You are given volume of the gas, not number of moles, so you can't plug that number into [itex]V=\frac{nRT}{P}[/itex] (besides, this is formula for finding volume, so the answer can't be in grams).

You are right in starting with the ideal gas law, you just need to solve it for n and plug the given volume in. Once you know number of moles of hydrogen, just follow the stoichiometry.