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
