The solubility of gas is realted to the mole fraction in aqueous solution. The mole fraction of the gas in solution is calculated by applying Henry's Law
Thus :
According to Henry's law :
χHe=pHeKH=0.5 bar145000 bar=3.45×10−6
As 1 litre of water contains 55.55 mol of it, therefore if n represents number of moles of He (g) in solution
χHe=nn+55.55
Since , n<<55.55∴n55.55=3.45×10−6n=1.91×10−4 moln=1.91×10−4×1000 mmoln=0.191 mmol