Aballoon is filled with 3.8 l of helium gas at stp. approximately how many moles of helium are contained in the balloon? 72 mol 0.17 mol 85 mol 0.26 mol

We can use the idea gas law equation to find the number of moles of He in the balloon at STP
PV = nRT
where
P - standard pressure - 10⁵ Pa
V - volume - 3.8 x 10⁻³ m³
n - number of moles 
R - universal gas constant - 8.314 Jmol⁻¹K⁻¹
T - standard temperature - 273 K
substituting these values in the equation 
10⁵ Pa x 3.8 x 10⁻³ m³ = n x 8.314 Jmol⁻¹K⁻¹ x 273 K
n = 0.17 mol 
number of He moles are 0.17 mol