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 P1V1 = P2V2 (T fixed)

Charles Law V1/T1 = V2/T2 (P fixed)

Gay-Lussacs P1/T1 = P2/T2 (V fixed) Avogadros Law V1/n1 = V2/n2 (T&P fixed)

STP - Standard Temperature and Pressure (Gases)

T = 273 K (0 oC) 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:

P1V1 = P2V2

n1T1 n2T2

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 25oC (approx 77oF). Assuming constant pressure, what is the volume of the balloon when you walk outside on a cold winter day with a temperature of

-20oC ( -4oF).

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 24oC. If the can is thrown into a fire (485oC), 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 24oC at a pressure of 745mmHg. A balloon with a volume of 425mL drifts into the airlock where the temperature is -95oC and the pressure is 0.115 atm. What is the new volume of this balloon (assume it’s very elastic)?

Dinitrogen oxide (N2O), laughing gas, is used by dentists as an anesthetic. If a 20.0 L tank of laughing gas contains 2.86 mol N2O at 23°C, what is the pressure in the tank?

Learning Check

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

A. 0.270 atm

B. 3.48 atm

C. 1.74 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 O2 gas is needed to completelyreact with 15.0 g of aluminum at STP?

4Al(s) + 3O2 (g) 2Al2O3(s)

Gas Equations and Reactions

What volume (L) of Cl2 gas at 1.2 atm and 27°C is needed to completely react with 1.5 g of aluminum?

2Al(s) + 3Cl2 (g) 2AlCl3(s)

Gas Laws and Stoichiometry Butane, C4H10, burns according to the equation:

2C4H10 (g) + 13O2 (g) 8CO2 (g) + 10 H2O (l)

What volume of CO2 measured at STP would be produced by

85.0 g of C4H10?

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: 2KNO3 (s) 2KNO2 (s) + O2 (g)

What mass of KNO2 forms along with 14.5 L of O2 measured at 1.00atm and 25 oC?

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 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

Molar Mass of a Gas

Density of a Gas continued….

What is the density of ammonia gas, NH3(g), at 745 mmHg and 65oC?

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 O2 gas at STP.