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Grade 11Physical Chemistry

One litre flask cointaing vapour of methyl alchol (MOL mass 32) at a pressure of 1 atm and 25 c was evacuated till the final pressure was 10-3 mm.How many molecules of methyl alchol were left in the flask

Profile image of Mahir Shah
7 Years agoGrade 11
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1 Answer

Profile image of Arun
7 Years 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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