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 Law Charles Law Gay-Lussacs Avogadros Law P 1 V 1 = P 2 V 2 (T fixed) V 1 /T 1 = V 2 /T 2 (P fixed) P 1 /T 1 = P 2 /T 2 (V fixed) V 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 = nT constant Useful form of the expression: P 1 V n 1 T 1 1 = P n 2 2 V T 2 2

Ideal Gas Law

 Let’s now solve for the constant:  PV/nT = constant = R = 0.0821atm 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 0.115 atm. What is the new volume of this balloon (assume it’s very elastic)?

Learning Check

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? A. 0.270 atm B. 3.48 atm C. 1.74 atm Copyright © 2005 by Pearson Education, Inc.

Publishing as Benjamin Cummings

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 85.0 g of C 4 H 10 ? measured at STP would be produced by  A. 131L  B. 32.8L

 C. 65.5 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 and 25 o C? forms along with 14.5 L of O 2 measured at 1.00atm

Ideal Gas Law - expanded

 PV = nRT  n = g/MM ; n = grams/Molar Mass  PV = gRT MM

Molar Mass of a Gas

What is the molar mass of a gas if 0.250 g of the gas occupies 215 mL at 0.813 atm and 30.

°C?

A. 3.52g/mol B. 17.8g/mol C. 35.6g/mol

Density of a Gas continued….

 What is the density of ammonia gas, NH 3 (g), at 745 mmHg and 65 o C?

Density of a Gas at STP

Density = Molar mass Molar volume Density = grams/mole L/mole = grams L

Density of a Gas

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