# Homework Help: Need help using Ideal Gas Law

1. Sep 18, 2011

### MastersBound

1. The problem statement, all variables and given/known data
A rigid tank contains 5 kg of an ideal gas at 4 atm and T=40 deg C. Now, a valve is opened and half of the mass of the gas is allowed to escape. If the final pressure in the tank is 1.5 atm, the final temperature in the tank is?

2. Relevant equations

pV=mRT

3. The attempt at a solution

I know pV=mRT but I don't know how to use this equation for to different states i.e. beginning and end state.

2. Sep 18, 2011

### vela

Staff Emeritus
The equation holds for the two different states, so you have
\begin{align*}
p_i V_i &= m_i R T_i \\
p_f V_f &= m_f R T_f
\end{align*}
What are the quantities you know? What other relationship were you given that relates the quantities?

3. Sep 18, 2011

### MastersBound

That is it. That is the whole problem statement. No specific gas or fluid was given.

4. Sep 18, 2011

### vela

Staff Emeritus
I know. There's enough information to solve the problem. Those questions were meant for you to answer.