First, we need to state the chemical equation for the combustion of PH3

And the mass of PH3 is 17.0 grams and we need to know the moles.
In the periodic table, the atomic mass of the P (phosphorus) is 31 and the atomic mass of the H (hydrogen) is 1.
So, you sum the mass of P to the mass of H multiplied by 3 and you obtain this:

With this data, we can search the moles of PH3:
A helium balloon displaces the amount of air. The weight of the helium in the balloon + the balloon fabric = lighter than the air that it displaces. Helium is lighter than air, so therefore, if it's in a balloon, the balloon will float.
According to
Graham's Law of Diffusion," Diffusion of Gas is inversely proportional to square root of its Molecular Mass or Density".
rᵇ/rᵃ =

Or,
rᵇ/rᵃ =

----- (1)
As,
Ma = 275 g/mol
Mb = 205 g/mol
Putting Values in eq.1,
rᵇ/rᵃ =

rᵇ/rᵃ = 1.15
Result: Perfume B will diffuse 1.15 times faster than
Perfume A. Hence, Perfume B will be first smelled by the person.