2.572 g
From the question;
Volume of Helium gas, V is 18.5 L
Pressure,P is 85.5 kPa
Temperature, T is 296 K
We are required to calculate the mass of Helium gas
Rearranging the formula;
[tex]n=\frac{PV}{RT}[/tex]
Thus, [tex]n=\frac{(85.5kPa)(18.5L)}{(8.314)(296K)}[/tex]
[tex]n=0.6427[/tex]
n = 0.643 moles
Therefore, moles of Helium are 0.643 moles
To calculate the mass we multiply the number of moles by the molar mass.
Mass = Number of moles × Molar mass
Molar mass of Helium = 4.00 g/mol
Therefore;
Mass of He = 0.643 moles × 4.00 g/mol
= 2.572 g
Hence, mass of helium gas is 2.572 g