chapter 8 gases - chemistry121.pbworks.com

General, Organic, and Biological Chemistry: Structures of Life, 5/e

Karen C. Timberlake

© 2016 Pearson Education, Inc.

Karen C. Timberlake

Lecture Presentation

Chapter 8

Gases


Karen C. Timberlake


Chapter 8 Readiness

Key Math Skills

• Solving Equations (1.4D)

Core Chemistry Skills

• Using Significant Figures in Calculations (2.3)

• Writing Conversion Factors from Conversion Equalities (2.5)

• Using Conversion Factors (2.6)

• Using Molar Mass as a Conversion Factor (7.5)

• Using Mole–Mole Factors (7.6)


Karen C. Timberlake


8.1 Properties of Gases

Generally molecules with fewer than five atoms from the first

two periods in the periodic table are gases at room temperature. In addition, the following are also gases:

Learning Goal Describe the kinetic molecular theory of gases and the units of measurement used for gases.

• H2, N2, O2, F2, and Cl2

• oxides of the nonmetals on the upper-right corner of the periodic table: CO, CO2, NO, NO2, SO2, and SO3

• noble gases

He

Ne

Ar

Kr

Xe

Rn

FONC

S Cl

H


Karen C. Timberlake


Kinetic Molecular Theory

A gas consists of small particles that

1. move randomly with high velocities.

2. have very small attractive (or repulsive) forces between molecules.

3. occupy a much larger volume than the volume of the molecules alone.

4. are in constant motion, moving rapidly in straight lines.

5. have a Kelvin temperature proportionate to the average kinetic energy of the molecules.

Gas particles which move in straight lines within a container, exert pressure when they collide with the walls of the container.


Karen C. Timberlake


Properties That Describe a Gas

Gases can be quantified in terms of four properties: pressure

(P), volume (V), temperature (T), and amount of particles (n =

moles).


Karen C. Timberlake


Gas Volume

The volume of a gas

• is the same as the volume of the container it occupies.

• is usually measured in liters or milliliters.

• increases with an increase in temperature at a constant pressure. Gas particles which

move in straight lines within a container, exert pressure when they collide with the walls of the container.


Karen C. Timberlake


Gas Temperature

The temperature of a gas is directly related to the average kinetic energy (motion) of the molecules and is measured in the Kelvin (K) temperature scale.

**Note: The Kelvin scale must used as calculations using the

Celsius scale could result in negative volume or pressure of a gas � that’s impossible!!!

When the temperature of a gas is

• decreased, the particles are moving more slowly and have fewer collisions.

• increased, the particles are moving faster have more collisions.


Karen C. Timberlake


Gas Pressure

Gas Pressure is a measure the number of collisions of the gas particles with the walls of its container, and the force of those collisions, and is measured in various units:

• millimeters of mercury, mmHg or torr.

• atmospheres, atm.

• pascals, Pa, or kilopascals, kPa (the SI unit of measurement)

• pounds per square inch, psi.

• At weather stations in the U.S., inches of Hg are used


Karen C. Timberlake


Gas Pressure

Gas particles in the air exert pressure on us called atmospheric pressure.

In 1643, Italian physicist EvangilistaTorricelli invented a device to measure the pressure of the atmosphere = Barometer


Karen C. Timberlake


Barometers Measure Pressure

A barometer

• measures the pressure exerted

by the gases in the atmosphere.

• indicates atmospheric pressure as the height in mm of the mercury column.

760 mmHg = 1 atm = 760 Torr

The barometer was invented by Evangelista Torricelli, at exactly 1 atmthe barometer tube measures exactly760 mm high.


Karen C. Timberlake


Units of Pressure


Karen C. Timberlake


Atmospheric Pressure

Atmospheric pressure

• is the pressure exerted by a

column of air from the top of

the atmosphere to the

surface of Earth.

• decreases as altitude

increases.

• about 1 atm at sea level.


Karen C. Timberlake


Altitude and Atmospheric Pressure

Atmospheric pressure changes with variations in weather and altitude.

• On a hot, sunny day, the mercury column rises, indicating a higher atmospheric pressure.

• On a rainy day, the atmosphere exerts less pressure, which causes the mercury column to fall.


