Answer :

Mole fraction of He in ppm is 49.9975

Data known :

nHe = helium gas moles

ngas =  moles of gas

Na = Avagrado number = 6.02 × [tex]10^{23}[/tex]

Xhe = helium gas mole fraction

Applying the formula :

nHe = number of He atoms/ Na

nHe = 6.02 × [tex]10^{19}[/tex] / 6.02 × [tex]10^{23}[/tex]

nHe = [tex]10^{-4}[/tex] = 0.0001

Xhe = nHe / nHe+ngas

Xhe = 0.0001 / 2.0001

Xhe =  4.99975 × [tex]10^{-5}[/tex]

Mole fraction of He in pmm is 49.9975

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