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College Physics (Urone)
Found in: Page 472

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Short Answer

Question: Air in human lungs has a temperature of 37.0ºC and a saturation vapor density of 44.0 g/m3. (a) If 2.00 L of air is exhaled and very dry air inhaled, what is the maximum loss of water vapor by the person? (b) Calculate the partial pressure of water vapor having this density, and compare it with the vapor pressure of 6.31×103 N/m2.


The maximum water vapor exhaled by the person is 0.088 g and the partial pressure of water vapor is equal to the vapor pressure.

See the step by step solution

Step by Step Solution

Step 1: Concept

Density is the ratio of mass to volume. Therefore, mass is the product of density and volume.

Step 2: Solution for part a

The loss of water vapor from the person’s body is equal to the mass of oxygen exhaled by the person. Here, volume and density are given.

density=44.0 g/m3volume=200L=2.00L1 m31000 L=0.002 m3

Mass=density×volume =44.0 g/m3×0.002m3 =0.088g

Therefore, 0.088 g of water vapor was lost from the person.

Step 3: Derive an expression for pressure from the ideal gas equation

According to the gas law,

PV = nRTPV = massmolarmassRTP = massmolarmass×v×RT= densitymolarmassRT

Here, is the pressure, is the volume, is the number of moles, is the universal gas constant and is the temperature.

Step 4: Solution for part b

The value of density and temperature are given.

Density=44.0g/m3 T=37.0C=(37.0+273.15) K=310.15K

The value of the universal gas constant is 8.314 JK-1mol-1 and the molar mass of water is 18g. The partial pressure of water vapor can be calculated by substituting these values in the expression of pressure.

P=44.0g/m318g×8.314 JK-1mol-1×310.15 =6.303×103N/m3

This value is almost equal to the vapor pressure which is equal to 6.31×103 N/m2. So, partial pressure of water vapor and vapor pressure are equal.


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