The normal boiling point of liquid butanol is 391 K. Assuming that its molar heat of vaporization is constant at 45.9 kJ/mol, the boiling point of C4H9OH when the external pressure is 0.647 atm is

Answer 1

This is another Clausius-Clapeyron problem You need to remember the definition of boiling point. At the normal boiling point, the vapor pressure equals atmospheric pressure; i.e., at 391 K the vapor pressure is 1 atm or 760 torr. Plug in the numbers and turn the crank.