Karen C. Timberlake


Study Check

1. What is 475 mmHg expressed in atm?

A. 475 atm

B. 0.625 atm

C. 3.61 × 105 atm

2. The pressure in a tire is 2.00 atm. What is thispressure in mmHg?

A. 2.00 mmHg

B. 1520 mmHg

C. 22 300 mmHg


Karen C. Timberlake


8.2 Gas Pressure and Volume, (Boyle’s Law)

The inverse relationship between

the pressure and volume of a gas is known as Boyle’s law.

Changes occur in opposite directions. When volume increases, the pressure decreases, provided the temperature and moles of the gas remains constant.

Learning Goal Use the pressure–volume relationship (Boyle’s law) to determine the final pressure or volume when the temperature and amount of gas are constant.

The anesthetic N2O gas, is used for pain relief.


Karen C. Timberlake



In the late 1600s, Irish chemist

Robert Boyle performed experiments in order to determine the relationship of the pressure of a gas and its volume under conditions of constant pressure and amount of gas. When he graphed the pressure of a gas as a function of its volume, the result was the graph to the right

Notice that there is not a linear relationship between the variables. This downward sloping curve can indicate an inverse relationship


Karen C. Timberlake



Boyle re-graphed his data, this time

plotting the pressure of the gas as a function of the inverse of its volume.

Notice the graph to the right now shows a linear relationship

So, the pressure of a gas, P, at constant temperature and amount of gas, varies directly with the inverse of its volume


Karen C. Timberlake


8.2 Pressure and Volume, (Boyle’s Law)

The inverse relationship between

the pressure and volume of a gas is known as Boyle’s law.

Changes occur in opposite directions. When volume increases, the pressure decreases, provided the temperature and moles of the gas remains constant.

Mathematical Expression of Boyle’s Law: P1V1 = P2V2


Karen C. Timberlake


Boyle’s Law

Boyle’s law states that

• the pressure of a gas is inversely related to its volume when T is constant.

• the product P × V is constant when temperature and amount of a gas is held constant

• if volume decreases, the pressure increases.

P1V1 = P2V2


Karen C. Timberlake


Boyle’s Law: PV = Constant

Pressure × volume is a constant, provided the

temperature and amount of the gas remains

the same.

P1V1 = 8.0 atm × 2.0 L = 16 atm L

P2V2 = 4.0 atm × 4.0 L = 16 atm L

P3V3 = 2.0 atm × 8.0 L = 16 atm L

Boyle’s law can be stated as

P1V1 = P2V2 (T is constant.)


Karen C. Timberlake


Chemistry Link to Health: Boyle’s Law and Breathing

During an inhalation,

• the lungs expand.

• the pressure in the lungs

decreases.

• air flows toward the lower

pressure in the lungs.


Karen C. Timberlake


Chemistry Link to Health: Boyle’s Law and Breathing

During an exhalation,

• lung volume decreases.

• pressure within the lungs

increases.

• air flows from the higher

pressure in the lungs to

the outside.


Karen C. Timberlake


Guide to Using Gas Laws


Karen C. Timberlake


Calculations Using Boyle’s Law

Freon-12, CCl2F2, was used in refrigeration systems. What is

the new volume of an 8.0 L sample of Freon gas initially at 550 mmHg after its pressure is changed to 2200 mmHg at constant temperature and moles?

STEP 1 Organize the data in a table of initial and final conditions.

Temperature and moles remain constant.

ANALYZE Conditions 1 Conditions 2 Know Predict

THE P1 = 550 mmHg P2 = 2200 mmHg P increases

PROBLEM V1 = 8.0 L V2 = ? V decreases


THE P1 = 550 mmHg P2 = 2200 mmHg P increases

PROBLEM V1 = 8.0 L V2 = ? V decreases


Karen C. Timberlake


Calculations Using Boyle’s Law

STEP 2 Rearrange the gas law equation to solve for the

unknown quantity.

P1V1 = P2V2 Boyle’s lawTo solve for V2 , divide both sides by P2.

STEP 3 Substitute values into the gas law equation and calculate.

×

×


Karen C. Timberlake


Study Check

A sample of oxygen gas has a volume of 12.0 L at 600. mmHg.

