Concept explainers
(a)
The initial pressure in the vessel.
(a)

Answer to Problem 72AP
Explanation of Solution
Section1:
To determine: The constant volume occupied by the gas.
Answer: The volume of the gas is 0.524 m3.
Explanation:
Given Info:
The radius of the spherical vessel is 0.500 m.
Formula to calculate the volume of the gas is,
V=43πr3
- r is the radius of the vessel
Substitute 0.500 m for r to find the volume.
VSun=43π(0.500 m)3=0.524 m3
Thus, volume of the gas is 0.524 m3.
Conclusion:
From section 1,
The volume of the gas is 0.524 m3
The amount of hydrogen in the container is 14.4 mol.
The amount of oxygen in the container is 7.2 mol.
The initial temperature is 20 °C
To find the initial pressure of the vessel, consider the components in the gas mixture in container as ideal gases. Hence, the total pressure the mixture exerts on the container will be the sum of pressure of the components of the mixture
The pressure exerted by the hydrogen molecule is,
PH2=nH2RTV (I)
- nH2 is the amount of hydrogen molecules in moles
- R is the universal gas constant
- T is the absolute temperature
- V is the volume of the container
The pressure exerted by the oxygen molecule is,
PO2=nO2RTV (II)
- nO2 is the amount of oxygen molecule in moles
The total pressure exerted on the container is the sum of equation (I) and (II).
Therefore,
P=PH2+PO2=(nH2+nO2)RTV
Substitute 14.4 mol for nH2, 7.2 mol for nO2, 8.31 J/mol⋅K for R, 20 °C for T and 0.524 m3 for V to find the initial pressure of the container,
P=(14.4 mol+7.2 mol)(8.31 J/mol⋅K)((20+273)K)0.524 m3=1.00×103 Pa
Thus, the initial pressure in the vessel is 1.00×103 Pa.
(b)
The initial internal energy of the gas.
(b)

Answer to Problem 72AP
Explanation of Solution
Given Info:
The amount of hydrogen in the container is 14.4 mol.
The amount of oxygen in the container is 7.2 mol.
The molar specific heat of hydrogen at constant volume is 20.4 J/mol.K.
The molar specific heat of oxygen at constant volume is 21.1 J/mol⋅K.
The internal energy of the gas is the sum of the internal energy of the components of the gas mixture.
Formula to calculate the internal energy of the gas is,
U=UH2,i+UO2,i=(nO2Cv,o2+nH2Cv,H2)Ti
- nO2 is the amount of oxygen in moles
- Cv,o2 is the molar specific heat of oxygen in constant volume
- nH2 is the amount of hydrogen in moles
- Cv,H2 is the molar specific heat of hydrogen in constant volume
Substitute 14.4 mol for nH2, 20.4 J/mol.K for Cv,H2, 7.2 mol nO2 and 21.1 J/mol⋅K for Cv,o2 to find the initial internal energy of the gas,
Ui=|(14.4 mol)(20.4 J/mol⋅K)+(7.2 mol)(21.1 J/mol⋅S)|=1.31×105 J
Thus, the initial internal energy of the gas is 1.31×105 J
Conclusion:
The initial internal energy of the gas is 1.31×105 J
(c)
The energy produced if the gas burns out to water vapor completely.
(c)

Answer to Problem 72AP
Explanation of Solution
Given Info:
The amount of hydrogen in the container is 14.4 mol.
The amount of oxygen in the container is 7.2 mol.
The initial temperature is 20 °C
If the gas burns out to water vapor completely, instead of each one mole of hydrogen; one mole water will produce. In the container since the volume is constant, no other work done will be there. The water produced is equal to the amount of hydrogen.
Thus, the amount of water produced is 14.4 mol.
The chemical energy required to produce one mole of water is 241.8 kJ.
Formula to calculate the energy produced in combustion is,
ΔU=(241.8 kJ/mol)n
- n is the number of moles of water produced
Substitute 14.4 mol for n to find the energy produced,
ΔU=(241.8 kJ/mol)(14.4 mol)=3.48×106 J
Thus, the energy produced is 3.48×106 J.
Conclusion:
The energy produced if the gas burns out to water vapor completely is 3.48×106 J.
(d)
The temperature and pressure of the steam.
(d)

