What is the pressure inside the cylinder?

[dragon162]

Homework Statement

A 22 cm diameter vertical cylinder is sealed at the top by a frictionless 25 kg piston. The piston is 75 cm above the bottom when the gas temperature is 316 C. The air above the piston is at 1.00 atm pressure.

1)What is the gas pressure inside the cylinder?
2)What will the pressure be if the temperature is lowered to 12 C?
3)What will the height of the piston be if the temperature is lowered to 12 C?

Homework Equations

pV=nRT

The Attempt at a Solution

Ill be honest I am completely lost in this problem and really need some help.

 

[The legend]

is the system in equilibrium?...if so what can you say about the pressure?

 

[dragon162]

is the system in equilibrium?...if so what can you say about the pressure?

hmm if the system is in equilibrium can't we say that the pressure won't change or is constant ?

 

[The legend]

umm... see if something ain't moving(is in static equilibrium) then what should be the case?
from both sides ?

think buddy, think.

 

[dragon162]

umm... see if something ain't moving(is in static equilibrium) then what should be the case?
from both sides ?

well if its in static equilibrium we can say the gas the pressure is not changing because there is balance? Sorry if I am not getting what you are trying to say, very lost with this problem.

edit: then wouldn't the pressure on the outside be acting in and the pressure on the inside acting out?

 

[The legend]

yes something like that... pressure from both sides is equal..thats it... now you have pressure , volume and temperature ... go on calculate no of moles...then substitute n in case of next 2 questions along with temperature and get your answer

 

[dragon162]

yes something like that... pressure from both sides is equal..thats it... now you have pressure , volume and temperature ... go on calculate no of moles...then substitute n in case of next 2 questions along with temperature and get your answer

so pressure=1 atm, temperature=316c+273=589k, and volume would be the volume of the cylinder right? v=pi *r^2*h=pi*(121)(75)=9075pi. Then i plug this in pV=nRT solve for n. Now that i have n i use that number to solve for temperature in the other problem and then volume in the last to find the height?

 

[Pottsy1994]

The legend is an incorrect smartass for anyone who reads this thread.

Need to also factor in the weight of the piston, ie, equate force due to pressure inside the cylinder with force due to pressure outside the cylinder + weight force of the piston.