What is the new pressure when the volume changes to 36.0 L at a constant T and n?

A. 200. mmHg

B. 400. mmHg

C. 1200 mmHg


Karen C. Timberlake


Solution






Karen C. Timberlake


Solution



STEP 2 Rearrange the gas law equation to solve for the unknown quantity.

×


Karen C. Timberlake


Solution




The answer is A, 200. mmHg.

×


Karen C. Timberlake


Study Check

For a cylinder containing helium gas, indicate if cylinder A

or cylinder B represents the new volume for the followingchanges (n and T are constant).

1. pressure decreases

2. pressure increases


Karen C. Timberlake


Solution

For a cylinder containing helium gas, indicate if cylinder A

or cylinder B represents the new volume for the followingchanges (n and T are constant).

1. pressure decreases cylinder B

2. pressure increases cylinder A


Karen C. Timberlake


Study Check

If a sample of helium gas has a volume of 120 mL and apressure of 850 mmHg, what is the new volume if the pressureis changed to 425 mmHg at a constant T and n?A. 60 mL

B. 120 mL

C. 240 mL


Karen C. Timberlake


Solution

If a sample of helium gas has a volume of 120 mL and apressure of 850 mmHg, what is the new volume if the pressureis changed to 425 mmHg at a constant T and n?




Karen C. Timberlake


Solution

If a sample of helium gas has a volume of 120 mL and apressure of 850 mmHg, what is the new volume if the pressureis changed to 425 mmHg at a constant T and n?

STEP 2 Rearrange the gas law equation to solve for the unknown quantity.


×

×


Karen C. Timberlake


Study Check

A sample of helium gas in a balloon has a volume of 6.4 L at a

pressure of 0.70 atm. At 1.40 atm (T and n are constant), is the new volume represented by A, B, or C?


Karen C. Timberlake


Solution

A sample of helium gas in a balloon has a volume of 6.4 L at a

pressure of 0.70 atm. At 1.40 atm (T and n are constant), is the new volume represented by A, B, or C?

At a higher pressure (T and n constant), the new volume is represented by the smaller balloon A.


Karen C. Timberlake


Solution

1. What is 475 mmHg expressed in atm?

The answer is B, 0.625 atm.

2. The pressure in a tire is 2.00 atm. What is this pressure in mmHg?

The answer is B, 1520 mmHg.

×

×


Karen C. Timberlake


Study Check

1. The downward pressure on the Hg in a barometer

is _____ the pressure of the atmosphere.

A. greater than B. less than C. the same as

2. A water barometer is 13.6 times taller than an Hg

barometer (dHg = 13.6 g/mL) because

A. H2O is less dense than mercury.

B. H2O is heavier than mercury.

C. air is more dense than H2O.


Karen C. Timberlake


Solution

1. The downward pressure on the Hg in a barometer

is _____ the pressure of the atmosphere.

The answer is C, the same as.

2. A water barometer is 13.6 times taller than an Hg barometer (dHg = 13.6 g/mL) because

The answer is A, H2O is less dense than mercury.


Karen C. Timberlake


8.3 Gas Temperature and Volume (Charles’s Law)

If we increase the temperature of a gas sample, kinetic molecular theory states that the motion (kinetic energy) of the gas particles will also increase.

If the amount and pressure of the gas is held constant, the volume of the container will increase.

Learning Goal Use the temperature–volume relationship (Charles’s law) to determine the final temperature or volume when the pressure and amount of gas are constant.


Karen C. Timberlake


Charles’s Law

In Charles’s law,

• the Kelvin temperature (K)

of a gas is directly related to its volume.

• pressure and moles of gas are constant.

• when the temperature of a sample of gas increases, its volume increases at constant pressure.


Karen C. Timberlake


Charles’s Law: V and T

• For two conditions, Charles’s law is written

(P and n are constant)

• Rearranging Charles’s law to solve for V2:

× × ×


Karen C. Timberlake


Study Check

Solve Charles’s law expression for T2.


