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Mechanical Properties Of Fluids

Question
CBSEENPH11019256

An oxygen cylinder of volume 30 liters has an initial gauge pressure of 15 atm and a temperature of 27°C. After some oxygen is withdrawn from the cylinder, the gauge pressure drops to 11 atm and its temperature drops to 17°C. Estimate the mass of oxygen taken out of the cylinder. (R= 8.3 J/mol/ K, molecular mass of O2=32].

Solution
Ideal gas equation is given by, PV=nRT
∴                         n=PVRT     ... (1)

Volume of oxygen gas cylinder, V = 30 l = 30×10-3 m3 
Pressure, P = 15 atm + 1 atm = 16 atm = 16×1.01×105 Pa
Temperature, T = 27o C = 300 K

Putting this in equation (1), we have
n = 16×1.01×105×30×10-3300×8.3 = 19. 5 moles

After some amount of oxygen is withdrawn from the cylinder, 

New pressure, P' = 11 atm + 1 atm = 12 atm
Temperature, T' = 17o C = 290 K 

So, number of moles = 12×1.01×105×30×10-3290×8.3 = 15.1 moles

So, number of moles withdrawn from the cylinder is given by,
n = 19.5-15.1 = 4.4 moles 

So, mass of O2 taken out is given by, 
m = 4.4 × 32 gm = 140.8 gm