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CBSE Class 11-science Answered

If we have a sample of water in a closed container, some of the water will evaporate until pressure of the water vapour at 22°C is 19.8 mm Hg. How many molecules of water per cubic centimeters exist in the vapour phase?
Asked by amangeneralstore27 | 21 Dec, 2019, 11:03: AM
answered-by-expert Expert Answer
Let us use ideal gas equation,   p V = n R T .............(1)
 
where p is pressure, p = ρgh ,
 
ρ is density of mercry, g is acceleration due to gravity and h is height of mercury column
 
p = 13590 × 9.8 × 19.8 × 10-3 = 2637 Pa
 
V is volume , V = 1 cm3 = 10-6 m3

n is number of moles,   R is universal gas constant, R = 8.314 J / mole /K

T is temperature in Kelvin
 
hence number of moles in 1 cm3 water vapour is given by, 
 
n  = ( p V ) / ( R T )  = 2637 × 10-6 / ( 8.314 × 295) = 1.075 × 10-6
 
Number of molecules in 1 cm3 water vapour is obtained by multiplying number of moles by Avagadro number
 
Number of molecules in 1 cm3 water vapour = 1.075 × 10-6 × 6.022 × 1023 = 6.475 × 1017
Answered by Thiyagarajan K | 23 Dec, 2019, 08:13: PM
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