Karen C. Timberlake


Solution

Solve Charles’s law expression for T2:

Cross-multiply to give V1T2 = V2T1

Isolate T2 by dividing through by V1:

×


Karen C. Timberlake


Calculations Using Charles’s Law

A balloon has a volume of 785 mL at 21 °C. If the temperature drops to 0 °C, what is the new volume of the balloon at constant pressure and moles?


Pressure and moles remain constant.


Karen C. Timberlake


Calculations Using Charles’s Law

STEP 2 Rearrange to solve for unknown quantity: V2.

STEP 3 Substitute the values into the gas law equation and calculate.

×

×

××


Karen C. Timberlake


Study Check

A sample of oxygen gas has a volume of 420 mL at a temperature of 18 °C. At what temperature (in °C) will the volume of the oxygen be 640 mL (P and n are constant)?

A. 443 °C

B. 170 °C

C. −82 °C


Karen C. Timberlake


Solution

A sample of oxygen gas has a volume of 420 mL at a temperature of 18 °C. At what temperature (in °C) will the

volume of the oxygen be 640 mL (P and n are constant)?

STEP 1 Organize the data into a table of initial and final conditions.

Pressure and moles remain constant.


THE V1 = 420 mL V2 = 640 mL V increases

PROBLEM T1 = 18 °C = 291 K T2 = ? T increases


THE V1 = 420 mL V2 = 640 mL V increases

PROBLEM T1 = 18 °C = 291 K T2 = ? T increases


Karen C. Timberlake


Solution

A sample of oxygen gas has a volume of 420 mL at a temperature of 18 °C. At what temperature (in °C) will the

volume of the oxygen be 640 mL (P and n are constant)?

STEP 2 Rearrange to solve for unknown quantity: T2.


The answer is B.

×

× -


Karen C. Timberlake


Study Check

Use the gas laws to complete each sentence with increases

or decreases.

A. Pressure _______ when V decreases at constant temperature and moles.

B. When T decreases, V _______ at constant pressure andmoles.

C. Pressure _______ when V changes from 12 L to 24 L at constant temperature and moles.

D. Volume _______when T changes from 15 °C to 45 °C at

constant pressure and moles.


Karen C. Timberlake


Solution

Use the gas laws to complete each sentence with increases

or decreases.

A. Pressure increases when V decreases at constant temperature and moles.

B. When T decreases, V decreases at constant pressure and moles.

C. Pressure decreases when V changes from 12 L to 24 L at constant temperature and moles.

D. Volume increases when T changes from 15 °C to 45 °C at

constant pressure and moles.


Karen C. Timberlake


8.4 Temperature and Pressure(Gay-Lussac’s Law)

Gay-Lussac’s law:

When the Kelvin temperature of a gas doubles at constant volume and amount of gas, the pressure also doubles.

Learning Goal Use the temperature–pressure relationship (Gay-Lussac’s law) to determine the final temperature or pressure when the volume and amount of gas are constant.


Karen C. Timberlake


Gay-Lussac’s Law

In Gay-Lussac’s law,

• the pressure exerted by a gas is directly related to the Kelvin temperature of the gas.

• volume and amount of gas are constant.


Karen C. Timberlake


Study Check

Solve Gay-Lussac’s law for P2.


Karen C. Timberlake


Solution

Solve Gay-Lussac’s law for P2.

Multiply both sides by T2 and cancel:

× × ×


Karen C. Timberlake


Calculations Using Gay-Lussac’s Law

A gas has a pressure at 2.0 atm at 18 °C. What is the new

pressure when the temperature is 62 °C (constant volume

and moles)?

STEP 1 Organize the data in a table of initial and final

conditions.

Volume and moles remain constant.


Karen C. Timberlake


Calculations Using Gay-Lussac’s Law

STEP 2 Rearrange to solve for unknown quantity P2.

Solve Gay-Lussac’s law for P2:

STEP 3 Substitute the values into the gas law equation

and calculate.

×

× ××


Karen C. Timberlake


Study Check

A gas has a pressure of 645 Torr at 128 °C.

What is the temperature in Celsius if the pressure increases to 824 Torr (V and nremain constant)?


Karen C. Timberlake


Solution

A gas has a pressure of 645 Torr at 128 °C. What is the

temperature in Celsius if the pressure increases to 824 Torr(V and n remain constant)?


Volume and moles remain constant.


