Calculating Weight of Argon in a Cylinder

[simonak]

HI,

Here's the question :

A cylinder is fill with Argon (Ar) at 25 °C and 102,4 kPa with a total weight of 312,50 g. The dimension of the cylinder is 32,4 mm of diameter and 1,2162 m long. We remove Argon with a vacuum pump. What will be the total weight of the cylinder if the inside pressure is 18,2 mmHg and the temperature 35 °C ?

First, I don't know why they told us the dimension of the cylinder because the volume will stay the same. Second, I started to convert 18,2 mmHg to 2,42606 kPa, 25 °C to 298 K and 35 °C to 308 K.

After that, I wrote my equation :

P1 / (n1 . T1) = P2 / (n2 . T2)

Now, the only thing that is missing is the n2 so if whe isolate n2 it is :

n2 = 0.179317 mol

and we know that n = m (weight) / M (atomic weight)
so m2 = n2 * M2
...
m2 = 0.179317 mol * 39.948 g/mol
m2 = 7.16337 g
but the answer is
m2 = 310.89 g

The only thing that I'm not really sure is that in the question they told us the weight of the totat cylinder...I use this weight instead of the weight gas.

anyway, maybe it's juste a little detail but if someone could help me it would be very appreciated.

Thank you !

 

[Physics Monkey]

You're given the total weight of the cylinder plus the gas. You can calculate the total weight of the gas initially, right? What should you do from here?

 

[simonak]

I don't know,... they gave me the total weight before and I need the total weight after...I don't know how to find the weight of the gas or the weight of the cylinder...they juste gave me the total.

 

[Physics Monkey]

I don't know how to find the weight of the gas

Sure you do. You know the pressure, temperature, and volume of the gas initially, is that enough to determine the mass of the gas (incidently, this should answer your question as to why they told you the volume)? If so, how would you do it?