In 1911 G. Kamerlingh Onnes, when measuring the resistance of mercury cooled by liquid helium, discovered that when helium vapor is pumped out of the cryostat, the resistance of mercury disappears. This is how the phenomenon of superconductivity was discovered. Calculate to what pressure it was necessary to pump out helium vapor, if at a pressure pk = 1 atm the boiling point of helium is Tk = 4.22 K, the heat of evaporation at this pressure is q = 84 J/mol, and the critical temperature of the superconducting transition of mercury is Tc = 4, 15 K.

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