Re: Why do pressures have to be equal in water displacement methods?

If the pressure is different, it is more difficult to measure the molar 
quantity of gas.  Ideally, PV = nRT where P = absolute pressure, V = 
volume, n = moles of gas, T = absolute temperature (Kelvin), and R is the 
ideal gas constant which is approximately equal to 0.082057 Liter*Atm/Mole 
*Kelvin.  Depending on the units you are working with, the units of the 
ideal gas constant vary.  

By setting the pressure of the graduated cylinder to the outside pressure 
you eliminate a variable that needs to be measured.  Therefore, knowing P ~ 
1 atm, T~ room temperature, R is constant, V = volume of space used in the 
graduated cylinder, the moles of gas are easily calculated.  

Thus, n = PV/RT.

I hope this is the answer you were looking for.  If you would like to send 
me additional information on the method, I would be happy to give you more 
explanation.

Jared