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Ideal Gas Law



The ideal gas law is the most important gas law for you to know: it combines all of the laws you learned about in this unit thus far, under a set of standard conditions. The four conditions used to describe a gas—pressure, volume, temperature, and number of moles (quantity)—are all related, along with R, the universal gas law constant, in the following formula:

PV = nRT

where P = pressure (atm), V = volume (L), n = number of moles (mol),T = temperature (K)  and R = 0.08206 L ˇ atm/mol ˇ K,   

Take a look at the units of R, they tell you what the other units have to be in. If the units are different, simply convert it.


A 16.0 g sample of methane gas, CH4, the gas used in chemistry lab, has a volume of 5.0 L at 27ēC. Calculate the pressure.


Looking at all the information given, you have a mass, a volume, and a temperature, and you need to find the pressure of the system. As always, start by checking your units. You must first convert 16.0 g of CH4 into moles: 16.0 g CH4 x 1 mol CH4/16.0 g CH4 = 1.00 mol of methane. The volume is in the correct units, but you must convert the temperature into Kelvins: 27 + 273 = 300.K. Now you’re ready to plug these numbers into the ideal gas law equation:

PV = nRT

(P) (5.0 L) = (1.00 mol) (0.0821 L atm/mol K) (300.K), so P = 4.9 atm


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