Question

5. One litre flask containing vapours of methyl alcohol (Mol mass 32) at a pressure of 1 atm. and 25°C was
evacuated till the final pressure was 10^-3mm. How many molecules of methyl alcohol were left in the
flask?​

Answer 1

Hi,

Answer:

3.52 * 10¹⁶ molecules of methyl alcohol were left in the flask.

Explanation:

Given data:

Volume of methyl alcohol in the flask = 1 liter

Initial Pressure, P1 = 1 atm

Final Pressure, P2 = 10^-3 mmHg = 1.315 * 10^-6 atm [∵ 1 atm = 760 mmHg]

To find: No. of molecules of methyl alcohol left in the flask

We know that according to STP conditions of a substance, at standard pressure of 1 atm 1 mole of gas occupies 22.4 L of volume which contains 6.022 * 10^23 no. of molecules.

Therefore, at P1 = 1 atm

No. of moles of methyl alcohol = 1 / 22.4 = 0.0446

And,  

No.of molecules of methyl alcohol = 0.0446 * 6.022 * 10²³ = 2.68 * 10²²

Now, at P2 = 1.315 * 10⁻⁶ atm

No. of molecules of methyl alcohol left in th flask = 2.68 * 10²² * 1.315 * 10⁻⁶ = 3.52 * 10¹⁶

Hope it helps!!!!