Mechanical Engineering : Past Year Questions: Properties Of Pure Substances Notes | EduRev
Question 1:2 moles of oxygen are mixed adiabatically with another 2 moles of oxygen in a mixing chamber, so that the final total pressure and temperature of the mixtures become same as those of the individual constituents at their initial states.The universal gas constant is given as R. The change in entropy due to mixing, per mole of oxygen, is given by
Since, DQ = 0, therefore S = ∫ dQ/T = 0
Question 2:Water has a critical specific volume of 0.003155 m3/kg. A closed and rigid steel tank of volume 0.025 m3 contains a mixture of water and steam at 0.1 MPa. The mass of the mixture is 10 kg. The tank is now slowly heated. The liquid level inside the tank
vc = 0.0031 m3/k g
v2 = 0.025/10 = 0.0025 m3 kg
as (vc > v)
As heat is supplied the amount of steam in mixture will decrease and pressure will increase, because volume of mixture is less than critical volume. The final state will move towards saturated liquid line. Hence level of liquid in tank will rise.
Question 3:Which combination of the following statements is correct?
P : A gas cools upon expansion only when its Joule-Thomson coefficient is positive in the temperature range of expansion.
Q : For a system undergoing a process, its entropy remains constant only when the process is reversible.
R : The work done by a closed system in an adiabatic is a point function.
S : A liquid expands upon freezing when the slope of its fusion curve on pressureTemperature diagram is negative.
First law of thermodymnamics,
dU = – W
Hence work acts as the point function.
2. Liquid expands upon freezing when the liquid is water. In case of water the slope of fusion curve is negative.
Question 4:When wet steam flows through a throttle valve and remains wet at exit
Throttling always takes place in direction of decreasing pressure
Hence we can say during throtting temperature also decreases quality increases.
Question 5:Constant pressure lines in the superheated region of the Mollier diagram will have
Mollier diagram is h-s diagram
Tds = dh - vdh
dp = 0
Tds = dh
dh/ds = T
∵ T > 0
dh/ds > 0
slope > 0
Question 6:Which one of the following statements is correct for a superheated vapour?
Question 7:The INCORRECT statement about the characteristics of critical point of a pure substance is that
Question 8:A rigid container of volume 0.5 m3 contains 1.0 kg of water at 120°C (vf = 0.00106 m3/kg, vg = 0.8908 m3/kg). The state of water is
V = 0.5/1m3 / kg = 0.5 m3 / kg
Since Vf < V < Vg the state of water is mixture of saturated water and saturated vapour.
Question 9:At the triple point of a pure substance, the number of degrees of freedom is
According to Gibb's phase rule
P + F = C + 2
at triple point C = 1, P = 3
3 + F = 1 + 2
F = 0
Question 10:During the phase change of a pure substance
dP = 0
During phase change
ΔP = 0, ΔT = 0