Solving a Thermodynamics Problem: Heating Water in a Piston-Cylinder

[apamnani1]

Hi guys,

Little stuck on the following thermodynamics question;

A piston-cylinder contains 1kg of water at 100kPa in a 2-phase mixture (liquid and vapour). Heat is transferred to the water by a resisance heater The piston is free to move and the cylinder is insultated. The water is heated until it is in a superheater state. The motion of the piston holds he pressure constant throughout the process.

A) write the first law of thermo applied (I have done this)
b) What is the initial temp (I have worked this out to be 99.6 degrees C using steam tables)
c) If the initial volume of the cylinder is 0.1m^3, how much heat must be transferred to the system to reach saturated vappour state? (STUCK HERE I tried to use Q = mcdeltat) :(

 

[jbsteele]

Any help would be greatly appreciated! Since the pressure is kept constant, you can use the enthalpy of vaporization to calculate the amount of heat required. The enthalpy of vaporization is the amount of energy required to turn a unit mass of liquid into a gas at constant pressure. So, you need to find the enthalpy of vaporization of water at 100 kPa and then multiply it by the mass of the water in the cylinder (1 kg). This will give you the total energy required to turn the liquid water into vapor.