Question

Determination of the Heat of Vaproization of Water

1) If air in a sealed flask at 20 degrees C has a pressure of 101 kPa, what is the value of k using the law of Gay-Lussac?

2) Use the value of k from question 1 to caclulate the pressure of the air in the flask at 50 degrees C.

3) Why do we use 8.314 (J/mole x K) as the value for R in the determination of the heat vaporization of water?

Answer 1

1) T = 20 + 273 = 293 K

   P = 101 kPa

      = 0.997 atm

P / T = constant = k

0.997 / 293 = k

k = 3.4 x 10^-3 atm / K

2) T = 273 + 50 = 323 K

     P = ?

P / T = k

P = k T

    = 3.4 x 10^-3 x 323

    = 1.098

P = 1.098 atm

3) in the vapour pressur calcualtion Hvap is value is in jouls or kilo jouls.

so we need take corresponding R value selctively.

R values = 0.0821 L-atm / K mol

                = 8.314 x 10^-7 erg / K mol

               = 8.314 J / K mol

                = 8.314 x 10^-3 kJ / K mol

                = 1.97 cal / K mol

among all R value bolded values are suitable

in the vapour pressure calculation ( Hvap / R ) comes . so Hvap and R units must be same