Practice Problems – Exam 3
1.
Heat Loss During Breathing
Estimate heat loss during breathing. Assume that a normal
person inspires about 6 L/min of bone dry air at 20
º
C. Assume
that expired air is saturated with warm vapor and is at 37
º
C. The
molecular weight of air is 28.9 g/mol and the density of air is
0.0012 g/cm
3
. The molal humidity for saturated water vapor at
37
º
C is 6.7%; therefore, when saturated, there are approximately
0.041 g of water per 1 g of dry air. The enthalpy change involved
with heating air from 20
º
C to 37
º
C is 494 J/mol and the enthalpy
change associated with the vaporization of water to saturated air
is 577 cal/g.
Heat lost due to respiration _________________________
Atomic weights: H = 1, C = 12, N = 14, O = 16
1 atm = 101.3 kPa
1 cal = 4.814 J
1 N/m
2
= Pa
1 J = 1 N/m
1000 cm
3
= 1 L = 0.001 m
3
Gravitational acceleration = 9.81 m/s
2
= 32.174 ft/s
2
Gas constant = 8.314 m
3
·Pa/(mol·K) = 8.314 J/(mol·K) = 0.08206 L·atm/(mol·K)
Heat of fusion of pure ice: 6009.5 J/mol
Heat of vaporization of pure water: 40656.0 J/mol
6 L/min
saturated air,
37ºC
6 L/min dry air,
20ºC
3
1
2
Water from
nasal tissue
Lungs
Heat lost due to
respiration
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Hydroelectric Power
Suppose that water from a reservoir flowing into a
hydroelectric power plant has a velocity of
1.0 m/s.
After flowing through the plant, the water enters a
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 Fall '08
 Staff
 Thermodynamics, Mole, 1.0 m/s, 4.18 kJ, 8.314 J, 4.814 J

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