250+ TOP MCQs on Pure Substance and Answers

Thermodynamics Multiple Choice Questions on “Pure Substance”.

1. Which of the following represents the specific volume during phase transition.
a) Vf-Vg
b) Vg-Vf
c) Vf+Vg
d) none of the mentioned

Answer: b
Clarification: Here Vg is the specific volume of the saturated vapour and Vf is the specific volume of the saturated liquid.

2. At critical point, value of Vg-Vf is
a) two
b) one
c) zero
d) infinity

Answer: c
Clarification: As pressure increases, there is a decrease in Vg-Vf and at critical point its value becomes zero.

3. Above the critical point, the isotherms are continuous curves.
a) true
b) false

Answer: a
Clarification: These continuous curves approach equilateral hyperbolas at large volumes and low pressures.

4. A rigid tank contains 50 kg of saturated liquid water at 90°C. Determine the pressure in the tank and the volume of the tank.
a) 0.0518 m³
b) 0.0618 m³
c) 0.0718 m³
d) 0.0818 m³
Answer: a

5. A piston –cylinder device contains 0.06m³ of saturated water vapour at 350 kPa pressure. Determine the temperature and mass of the vapour inside the cylinder.
a) 0.104 kg
b) 0.124 kg
c) 0.134 kg
d) 0.114 kg
Answer: d

6. A rigid tank contains 10 kg of water at 90°C. If 8 kg of the water is in the liquid form and the rest is in the vapour form, determine the pressure in the tank.
a) 60.183 kPa
b) 70.183 kPa
c) 80.183 kPa
d) 90.183 kPa
Answer: b

7. A rigid tank contains 10 kg of water at 90°C. If 8 kg of the water is in the liquid form and the rest is in the vapour form, determine the volume of the tank.
a) 1.73 m³
b) 2.73 m³
c) 3.73 m³
d) 4.73 m³
Answer: d

8. An 80 litre vessel contains 4 kg of R-134a at a pressure of 160 kPa. Determine the temperature.
a) -10.60°C
b) -13.60°C
c) -15.60°C
d) -19.60°C
Answer: c

9. An 80 litre vessel contains 4 kg of R-134a at a pressure of 160 kPa. Determine the quality.
a) 0.127
b) 0.137
c) 0.147
d) 0.157

Answer: d
Clarification: v = V/m = 0.080 m³/4 kg = 0.02 m³/kg
@ 160kPa, vf = 0.0007437 m³/kg; vg = 0.12348 m³/kg.
vf < v < vg
x = (v –vf)/ vfg = 0.157.

10. An 80 litre vessel contains 4 kg of R-134a at a pressure of 160 kPa. Determine the volume occupied by the vapour phase.
a) 0.0775 m³
b) 0.0575 m³
c) 0.0975 m³
d) 0.0375 m³

Answer: a
Clarification: v = V/m = 0.080 m³/4 kg = 0.02 m³/kg
@ 160kPa, vf = 0.0007437 m³/kg; vg = 0.12348 m³/kg
vf < v < vg
x = (v –vf)/ vfg = 0.157
mg = x*m(total) = 0.628kg
Vg = mg*vg = 0.0775 m³ or 77.5 litre.

11. Determine the specific volume of R-134a at 1 MPa and 50°C, using ideal gas equation of state.
a) 0.022325 m³/kg
b) 0.024325 m³/kg
c) 0.025325 m³/kg
d) 0.026325 m³/kg

Answer: b
Clarification: v = RT/P = (0.0815 kJ/kg.K)* (323 K)/(1000 kPa)
= 0.026325 m³/kg.