Helium boils at 4.22 K at atmospheric pressure, 101.3 kPa, with hFG 83.3 kJ/kmol. By pumping a

Question:

Helium boils at 4.22 K at atmospheric pressure, 101.3 kPa, with hFG 83.3 kJ/kmol. By pumping a vacuum over liquid helium, the pressure can be lowered and it may then boil at a lower temperature. Estimate the necessary pressure to produce a boiling temperature of 1 K and one of 0.5 K.