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A flask of volume 1l contains vapour of CH3OH at a pressure of 1atm and 25`C. The flask was then evacuated till the final pressure dropped to 10^-4 mm. Find the number of molecules of methyl alcohol left in the flask.

A flask of volume 1l contains vapour of CH3OH at a pressure of 1atm and 25`C. The flask was then evacuated till the final pressure dropped to 10^-4 mm. Find the number of molecules of methyl alcohol left in the flask.

Grade:11

1 Answers

Arun
25763 Points
10 months ago
The final pressure after evacuation is given in mm Hg and hence has to be converted to atm.
 
 760 mm Hg = 1 atm
thus 0.001 mm Hg = 1. 31 x 10-6 atm.
 
As we know that at STP conditions 1 Mole of a substance occupies 22.4 L of volume and contains 6.022 x 1023 molecules
 
Here the volume occupied at STP = 1 Litre.
 
Thus no. of moles = 1/22.4 = 0.044 Moles of methanol and no. of molecules = 0.044 x (6.022 x 1023) = 2.68 x 1022
 
Hence, at 1.31 x 10-6 atm pressure, no. of molecules will be
 
= (2.68 x 1022 ) x (1.31 x 10-6)
 
Thus no. of molecules = 3.51 x 1016

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