Question

one litre flask containing vapours of methyl alcohol molecular mass 32 at a pressure of 1 atmosphere and 25 degree Celsius was evacuated till the final pressure was 10 raise to power minus 3 ml how many molecules of methylalcohol were left in the flask ​

Answer 1

• pv=nRT
• 0.1.2=n0.8231.298
• n=16n=w/m
• n=2

Answer 2

Solution:

The final pressure which came after evacuation is given in the sum as mm Hg and hence needs to be converted into the atm.

 760 mm Hg = 1 atm

so, 0.001 mm Hg

 As you know that at STP, 1 mole of a gas occupies 22.4 L of vol and has

$6.022 \times 10^{23}$ molecules in it.

Hence the volume that it occupies at STP = 1 Litre.

Thus number of moles $=\frac{1}{22.4}=0.044$ Moles of methyl alcohol and,

no. of molecules $=0.044 \times\left(6.022 \times 10^{23}\right)=2.68 \times 10^{22}$

so, at$1.31 \times 10^{-6}$atm pressure, number. of molecules will be is

$=\left(2.68 \times 10^{22}\right) \times\left(1.31 \times 10^{-6}\right)$

 Thus, no. of molecules remaining in the jar $=3.51 \times 10^{16}$