Question

A helium-filled balloon at 315.0 K and 1 atm, contains 0.30 g He, and has a volume of 1.29 L. It is placed in a freezer (T = 250.0 K), and its volume decreases to 0.93 L. Find ΔE for the gas (in joules). (Cp of He = 20.8 J/mol K.)

Answer 1

Solution :-

moles of He = mass / molar mass

                   = 0.30 g He / 4.0026 g per mol

                  = 0.075 mol He

Lets first calculat the changein the energy of the gas

q = n*Cp*delta T

= 0.075 mol * 20.8 K per mol K * ( 250.0 K - 315.0 K)

= -101.4 J

Now lets calculate the work

W= - P*DeltaV

= - 1 atm * (0.93 L -1.29 L)

= 0.36 L atm

0.36 L atm * 101.3 J / 1 L atm = 36.5 J

Delta E= q+w

delta E = -104.4 J + 36.5 J

            = -67.9 J