First, we need to get moles of He:
moles of He = mass/molar mass of He
when the molar mass of He = 4 g/mol
          Â
and when the mass = 2 gÂ
Â
by substitution:
 moles of He = 2 g / 4 g/molÂ
            = 0.5 moles
and when V = nRT/P and n is the number of moles
so, V1 = 0.5RT/P
then, we need moles of H2 = mass / molar massÂ
                        = 2.74g / 2g/mol
                        = 1.37 moles
∴ moles of He + moles of H2 = 0.5 moles + 1.37 molesÂ
                         = 1.87 moles
so, V2= 1.87RT/P
from the V1 and V2 formula:
∴V2/V1 = 1.87 / 0.5
       = 3.74Â
∴ the party balloon is 3.74 times bigger