Applying the Ideal Gas Law, we first want to check if all units match
R 0.082057 L atm mol^-1 K^-1

Celcius

We'll have to convert temperature 25.00 °C + 298.15 = 295.15 K pressure from mm Hg to atm 745 mm Hg 1 atm / 760 mm Hg = 0.98 atm and adjusting volume measurement 500 mL 1 L / 1000 mL = 0.5 L

witchery & chemistry

Now, we're ready for pV = nRT (0.98 atm)(0.5 L) = (x) (0.082057 L atm) /             moles K (298.15 K) 0.49 atm L = (x)(24.46529 L atm) / mole 0.49 atm L moles / 24.46529 Latm) = x

Kelvin

x = 0.020028 moles

There are 0.020 moles of He