9 Combined Gas Law So, why struggle to remember all those people and laws whenWe can summarize themWith the combined gas law.This law can be used to solveProblems with ANY of theBoyle-Charles-Gay-LussacConditions. Just leave out theVariable that is constantAnd only 1 law to remember!
10 Using the Ideal Gas Law What is an ideal gas? How do gases deviate from the ideal?No gas is really ideal. However, this law gives good approximation of gas behavior.It does not work well when the temperature is low (near boiling point) or when thePressure is high. Both of those conditions mean that the particles are very closeTogether and will experience forces between them. Molecules with high molar massesAlso tent away from the ideal.
11 PV =nRT The ideal gas law! This law uses the relationships from the combined law and add molar ratio This is often used to find molar mass or density Remember that molar mass is g/mole so g/n And density is mass/volume n/V x g/mole (molar mass)
12 Which Law to Use?P1V1 = P2V2T1 T2PV = nRTConditions are changing Example: A piston (volume changes as it moves in and out) A balloon moving up in the atmosphere (the outside P decreases)Conditions are constant But one variable is unknown Finding the number of moles of a gas at known temperature, volume and pressure (STP for one) Finding any variable for a given chemical reaction
13 Stoichiometry Combined with Gas Laws Example: For the reaction: 2H2 + O2 2H2OHow many liters of oxygen are needed to react with 2.56 moles of hydrogen at STP?Find the moles of O2 needed. Use the molar ratio.2.56 moles mole = moles O22 molesChange the moles to litersSub into PV = nRT V = nRT/PV = (1.28 moles)( L atm/Kmol)(273K)/(1atm)V = L; with sig figs 28.7 L
14 An unknown gas has a density of 0. 89 g/ml at a pressure of 0 An unknown gas has a density of 0.89 g/ml at a pressure of atm and temperature of 25 C. What is the molar mass?Use PV = nRT molar mass = g/moln/V = P/RT sub inn/V = (0.900 atm)/( Latm/Kmol)( K)n/V = mol/LWe want g/mol for molar mass0.89 g x ml x 1 L = g/molml L mol