How Do You Calculate the Initial Number of Gaseous Molecules in a Closed System?

[tunabeast]

Homework Statement

In a closed system shown in the Figure below (http://i18.tinypic.com/7x1r8mo.jpg"), the piston is freely moving without friction and the system is initially filled with gaseous molecules A only. The mass of a piston is 100 g, its area is 10 cm2 and it is 10 cm from the bottom in the initial state of equilibrium. The pressure outside the system is 1 atm; temperature inside the system is 900 ºC.

a) Calculate the initial number of molecules in the system;
b) Write the equation describing the initial rate of decrease in the number of gaseous
molecules if it is known that molecules A stick to the surface of the piston upon
collision with it (molecules do not stick to any other surfaces; the processes when
gaseous molecules react with a hot substrate surface are the basis of the chemical
vapour deposition technology). Obtain numerical value of the initial rate.

Homework Equations

The Attempt at a Solution

I'm really struggling with where to start on this. I'v been handed a large physical chemistry book and basically been left to my own devices. I understand the problem relates to molecular theory of gasses but could you please steer me in the right direction.

 

[chemisttree]

You can start by determining the pressure exerted by the piston on the confined "Molecules A, 900 C". The external pressure is 1 atm but the piston has mass as well.