Section 18.1 states that ordinarily, pressure, volume, and temperature cannot change individually without one affecting the others. Yet when a liquid evaporates, its volume changes, even though its pressure and temperature are constant. Is this inconsistent? Why or why not?

Section 18.1

A 20.0-L tank contains 4.86 x 10^-4 of helium at 18.0° C. The molar mass of helium is 4.00 g/mol.

(a) How many moles of helium are in the tank? 

(b) What is the pressure in the tank, in pascals and in atmospheres?