Chemistry 103 Lecture 19

General Course structure - WEEK 5 Atoms ---> Compounds ---> Chemical Reactions GAS LAWS SOLUTIONS

Outline I. GAS LAWS continued ….. - Ideal/Combined Gas Law - Stoichiometry - modified gas expression - Dalton’s law of Partial Pressure II. SOLUTIONS - INTRODUCTION

Important Simple Gas Relationships Boyles LawP 1 V 1 = P 2 V 2 (T fixed) Charles LawV 1 /T 1 = V 2 /T 2 (P fixed) Gay-LussacsP 1 /T 1 = P 2 /T 2 (V fixed) Avogadros LawV 1 /n 1 = V 2 /n 2 (T&P fixed) STP - Standard Temperature and Pressure (Gases) T = 273 K (0 o C) and P = 1 atm (760 mmHg) 1 mole of a gas at STP = 22.4L

Combined Gas Laws Relates the changes in pressure, volume and temperature, and quantity change of a gas PV = constant nT Useful form of the expression: P 1 V 1 =P 2 V 2 n 1 T 1 n 2 T 2

Ideal Gas Law Let’s now solve for the constant: PV/nT = constant = R = atm L mol K IDEAL GAS LAW : PV=nRT

Gas Laws A balloon contains 2.5L of Helium gas when the inside temperature is 25 o C (approx 77 o F). Assuming constant pressure, what is the volume of the balloon when you walk outside on a cold winter day with a temperature of -20 o C ( -4 o F). A.48 B.2.9 C.2.1

Gas Laws A spray can is empty except for the propellant gas, which exerts a pressure of 1.2 atm at 24 o C. If the can is thrown into a fire (485 o C), what will be the pressure (atm) inside the hot can? A.3.1 B.24 C.0.041

Gas Laws Your space ship has docked at a space station above Mars. The temperature inside the space station is 24 o C at a pressure of 745mmHg. A balloon with a volume of 425mL drifts into the airlock where the temperature is - 95 o C and the pressure is atm. What is the new volume of this balloon (assume it’s very elastic)?

Dinitrogen oxide (N 2 O), laughing gas, is used by dentists as an anesthetic. If a 20.0 L tank of laughing gas contains 2.86 mol N 2 O at 23°C, what is the pressure in the tank? Learning Check Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings A atm B atm C atm

Gases in Chemical Equations The volume or amount of a gas in a chemical reaction can be calculated from: STP conditions or the Ideal Gas Law Mole factors from the balanced equation

STP and Gas Equations What volume (L) of O 2 gas is needed to completely react with 15.0 g of aluminum at STP? 4Al(s) + 3O 2 (g) 2Al 2 O 3 (s)

Gas Equations and Reactions What volume (L) of Cl 2 gas at 1.2 atm and 27°C is needed to completely react with 1.5 g of aluminum? 2Al(s) + 3Cl 2 (g) 2AlCl 3 (s)

Gas Laws and Stoichiometry Butane, C 4 H 10, burns according to the equation:  2C 4 H 10 (g) + 13O 2 (g)  8CO 2 (g) + 10 H 2 O (l) What volume of CO 2 measured at STP would be produced by 85.0 g of C 4 H 10 ? A. 131L B. 32.8L C L

Gas Laws and Stoichiometry, II Oxygen gas can be prepared in the lab by the decomposition of potassium nitrate according to the equation:  2KNO 3 (s)  2KNO 2 (s) + O 2 (g) What mass of KNO 2 forms along with 14.5 L of O 2 measured at 1.00atm and 25 o C?

Ideal Gas Law - expanded PV = nRT; n = grams/Molar Mass n = g/MM PV = gRT MM

What is the molar mass of a gas if g of the gas occupies 215 mL at atm and 30.°C? A. 3.52g/molB. 17.8g/molC. 35.6g/mol Molar Mass of a Gas

Density of a Gas continued…. What is the density of ammonia gas, NH 3 (g), at 745 mmHg and 65 o C?

Density = Molar mass Molar volume Density = grams/mole =grams L/mole L Density of a Gas at STP

Density of a Gas Calculate the density in g/L of O 2 gas at STP.