[danshodan]

hello everyone,
i have the following problem:
When a rigid hollow sphere containing 680L of helium gas is heated from 300K to 600K, the pressure of the gas increases to 18 atm. How many moles of helium does the sphere contain?

I know I need to use the formla PV=nRt, which from there I can say n=PV/RT; the problem I am having is how to know which temperature to choose. I went with the 600 as it is the final temp??? not sure bout that, do u need to use both?
I ended up with 18 atmX 680L/ 0.0821 L*atm       600K
                                            ______________  /
                                                   mol*K
and my final answer was 249 mol.
So am i correct, and how do u know which one to choose

[AWK]

You should use the temperature in Kelvins

[arnyk]

Also depending on what constant *R* you use, pressure is most commonly measured in kPa.

[Donaldson Tan]

since Helium is contained in a rigid container, then volume is constant
PV = nRT => P/T = nR/V = constant

=> P₁T₂ = P₂T₁

since pressure is increased by 18atm after heating, then
P₂ = P₁ + 18

P₁(600) = (P₁ + 18)(300)
(P₁ + 18)/P₁ = 600/300 = 2
1 + 18/P₁ = 2
P₁ = 18atm = 18(1.013E5) Pa = 1.82E6 Pa

since initial pressure is now known, and we know T₁, then number of moles of He gas is given as:

n = P₁V/R.T₁ = (1,82E6)(680E-3)/(8.314)(300) = 730moles