Karen C. Timberlake


Solution

A gas has a pressure of 645 Torr at 128 °C. What is the

temperature in Celsius if the pressure increases to 824 Torr(V and n remain constant)?

STEP 2 Rearrange to solve for unknown quantity T2

Solve Gay-Lussac’s law for T2:

STEP 3 Substitute the values into the gas law equation

and calculate.

×

× -


Karen C. Timberlake


Vapor Pressure and Boiling Point

When liquid molecules with sufficient kinetic energy break away from the surface of a liquid, they become a vapor.

• In an open container, all the liquid will eventually evaporate.• In a closed container, the vapor accumulates and creates

pressure called vapor pressure.

A liquid• exerts its own vapor pressure at a given temperature.• boils when its vapor pressure becomes equal to the external

pressure.


Karen C. Timberlake


Altitude and Boiling Point

At high altitudes,• atmospheric pressure is

lower than 1 atm, 760 Torr.• the boiling point of water is

lower than 100 °C.

In a closed container, such as a pressure cooker, • a pressure greater than

1 atm, 760 Torr, can be obtained.

• water boils at a higher temperature than 100 °C.

Pressure and the boiling point of water.


Karen C. Timberlake


Vapor Pressure and Boiling Point


Karen C. Timberlake


Study Check

Explain why water boils at a lower temperature in the mountains than at sea level.


Karen C. Timberlake


Solution

Explain why water boils at a lower temperature in the mountains than at sea level.

Atmospheric pressure in the mountains is less than at sea level. The vapor pressure of the water reaches the atmospheric pressure at a lower temperature.


Karen C. Timberlake


8.5 The Combined Gas Law

Under water, the pressure on a

diver is greater than the atmospheric pressure.

The combined gas law comes from the pressure–volume–temperature relationships for gases that we have studied.

Learning Goal Use the combined gas law to calculate the final pressure, volume, or temperature of a gas when changes in two of these properties are given and the amount of gas is constant.


Karen C. Timberlake


The Combined Gas Law

The combined gas law uses the pressure–volume–

temperature relationships from Boyle’s law, Charles’s law, and Gay-Lussac’s law where n is constant.


Karen C. Timberlake


Calculations Using Combined Gas Law

A gas has a volume of 675 mL at 35 °C and 646 mmHg

pressure. What is the volume (mL) of the gas at −95 °C and a

pressure of 802 mmHg (n is constant)?


Moles of gas remain the same.

ANALYZE Conditions 1 Conditions 2

THE P1 = 646 mmHg P2 = 802 mmHg

PROBLEM V1 = 675 mL V2 = ?

T1 = 35 °C + 273 T2 = −95 °C + 273

= 308 K = 178 K


THE P1 = 646 mmHg P2 = 802 mmHg

PROBLEM V1 = 675 mL V2 = ?

T1 = 35 °C + 273 T2 = −95 °C + 273

= 308 K = 178 K


Karen C. Timberlake


Calculations Using Combined Gas Law

STEP 2 Rearrange to solve for unknown quantity V2.


× ×

× ×


Karen C. Timberlake


Study Check

A sample of helium gas has a volume of 0.180 L, a pressure of 0.800 atm, and a temperature of 29 °C. At what temperature (°C) will the helium have a volume of 90.0 mL and a pressure of 3.20 atm(n remains constant)?


Karen C. Timberlake


Solution

A sample of helium gas has a volume of 0.180 L, a pressure of

0.800 atm, and a temperature of 29 °C. At what temperature

(°C) will the helium have a volume of 90.0 mL and a pressure

of 3.20 atm (n remains constant)?


Moles of gas remain the same.


THE P1 = 0.800 atm P2 = 3.20 atm

PROBLEM V1 = 0.180 L (180 mL) V2 = 90.0 mL

T1 = 29 °C + 273 T2 = ?

= 302 K


THE P1 = 0.800 atm P2 = 3.20 atm

PROBLEM V1 = 0.180 L (180 mL) V2 = 90.0 mL

T1 = 29 °C + 273 T2 = ?

= 302 K


Karen C. Timberlake


Solution





STEP 2 Rearrange to solve for unknown quantity T2.

