# Find volume of pesticide after pressure decreases

## Homework Statement

Here is a pesticide spray vessel, of total volume of 10 litres, containing 8 litres of pesticide.
Valve B is closed first,and the space above the air is filled with air at a pressure of 200KPa until the desired pressure is reached, and then valve A is then closed.

The sprayer is then used for sometime until the liquid level falls and the pressure of the air in the space above the liquid is reduced
Calculate the volume of the liquid that would still be in the tank when air pressure above the tank is 150KPa and all temperatures reamin constant.

Assume all pressure differences due to hydrostatic pressure of liquid to be negligible; i.e pressure at upper surface of liquid = pressure at bottom of the tank.

(Atmospheric pressure =100Kpa)

P1V1=P2V2

## The Attempt at a Solution

Hi I originally thought the solution would be to use Boyle's law, but I realized its for the volume of pesticide not the air.

And normally I would think, if the initial pressure of the air pumped was 200Kpa, would I have to add atmospheric pressure to that, but that cant be since it's in a container?
I'm very stumped as this is one of my worst areas in physics.

*high school level*

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Chestermiller
Mentor
I think you are supposed to assume that those are absolute pressures.

Here is my attempt, I use boyle's law to find out how much litres of air is left after the spraying is done.
250000Pa*0.002m3=150000Pa*V2

V2= 0.0027m3 =2.7 litres

Therefore that is the volume of the air after.

Since the air previously was 2 litres, and the pesticide was 8, to find out how much pesticide remains I would just do 8L-0.7L = 7.3L

SO my answer is 7.3L, I'm not sure if it is correct.

Chestermiller
Mentor
Looks good.

• Richie Smash