your physics book). The result will be the number of grams of air, from which you can easily find the weight of the air.