Its for AP Chemistry homework. Any help would be much appreciated?

Use the gas equation to work out the moles of any ideal gas that you would need to fill this volume.

PV = nRT

P = pressure = 250.0 mmHg
V = volume = 100000 L
n = moles = ?
T = temp in Kelvin (always) = deg C + 273.15 = 238.15 K
R = gas constant which is different for different units of Pressure and volume and can be found here
http://en.wikipedia.org/wiki/Gas_constant
R = 62.36367 L mmHg mol^-1 K^-1

n = PV / RT
n = 250.0 mmHg x 100000 L  / 62.36367 x 238.15 K
n = 1683 moles

So you need 1683 moles of He (or any ideal gas) to fill the balloon

mass = molar mass x moles
mass He = 4.003 g/mol x 1683 mol
= 6738 g
= 6.738 kg (4 sig figs)