Gas law

[Otto_]

I have a vacuum tank of 500 L with an initial pressure of 450 mbarA. After opening a valve for 10 seconds the pressure increases to 600 mbarA. The question is how many litres of atmospheric air is entering the tank. We can assume that temperature in the tank is constant (298K).

I have started to calculate the number of moles by using the ideal gas law : n1= P1V1/RT1 = 45000*0.5 m3/RT1 = 9 mol

After 10 seconds the pressure has increased which gives me the following amount of moles: n2 = P2V2/RT1 = 60000*0.5 m3 /RT2 = 12 mol

This gives me the amount om mole entering the tank after 10 seconds which i call n3 = n2-n1 = 3 mol.

The final step is to calculate how many litres of atmospheric air (V3) is entering the tank. My question is which pressure shall I use? The formula will be like this: V3 = n3RT3/P = 3*8.314*298 / P.What pressure is the correct to use in this final step (I know we must do an assumption here) ? Is it the starting pressure with the highest vacuum or is it the final pressure?

Any comments would be highly appreciated.

 

[LittleInch]

"how many litres of atmospheric air (V3) is entering the tank" - therefore pressure is "atmospheric"

You know moles, you know P so can work out V.

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