# Homework Help: Thermal Physics

1. Nov 13, 2008

### unscientific

1. The problem statement, all variables and given/known data
A steel container contains 300g of ideal gas at a pressure of 1.35 x 106 Pa and temperature of 77oC. When the tank is checked later, the temperature is dropped to 22oC and the pressure has fallen to 8.7 x 105 Pa. How many grams of gas leaked out of the tank?

2. Relevant equations
PV = NkT

3. The attempt at a solution
first, converting: 77oC = 350.15K and 22oC = 295.15K

The problem is, i do not know how to link mass to the equation of PV = NkT
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 13, 2008

### Hootenanny

Staff Emeritus
Hint: Can you relate N - the number of particles to the mass?

3. Nov 14, 2008

### unscientific

Number of particles = Total mass / mass per molecule?

But i do not know the mass per molecule...

4. Nov 14, 2008

### unscientific

I really need an answer quick, my holidays are ending soon and this is my holiday assignment question that i only stuck with..

5. Nov 14, 2008

### aniketp

HINT 2 : Avogardro number.......

6. Nov 15, 2008

### unscientific

I tried it again, closest i could get was,

PV = nRT

n = Mtotal / Mpermole = Mtotal/(mNA)

Where m = mass per molecule..

7. Nov 15, 2008

### eys_physics

Hey

Assume that each particle have the mass $$m_0$$. Can you now relate the mass to the number of particles N?
Use this relations to express N in terms of m. Now you can relate the mass after $$m_2$$ to the mass before $$m_1$$ and this relation is independent of $$m_0$$ and only depends on known parameters.

8. Nov 15, 2008

### unscientific

Ok i solved it!!! :D:D The crux of this problem is that, the volume before and after do not change, and i managed to equate mass/molecule.

PV = NkT, PV = (Mtotal/mo)kT

After the reaction,

P'V' = N'kT', P'V' = (M'total/mo)kT

equating mo = mo,

I get

MtotalT/V = M'totalT'/V'

So i solved for M', giving 229.36 g. So, the mass that leaked = (300 - 229.3) g = 70.6g

I forgot that V was constant, that was perplexing me throughout!!

9. Nov 15, 2008

### Hootenanny

Staff Emeritus
Good job