Heya....
Here's your answer.....
PV = nRT... R is a constant...
so PV/nT = constant...
P1V1 / n1T1 = P2V2 / n2T2...P is constant.T is constant.. so
V1 / n1 = V2 / n2
since n = mass / mw
V1 / (mass 1 / mw) = V2 / (mass 2 / mw)
and since molecular mass = constant
V1 / mass 1 = V2 / mass 2
mass 2 = V2 x mass 1 / V1 = 2.90L x 2.00 g / 2.00 L = 2.9 g
mass 2 - mass 1 = 2.90 g - 2.00 g = 0.90 g
ie.. 0.90 g of He was added.
Thanks...!!!