Answer to Problem 72AP
Explanation of Solution
Section1:
To determine: The temperature of the steam
Answer: The final temperature of the steam is 9.28×103 K
Explanation:
Given Info:
The molar heat capacity for water vapor is 27.0 J/mol⋅K.
The initial internal energy of the gas is 1.31×105 J.
The energy produced if the gas burns out to water vapor completely is 3.48×106 J
For an ideal gas,
Formula to calculate the temperature of the steam is,
Tf=UfnwCv,w (I)
- Uf is the final internal energy
- nw is the amount of water in moles
- Cv,w is the molar heat capacity of water
Since, after the combustion; the finial internal energy is,
Uf=Ui+ΔU
Thus, the equation (I) is given by,
Tf=Ui+ΔUnwCv,w
Substitute 1.31×105 J for Ui, 3.48×106 J for ΔU, 14.4 mol for nw and 27.0 J/mol⋅K for Cv,w to find the temperature of the steam,
T=(1.31×105 J)(3.48×106 J)(14.4 mol)(27.0 J/mol⋅K)=9.28×103 K
Thus, the final temperature of the steam is 9.28×103 K
Section2:
To determine: The pressure of the steam
Answer: The pressure of the steam is 2.12×106 Pa
Explanation:
Given Info:
From section1,
The final temperature of the steam is 9.28×103 K
For an ideal gas,
Formula to calculate the pressure of the steam is,
P=nwRTV
- nw is the amount of water vapor in moles
- R is the universal gas constant
- T is the final temperature
- V is the volume occupied by the gas
Substitute 14.4 mol for nw, 8.31 J/mol⋅K for R, 9.28×103 K for T and 0.524 m3 for V to find the pressure of the steam,
P=(14.4 mol)(8.31 J/mol⋅K)(9.28×103 K)0.524 m3=2.12×106 Pa
Thus, the pressure of the steam is 2.12×106 Pa
Conclusion:
The temperature and pressure of the water vapor is 9.28×103 K and 2.12×106 Pa.
(e)
The mass of steam and steam’s density.
(e)

Answer to Problem 72AP
Explanation of Solution
Section1:
To determine: The mass of steam.
Answer: The mass of the steam is 0.259 kg.
Explanation:
Given Info:
The mass of water is 18.0×10−3 kg/mol
The amount of steam is 14.4 mol
Formula to calculate the mass of the steam after the combustion is,
ms=nsMw
- ns is the amount of steam in moles
- Mw is the mass of water in moles
Substitute 14.4 mol for ns and 18.0×10−3 kg/mol for Mw to find the mass of steam,
ms=(14.4 mol)(18.0×10−3 kg/mol)=0.259 kg
Thus, mass of the steam is 0.259 kg.
Conclusion:
From section1,
Mass of the steam is 0.259 kg.
The volume of the gas is 0.524 m3.
Formula to calculate the density of the steam is,
ρs=msV
- ms is the mass of the steam
- V is the volume of the gas
Substitute 0.259 kg for ms, 0.524 m3 for V to find the density of the steam,
ρs=0.259 kg0.524 m3=0.494 kg/m3
Thus, the steam’s density is 0.494 kg/m3.
(f)
The initial exhaust velocity of the exhaust steam.
(f)

Answer to Problem 72AP
Explanation of Solution
Given Info:
The Bernoulli’s equation is given by,
P2+12ρ2v22+ρsgh2=P1+12ρsv12+ρsgh1
Assume that the steam in the container is essentially at rest.
Thus,
v1=0, P2=0 and h2=h1.
Formula to calculate the exhaust velocity is,
v2=√2P1ρs
- P1 is the initial pressure of the gas
- ρs is the density of the steam
Substitute 2.12×106 Pa for P1 and 0.494 kg/m3 for ρs to find the exhaust velocity,
v2=√2(2.12×106 Pa)0.494 kg/m3=2.93×103 m/s
Thus, the exhaust velocity is 2.93×103 m/s.
Conclusion:
The initial exhaust velocity of the exhaust steam is 2.93×103 m/s.
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