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Using the Ideal Gas Law

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rvf263:
Hi,
I'm not understanding the Ideal Gas Law (PV=nRT), so I'm looking for help.

Here's a sample problem that I have questions about

"What is the pressure in atmosphere of 0.35 mole of N2O at 22 degree C contained in a 5.0 L tank?"

I organized the quantities:
P = ?
V = 5.0 L
n = 0.35 mole
T = 295 K (22+273)

I set the ideal gas law up to solve for P. "P = nRT/V"
I added the quantities and came up with:
P = (0.35 mole)(0.0821 L)(295K) / (5.0L)(mole x K) = 1.7 atm

My question is:
Where is the (0.0821L) and the (mole x K) coming from? I'm assuming it's the "R"?

Does "R" always = 0.0821 L / mole x K?

Seperate Question:
How do I solve for the number of moles using the ideal gas law?
Example:
"How many grams of Cl2 are in a 7.0L tank if the gas has a pressure of 760mmHg and a temp of 24 degree C?

I'm not understanding this Ideal Gas Law all the way.....Can someone maybe help explain it to me in a simple way?

Thanks
Robert

Donaldson Tan:
for your first question, the general ideal gas equation (PV = nRT) is derived from 3 gas law (Charle's, Boyle's, Guy's). This equation describes how each state property (P,V,T) varies with each other. A real gas behavior can be approximated as an ideal gas at low pressure or ambient conditions. the ideal gas law equation can be written in 2 equivalent forms:
1. PV = nRT
2. PV = NkT

R is the universal molar gas constant and k is the stefan-boltzman constant. n is the number of moles of the perfect gas, whereas N is the number of molecules, thus N/n = L (Avagadro's Constant) and R/k = L. Thus u can see why nR = Nk

for your 2nd question, the general ideal gas equation is a convenient mean to find the number of moles (n = PV/RT) of chlorine gas. Moreover, the conditions stated in the question corresponds to ambient condition, so the ideal gas approximation is valid. The mass of the chlorine gas is just the number of moles multiply by its molar mass.

Mitch:
Click miscellaneous under the general category in the menu to your left. Scroll down and you'll see we have 4 different R (Universal Gas Constant) values.

rvf263:
Thanks!

AWK:
R constants for any units used in the ideal gas law can by calculated from STP data, ie:
For 1 mole V= 22.4 L for p=1 atm and T=273
R=pV/T = 1  x 22.4 / 273.15 = 0.08206 L  x atm /(mol x K)
Put pressure = 760 mm Hg
R= 62.36 L x mmHg / (mol x K)

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