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Gay Lussac’s law of combining volumes of gases • When gases combine, they combine in simple whole number ratios. • These simple numbers are the coefficients of the balanced equation. N2 + 3H2 2NH3 • 3 volumes of hydrogen will produce 2 volumes of ammonia Mullis 1 Avogadro’s Law and Molar Volume of Gases • Equal volumes of gases (at the same temp and pressure) contain an equal number of molecules. In the equation for ammonia formation, 1 volume N2 = 1 molecule N2 = 1 mole N2 • One mole of any gas will occupy the same volume as one mole of any other gas • Standard molar volume of a gas is the volume occupied by one mole of a gas at STP. Standard molar volume of a gas is 22.4 L. Mullis 2 Sample molar volume problem • A chemical reaction produces 98.0 mL of sulfur dioxide gas at STP. What was the mass, in grams, of the gas produced? ***Turn mL to L first! (This way, you can can use 22.4 L) 98 mL 1 L 1000 mL 1 mol SO2 64.07g SO2 = 0.280g SO2 22.4 L 1 mol SO2 Mullis 3 Sample molar volume problem 2 What is the volume of 77.0 g of nitrogen dioxide gas at STP? 77.0 g NO2 1 mol NO2 22.4 L 46.01g NO2 1 mol NO2 Mullis = 37.5 L NO2 4 Ideal Gas Law • Mathematical relationship for PVT and number of moles of gas PV = nRT n = number of moles R = ideal gas constant P = pressure V = volume in L T = Temperature in K R = 0.0821 if pressure is in atm R = 8.314 if pressure is in kPa R = 62.4 if pressure is inMullis mm Hg 5 Sample Ideal Gas Law Problem • What pressure in atm will 1.36 kg of N2O gas exert when it is compressed in a 25.0 L cylinder and is stored in an outdoor shed where the temperature can reach 59°C in summer? V = 25.0 L T = 59+273 = 332 K P = ? R = 0.0821L-atm n = 1.36 kg converted to moles mol-K • 1.36 kg N2O 1000 g 1 mol N2O = 30.90 mol N2O 1 kg 44.02 g N2O • PV = nRT • P = 30.90 mol x 0.0821 L-atm x 332 K = 33.7 atm 25.0 L mol-K Mullis 6 Volume-Volume Calculations • Volume ratios for gases are expressed the same way as mole ratios we used in other stoichiometry problems. N2 + 3H2 2NH3 Volume ratios are: 2 volumes NH3 3 volumes H2 2 volumes NH3 3 volumes H2 1 volume N2 1 volume N2 Mullis 7 Sample Volume-Volume Problem • How many liters of oxygen are needed to burn 100 L of carbon monoxide? 2CO + O2 2CO2 100 L CO 1 volume O2 2 volume CO Mullis = 50 L O2 8 Sample Volume-Volume Problem 2 Ethanol burns according to the equation below. At 2.26 atm and 40° C, 55.8 mL of oxygen are used. What volume of CO2 is produced when measured at STP? C2H5OH + 3O2 2CO2 + 3H2O Number moles oxygen under these conditions is? PV = nRT: 2.26 atm(.0558L) = n = 0.0049 mol O2 (0.0821 L-atm)(313K) mol-K 0.0049 mol O2 2 mol CO2 22.4 L =0.073 L CO 2 3 mol O2 1 mol CO2 Mullis 9