× ×


Karen C. Timberlake


Solution






××

-


Karen C. Timberlake


8.6 Volume and Moles, Avogadro’s Law

The molar volume of a gas at STP is about the same as the volume of three basketballs.

The volume of 1 mole of gas is 22.4 liters.

Learning Goal Use Avogadro’s law to calculate the amount or volume of a gas when the pressure and temperature are constant.


Karen C. Timberlake


Avogadro’s Law: Volume and Moles

In Avogadro’s law,

• the volume of a gas is directly related to the number of moles (n) of gas.

• T and P are constant.


Karen C. Timberlake


Calculations Using Avogadro’s Law

If 0.75 mole of helium gas occupies a volume of 1.5 L, what

volume (L) will 1.2 moles of helium occupy at the same temperature and pressure?

A. 0.94 L

B. 1.8 L

C. 2.4 L


Karen C. Timberlake



If 0.75 mole of helium gas occupies a volume of 1.5 L, what

volume (L) will 1.2 moles of helium occupy at the same temperature and pressure?


Pressure and temperature remain constant.


Karen C. Timberlake



STEP 2 Rearrange to solve for unknown quantity V2.


The answer is C.

×

×


Karen C. Timberlake


Standard Temperature and Pressure

The volumes of gases can be compared at STP,

Standard Temperature and Pressure, when they have

• the same temperature.

• a standard temperature (T) of 0 °C or 273 K.

• the same pressure.

• a standard pressure (P) of 1 atm (760 mmHg).


Karen C. Timberlake


Molar Volume, STP

At standard temperature and pressure (STP), 1

mole of a gas occupies a volume of 22.4 L, which

is called its molar volume.

Use this equality as a conversion factor for gas

at STP:

22.4 L = 1 mole gas


Karen C. Timberlake


Molar Volume

Avogadro’s law indicated that1 mole of any gas at STP has a volume of 22.4 L.


Karen C. Timberlake


Guide to Using Molar Volume


Karen C. Timberlake


Calculations Using Molar Volume

What is the volume occupied by 2.75 moles of N2 gas at STP?

STEP 1 State the given and needed quantities.


STEP 2 Write a plan to calculate the needed quantity.

Moles N2 Volume N2


Karen C. Timberlake


Calculations Using Molar Volume

STEP 3 Write conversion factors including 22.4 L/mole at STP.

At STP, 22.4 L = 1 mole N2.

STEP 4 Set up the problem with factors to cancel units.

×


Karen C. Timberlake


Study Check

1. What is the volume at STP of 4.00 g of CH4?

A. 5.60 L B. 11.2 L C. 44.8 L

2. How many grams of He are present in 8.00 L of gas at STP?

A. 25.6 g B. 0.357 g C. 1.43 g


Karen C. Timberlake


Solution

STEP 1 State given and needed quantities.



2. How many grams of He are present in 8.00 L of gas at STP? Pressure and temperature remain constant.

ANALYZE Given Need

THE 4.00 grams CH4(g) volume CH4(g)

PROBLEM at STP at STP

ANALYZE Given Need

THE 4.00 grams CH4(g) volume CH4(g)

PROBLEM at STP at STP


Karen C. Timberlake


Solution

STEP 2 Write a plan to calculate the needed quantity.


mass CH4 moles CH4 volume CH4

2. How many grams of He are present in 8.00 L of

gas at STP?

volume He moles He mass He


Karen C. Timberlake


Solution

STEP 3 Write conversion factors including 22.4 L/mole

at STP.


22.4 L = 1 mole of CH4

1 mole of CH4 = 16.05 g CH4


Karen C. Timberlake


Solution

STEP 3 Write conversion factors including 22.4 L/mole

at STP.


22.4 L = 1 mole of He

1 mole of He = 4.00 g He


Karen C. Timberlake


Solution

STEP 4 Set up the problem with factors to cancel units.


The answer is A, 5.60 L


The answer is C, 1.43 g.

× ×

××


Karen C. Timberlake


8.7 Ideal Gas Law

When camping, butane is

used as a fuel for a portable gas stove. Given the pressure, volume, and temperature of the gas in the tank, we can use the ideal gas law equation to determine the amount of gas present.

