
Concept explainers
(a)
To Calculate: The number of moles of Helium gas contained in the balloon.
(a)

Explanation of Solution
Given:
Load, F=110 N
weight of the balloon’s envelope, w=50.0 N
Volume of the balloon when it is fully inflated, V=32.0 m3
Temperature of air, T=0°C = 273 K
Atmospheric pressure, P=1 atm =1.01×105Pa
Net upward force on balloon, Fnet=30.0 N
Formula Used:
Ideal gas law:
PV=nRT
Here, P is the pressure, V is the volume, R is the gas constant, n is the number of moles and T is the temperature.
Buoyant force can be obtained by:
Fb=ρVg
here, ρ is the density of the fluid, V is the volume of the displaced fluid and g is the acceleration due to gravity.
Calculations:
Fnet=Fb−F−w−wHeFb=ρaVgwHe=ρHeVg⇒Fnet=ρaVg−F−w−ρHeVg⇒V=Fnet+F+w(ρa−ρHe)gn=PVRT⇒n=PRTFnet+F+w(ρa−ρHe)g
Density of air, ρa=1.293 kg/m3
Density of helium, ρHe=0.179 kg/m3
Substitute the values and solve:
n=PRTFnet+F+w(ρa−ρHe)gn=(1atm)(30.0N+110 N+50.0 N)(8.21×10−2L⋅atm/mol⋅K)(273K)(1.293 kg/m3−0.179 kg/m3)(9.81m/s2)(1L/10−3m-3)n≈776 mol
Conclusion:
Thus, the number of moles of Helium gas contained in the balloon is 776 mol .
(b)
To Find:The altitudeat which the balloon would be fully inflated.
(b)

Explanation of Solution
Given:
Load, F=110 N
weight of the balloon’s envelope, w=50.0 N
Volume of the balloon when it is fully inflated, V=32.0 m3
Temperature of air, T=0°C = 273 K
Atmospheric pressure, P=1 atm =1.01×105Pa
Net upward force on balloon, Fnet=30.0 N
Formula Used:
Variation of pressure with altitude ( h ):
Ph=Poe−ChC=0.13 km-1
From ideal gas law:
PV=nRT
Here, P is the pressure, V is the volume, R is the gas constant, n is the number of moles and T is the temperature.
Calculations:
Ph=Poe−Ch⇒h=1Cln(PoPh)Ph=nRTVh=1Cln(PonRTV)⇒h=1Cln(PoVnRT)
Substitute the values and solve:
h=1Cln(PoVnRT)=10.13 km-1ln(1.00 atm×32 ×1 L10−3m3(776 mol)(8.206×10−2L⋅atm/mol⋅K)(273 K))=4.7 km
Conclusion:
Thus, the altitude at which the balloon would be fully inflated is 4.7 km.
(c)
Whether the balloon would ever reach the altitude at which it is fully inflated.
(c)

Explanation of Solution
Given:
Load, F=110 N
Weight of the balloon’s envelope, w=50.0 N
Volume of the balloon when it is fully inflated, V=32.0 m3
Temperature of air, T=0°C = 273 K
Atmospheric pressure, P=1 atm =1.01×105Pa
Net upward force on balloon, Fnet=30.0 N
Formula Used:
In order to reach the altitude at which the balloon is fully inflated, the buoyant force (Fb) must be at least equal to the total weight of the balloon wt or should be greater than it.
Fnet=Fb−wt≥0
wt=F+w+wHewt=F+w+ρHeVgFb=ρa,hVgPhPo=ρa,hρa⇒ρa,h=PhPoρaFb=ρa,hVg⇒Fb=PhPoρaVgPh=Poe−ChFnet=e−ChVg(ρa−ρHe)−F−w
Calculations:
Substitute the values and solve:
Fnet=e−ChVg(ρa−ρHe)−F−w=e−(0.13km-1)(4.7km)(32.0 m3)(9.81 m/s2)(1.293kg/m3−0.179 kg/m3)−110 N−50 N=30 N
Because Fnet>0 , the balloon would rise higher than the altitude where it is fully inflated.
Conclusion:
Yes, the balloon would ever reach the altitude at which it is fully inflated.
(d)
The maximum altitude attained by the balloon.
(d)

Explanation of Solution
Given:
Load, F=110 N
Weight of the balloon’s envelope, w=50.0 N
Volume of the balloon when it is fully inflated, V=32.0 m3
Temperature of air, T=0°C = 273 K
Atmospheric pressure, P=1 atm =1.01×105Pa
Net upward force on balloon, Fnet=30.0 N
Formula Used:
The balloon will rise till the net force on it is zero.
Fnet=Fb−wt=0
h=1Clnρaρa,hρa,h=FbVgh=1Cln[ρaFbVg]h=1Cln[VgρaFb]
Calculations:
Substitute the values and solve:
h=10.13 km-1ln[(32.0 m3)(9.81m/s2)(1.293 kg/m3)190.5 N]h=5.8 km
Conclusion:
The maximum altitude attained by the balloon is 5.8 km .
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