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Chemistry – Dr. May Two Kinds of Gas Law Problems The Combined Gas Law includes Boyle’s and Charles Law and can be used for any gas problem where conditions change. P1V1 ‗ P2V2 T1 T2 P = pressure, and calculations can be done when P is in millimeters of Hg (mm), kilopascals (kPa), or atmospheres (atm) as long as both pressures are in the same units. Standard pressure is 760 mm, 101.32 kPa, or 1 atm. T = temperature, and calculations can be done only when T is in Kelvins. Standard temperature is 273 K or 0oC. V = volume of gas and calculations can be done when V is in any unit as long as both volumes are in the same units. The most common units are liters (L), milliliters (ml), and cubic centimeters (cm3). One mole of gas at STP (0oC and 1 atm) occupies 22.4 liters. Sample Problem: A balloon in a hot vacuum oven at 60 oC and a pressure of 40 kPa has a volume of 60 liters. What would the volume be when the balloon is taken to standard conditions (STP)? P1V1 ‗ P2V2 T1 T2 P1 V1 T1 P2 V2 T2 = = = = = = ______________ ‗ ____________ ? V2 = 1 The Ideal Gas Law is used to determine the properties of a volume of gas under static conditions. PV = nRT We can express the equation as a function of each of the variables by solving for them algebraically. These equations can then be used to solve for any of the four variables when the other three are known and expressed in the proper units. P ‗ nRT V V ‗ nRT P T ‗ PV nR n ‗ PV RT It is very important that the numbers used in calculations in the Ideal Gas Law have the proper units. R is the universal gas constant and is always 0.0821 when P is in atmospheres and is always 8.3 when P is in kilopascals. V is always in liters P is in atmospheres or kilopascals ( 1 atm = 760 mm Hg or 1 atm = 101.32 kPa) and must be matched up with the proper universal gas constant. (With atmospheres use R = 0.0821 and with kilopascals use R = 8.3) n is always in moles T is always in Kelvins (To get Kelvins add 273 to oC) Anytime moles are involved, we must remember the following relationship: moles ‗ grams MW or grams = moles x MW Sample Problem: What is the volume of a vial containing 42 grams of nitrogen at a pressure of 290 kPa and a temperature of 29 oC. n R T P V = = = = = ? V V ‗ ‗ nRT P ______________________ 2 V =