Thermo FE Review

A closed rigid vessel contains 40% liquid water and 60% water vapor by volume. The liquid-vapor mixture is in equilibrium at 150 deg C. What is the quality of the mixture?

• 4.5 x 10^-3

• 0.40

• 0.60

• 0.996

A cylinder fitted with a frictionless piston contains an ideal gas at temperature T and pressure p. The gas expands isothermally and reversibly until the pressure is p/3. Which statement is true regarding the work done by the gas during expansion?

• It is equal to the change in enthalpy of the gas.

• It is equal to the change in internal energy of the gas.

• It is equal to the heat absorbed by the gas.

• It is greater than the heat absorbed by the gas.

An ideal heat engine is constructed by cycling 1 kg of oxygen gas (O2), initially at 1 atm and 25 deg C, through a sequence of four processes. The sequence consists of isothermal expansion to 2.35 m^3, followed by adiabatic expansion to 8.21 m^3, followed by isothermal compression to 2.67 m^3, and finally, adiabatic compression back to the initial state. All steps are reversible. For oxygen, the specific heat at constant pressure (cp) is 0.918 kJ/kg*K, and the specific heat at constant volume (cv) is 0.658 kJ/kg*K. What is the thermal efficiency of this heat engine?

• 20%

• 40%

• 53%

• 61%

Moist atmospheric air with a dry-bulb temperature of 30 deg C and a relative humidity of 40% is cooled under constant pressure until condensation of the water vapor is observed. What is the final temperature of the air?

• 10 deg C

• 16 deg C

• 20 deg C

• 30 deg C

How much energy per unit mass of dry air is released during the cooling process described in the previous problem?

• 0 kJ/kg

• 16 kJ/kg

• 63 kJ/kg

• 100 kJ/kg

A small gas turbine burns liquid octane (C8 H18) with 300% excess air. How many moles of nitrogen gas (N2) in the combustion products are produced per mole of fuel consumed?

• 47 N2

• 94 N2

• 141 N2

• 188 N2

What is the specific enthalpy of HFC-134a at 0.4 MPa and 85% quality?

• 241 kJ/kg

• 261 kJ/kg

• 333 kJ/kg

• 375 kJ/kg

An ideal gas at a gage pressure of 0.3 MPa and 25 deg C is heated in a closed container to 75 deg C. What is the final pressure?

• 0.35 MPa

• 0.47 MPa

• 0.90 MPa

• 1.20 MPa

All real gases deviate to some extent from the ideal behavior described by the equation pV = mRT. For which of the following conditions are the deviations smallest?

• high temperatures and low volumes

• high temperatures and low pressures

• high pressures and low volumes

• high pressures and low temperatures

Which thermodynamic property is the best measure of the molecular activity of a substance?

• enthalpy

• internal energy

• entropy

• external energy