Learning Goal Use the ideal gas law equation to solve for P, V, T, or n of a gas when given three of the four values in the ideal gas law equation. Calculate mass or volume of a gas in a chemical reaction.


Karen C. Timberlake


The Ideal Gas Law

The ideal gas law is the combination of the four

properties used in the measurement of a gas—

pressure (P), volume (V), temperature (T), and

amount of a gas (n)—to give a single expression,

which is written as

Ideal Gas Law PV = nRT

Core Chemistry Skill Calculating Mass or Volume

of a Gas in a Chemical Reaction


Karen C. Timberlake


R, Ideal Gas Constant

Rearranging the ideal gas law equation shows

that the four gas properties equal a constant, R.

• To calculate the value of R, we substitute the STP conditions (273 K, 1 atm) for molar volume into the expression: 1 mole of gas = 22.4 L at STP.

• Real gases show some deviations in behavior; however, the ideal gas law closely approximates the behavior of real gases at typical conditions.


Karen C. Timberlake


R, Ideal Gas Constant

The value for the ideal gas constant, R, is 0.0821 L

atm per mole K.

• If we use 760 mmHg for the pressure, we obtain

another useful value for R of 62.4 mmHg per

mole K.

• In working problems using the ideal gas law, the

units of each variable must match the units in the

R you select.


Karen C. Timberlake


Guide to Using the Ideal Gas Equation


Karen C. Timberlake


Study Check

Dinitrogen oxide, N2O, which is used in dentistry, is

an anesthetic also called laughing gas. What is the

pressure, in atmospheres, of 0.350 mole of N2O at 22 °C in a 5.00-L container?


Karen C. Timberlake


Solution






Karen C. Timberlake


Solution




STEP 2 Rearrange the ideal gas law equation to

solve for the needed quantity.


Karen C. Timberlake


Solution




STEP 3 Substitute the gas data into the equation

and the needed quantity.

× ×


Karen C. Timberlake


R, Unit Summary for Ideal Gas Constant


Karen C. Timberlake


Gas Laws and Chemical Reactions

Gases are involved as reactants and products in many chemical reactions.

Typically, the information given for a gas in a reaction is its pressure (P), volume (V), and temperature (T).

We can use the ideal gas law equation to determine

• the moles of a gas in a reaction if we are given the number of moles for one of the gases in a reaction.

• the moles of any other substance using a mole–mole factor.


Karen C. Timberlake


Guide to Using the Ideal Gas Law for Reactions


Karen C. Timberlake


Study Check

Nitrogen gas reacts with hydrogen gas to produce

ammonia (NH3) gas. How many liters of NH3 can be produced at 0.93 atm and 24 °C from a 16.0-g

sample of nitrogen gas and an excess of hydrogen

gas? N2(g) + 3H2(g) � 2NH3(g)


Karen C. Timberlake


Solution

Nitrogen gas reacts with hydrogen gas to produce ammonia

(NH3) gas. How many liters of NH3 can be produced at 0.93 atm and 24 °C from a 16.0-g sample of nitrogen gas and

an excess of hydrogen gas? N2(g) + 3H2(g) � 2NH3(g)



Karen C. Timberlake


Solution




STEP 2 Write a plan to convert the given quantity to the needed moles.

grams Molar moles Mole−mole moles litersof N2 mass of N2 factor of NH3 of NH3

Ideal

gas law


Karen C. Timberlake


Solution




STEP 3 Write the equalities for molar mass and mole–mole factors.


Karen C. Timberlake


Solution




STEP 4 Set up the problem to calculate moles of needed quantity.

× ×


Karen C. Timberlake


Solution




STEP 5 Convert moles of needed quantity to mass or volume using the molar mass or ideal gas law equation.

××


Karen C. Timberlake


8.8 Partial Pressure (Dalton’s Law)

Our cells continuously use

oxygen and produce carbon dioxide.

Both gases move in and out of the lungs through the membranes of the alveoli, the tiny air sacs at the ends of the airways in the lungs.

Learning Goal Use Dalton’s law of partial pressures to calculate the total pressure of a mixture of gases.


Karen C. Timberlake


Partial Pressure

The partial pressure of a gas is the pressure that each gas

in a mixture would exert if it were by itself in the container.

