How to Calculate Gas Usage in Pressure Drop Situation?

[prehisto]

Homework Statement

Pressure drops in welders tank of oxygen gas from p₁=150atm to p₂=120atm .
How much of the gas will be used ?

Homework Equations

The Attempt at a Solution

In my mind the simplest way of looking at this problem is to consider the process isothermal.
But then the volume of the gas increases when pressure drops (Ideal gas law). This contradicts the question.

Since I do not know the temperatures before and after, I don't know what to do. I am looking for different view at this problem, some help?

 

[Simon Bridge]

If you want to treat the gas as an ideal gas then state the ideal gas law.
Looking at the law, which of the variables is constant and which variable? List them.
i.e. does the volume of the tank change?

 

[Guneykan Ozgul]

Volume of the gas does not change if the tank is not elastic (I think not in this question) since the gas will always fill the whole tank.

 

[prehisto]

If you want to treat the gas as an ideal gas then state the ideal gas law.
Looking at the law, which of the variables is constant and which variable? List them.
i.e. does the volume of the tank change?

Volume of the gas does not change if the tank is not elastic (I think not in this question) since the gas will always fill the whole tank.

OK, I Think it is logical to assume that Volume of tank does not change. Now I can rethink my solution.
$P_1V=n_1RT_1$ and $P_2V=n_2RT_2$
$\frac {n_1RT_1} {P_1} = \frac {n_2RT_2} {P_2}$
$\frac {n_2} {n_1}=\frac {T_2P_1} {T_1P_2}$

But It seems that I need the temperature before and after, which i do not have. And to assume that T=const seems to be to only way, but it also seems not logical.

 

[Chestermiller]

OK, I Think it is logical to assume that Volume of tank does not change. Now I can rethink my solution.
$P_1V=n_1RT_1$ and $P_2V=n_2RT_2$
$\frac {n_1RT_1} {P_1} = \frac {n_2RT_2} {P_2}$
$\frac {n_2} {n_1}=\frac {T_2P_1} {T_1P_2}$

But It seems that I need the temperature before and after, which i do not have. And to assume that T=const seems to be to only way, but it also seems not logical.

You're supposed to assume that the tank is not insulated, so that the final and initial temperatures are equal to room temperature.