100A QUIZ 6

1. How much heat is required to evaporate 20-g water of 20ºC at 100ºC?

   Solution.

   The water is heated to 100°C, and evaporates.

   We find the heat from

             Q = m_waterc_waterDT_water + m_waterL_water  

                = (20 x 10^-3kg)(4186J/kg.°C)(100°C - 20°C) + (20 x 10^-3kg)(22.6 x 10⁵J/kg)

                = 5 x 10⁴J.

2. An ideal gas expands isothermally, performing 4.40 x 10³J of work in the process. Calculate (a) the change in internal energy of the gas, and (b) the heat absorbed during this expansion.

   Solution.

          (a) The internal energy of an ideal gas depends only on the temperature, U = 3/2nRT, so we have

                     DU = 3/2nRDT = 0.

          (b) We use the first law of thermodynamics to find the heat absorbed.

                      DU  = Q - W;

                           0 = Q - 4.40 x 10³J, which gives Q = 4.40 x 10³J.