Core Chemistry Skill Calculating Partial Pressure


Karen C. Timberlake


Dalton’s Law of Partial Pressures

Dalton’s law of partial pressures indicates that

• pressure depends on the total number of gas

particles, not on the types of particles.

• the total pressure exerted by gases in a mixture is

the sum of the partial pressures of those gases.

PT = P1 + P2 + P3 + ....


Karen C. Timberlake


Total Pressure

For example, at STP, 1 mole of a pure gas in a volume

of 22.4 L will exert the same pressure as 1 mole of a gas mixture in 22.4 L.

Gas mixtures

1.0 mole N2

0.4 mole O2

0.6 mole He

1.0 mole

0.5 mole O2

0.3 mole He

0.2 mole Ar

1.0 mole

1.0 atm1.0 atm 1.0 atm


Karen C. Timberlake


Total Pressure

The air we breathe

• is a mixture of different gases.• contains mostly N2 and O2, and contains small amounts of

other gases.What we call the atmospheric pressure is actually the sum of the partial pressures of the gases in the air


Karen C. Timberlake


Guide to Solving for Partial Pressure


Karen C. Timberlake


Solving for Partial Pressure

A scuba tank contains O2 with a pressure of 0.450 atm and He

at 855 mmHg. What is the total pressure in mmHg in the tank

(volume and temperature are constant)?

STEP 1 Write the equation for the sum of the partial pressures.

Ptotal = PO2+ PHe

STEP 2 Rearrange the equation to solve for the unknown pressure. Convert units to match.

Ptotal = PO2+ PHe

×


Karen C. Timberlake


Solving for Partial Pressure

A scuba tank contains O2 with a pressure of 0.450 atm and He

at 855 mmHg. What is the total pressure in mmHg in the tank

(volume and temperature are constant)?

STEP 3 Substitute known pressures and calculate the unknown partial pressure.

Ptotal = PO2 + PHe

Ptotal = 342 mmHg + 855 mmHg

= 1.20 x 103 mmHg×


Karen C. Timberlake


Study Check

For a deep dive, a scuba diver uses a mixture of

helium and oxygen with a pressure of 8.00 atm. If the

oxygen has a partial pressure of 1280 mmHg, what is

the partial pressure of the helium (volume and

temperature are constant)?

A. 520 mmHg

B. 2040 mmHg

C. 4800 mmHg


Karen C. Timberlake


Solution

For a deep dive, a scuba diver uses a mixture of helium and

oxygen with a pressure of 8.00 atm. If the oxygen has a partial pressure of 1280 mmHg, what is the partial pressure of the helium (volume and temperature are constant)?

STEP 1 Write the equation for the sum of the partial pressures.

Ptotal = PO2+ Phe

STEP 2 Rearrange the equation to solve for the unknown pressure. Convert units to match.

PHe = Ptotal − PO2 ×


Karen C. Timberlake


Solution

For a deep dive, a scuba diver uses a mixture of helium and

oxygen with a pressure of 8.00 atm. If the oxygen has a partial pressure of 1280 mmHg, what is the partial pressure of the helium (volume and temperature are constant)?

STEP 3 Substitute known pressures and calculate the unknown partial pressure.

PHe = 6080 mmHg – 1280 mmHg

= 4800 mmHg or 4.80 × 103 mmHgThe answer is C, 4800 mm Hg.


Karen C. Timberlake


Chemistry Link to Health: Blood Gases

• In the lungs, O2 enters the

blood, while CO2 from the blood is released.

• In the tissues, O2

enters the cells, which releases CO2 into the blood.


Karen C. Timberlake


Chemistry Link to Health: Blood Gases

In the body,

• O2 flows into the tissues because the partial

pressure of O2 is higher in blood and lower in

the tissues.

• CO2 flows out of the tissues because the partial

pressure of CO2 is higher in the tissues and

lower in blood.


Karen C. Timberlake


Chemistry Link to Health:Partial Pressures in Blood

Partial Pressures in Blood and Tissue


Karen C. Timberlake


Gas Exchange During Breathing

chapter 8 gases - chemistry121.pbworks.com

Documents