The problem says how many grams of nitrogen would fill a 240 L weather balloon to a pressure of 1.25 ATM at 29 degrees C

You want to use the Ideal gas Law for this problem since you have all the necessary information.

PV=nRT

Solve for n

PV/RT=n

plug in your information, remembering to use 0.08206 L·atm·mol−1·K−1 for R and to convert 29C to Kelvin.

Now you get how many moles of N you have.

12.09 moles N * 14.01g/mole N = 170 g. N

The key here is to see what information you have and to use the correct units. By always paying attention to the units, you can reduce most of your errors.