Behavior of Gas MoleculesBehavior of Gas Molecules

IDEAL GAS LAW AND GAS STOICHIOMETRY

Essential Questions

Since we ignore the volume and mass of gas molecules, can we assume anything about the volume all gases occupy?

Can we apply this information to solve for the three properties of gases: temperature, volume, and pressure?

Can we apply this to solve stoichiometry problems?

How can we increase volume

Quick: What are all of the ways you can think of to increase the volume of a balloon?

Avogadro’s Principle

Even though particles that make up gases vary greatly in size, KMT tells us to ignore particle size when talking about the volume a gas occupies.

Therefore, we can say 1000 krypton gas particles occupies the same volume as 1000 helium particles!

Avogadro first proposed this idea in 1811

Avogadro’s Principal

States that the same number of particles at the same temperature and pressure contain equal volumes of gases!

1 mol contains 6.02 x 1023 particles STP is Standard Temperature and Pressure

273K and 1.00 atm pressure (STP)

Avogadro’s Law states that one mole of any gas at Standard Temperature and Pressure (STP) occupies 22.4 Liters of volume

Therefore we can use 22.4 L/mol as a conversion factor

Molar Volume Conversions

1. Find the number of moles in a sample of gas that has a volume of 5.32 L at STP? Use the molar volume to convert

from volume to moles

5.32 L X 1 mol = 0.238 mol 22.4 L

Molar Volume Conversions

2. The main component of natural gas used for home heating and cooking is methane (CH4). Calculate the volume that 2.00kg of methane gas will occupy at STP.

Answer: 2,800 liters

Check for Understanding

Try some molar volume problems on your own

IDEAL GAS LAW

Now that we know that one mole of all gases occupy the same volume, we have a NEW Gas Law to introduce

While the Combined Gas Law deals with the relationship of gas properties under changing conditions

The Ideal Gas Law uses the basis of that relationship and applies it to a known set of conditions of those properties and the amount of gas that is present using Avogadro’s Law.

Ideal vs. Real GasIdeal vs. Real Gas

Ideal Gas Motion is totally

random and linear All gas particle

collisions are elastic Gas particles do not

have any intermolecular attraction

Gas particles have no volume

Real Gas Motion is almost but not

quite random Gas particle collisions

are not perfectly elastic Gas particles

experience very small intermolecular attraction

Gas particles do have small amount of volume

Ideal Gas Law

Known as the Ideal Gas Law because it works best for gases that obey the assumptions of the kinetic molecular theory!

P V = n R T 

We know what these stand for:P = PressureV = Volumen = Number of molesT = Temperature (in Kelvin)

But, what is R?

Notice there are no 1’s

and 2’s because the conditions are NOT

changing!

UNIVERSAL GAS CONSTANT - R

By selecting the values for standard conditions, the value of the constant can be determined:

Standard value for Temperature = 273K Standard value for Pressure = 1 atm Standard value for the amount of gas = 1

mole Standard value for volume = 22.4L

Let’s solve for R: 

Ideal Gas Law Practice

1. Calculate the number of moles of ammonia gas (NH3) contained in a 3.0 L vessel at 300K with a pressure of 1.5 atm.

Ideal Gas Law

2. What volume will 2.0 moles of nitrogen occupy at 720 torr and 20oC?

remember that 1.0 atm = 760 torr and that temperature must be in Kelvins

Ideal Gas Law

3. Find the number of grams of CO2 that exert pressure of 785 torrs at a volume of 32.5 L and a temperature of 32oC.

STOP: work on completing Chem Worksheet 14-4

Ideal Gas Law Demo

Glass Florence flask with stopper with glass tube.

Add a small amount of water and bring it to boiling

Invert over an ice bath

Demo As we heat the flask, the temperature

increases and the pressure inside the flask increases. Water vapor replaces the air in the flask.

After inverting, the temperature rapidly decreases, converting the water vapor back into liquid water therefore decreasing the volume of the gases.

Water rushes in to try to increase the pressure to make up for the decrease in temperature and volume.

Check for understanding

Try some ideal gas law problems

BE LAZY

And don’t memorize the gas laws! Tyler Dewitt http://www.youtube.com/watch?v

=UKUmYU6Q1cA (7:09)

GAS STOICHIOMETRY

Avogadro’s Law allows for the application of volume ratio to replace mole ratio to compare any two gases in stoichiometric calculations.

You will see two different types of problems:1. Volume to volume problems2. Volume to mass problems

GAS STOICHIOMETRY

1. Volume to Volume: What volume of hydrogen is necessary to react with 5L of nitrogen to produce ammonia? (assume constant temperature and pressure)a) balance the equationb) volume to volume ratioc) what volume of ammonia is produce?

___N2 + ___H2 NH3

GAS STOICHIOMETRY

2. Volume to Volume: If 20 liters of oxygen are consumed in the above reaction, how many liters of carbon dioxide are produced?

a) Balance the equationb) volume to volume ratio

___C3H8 + ___O2 ___CO2 + ___H2O

GAS STOICHIOMETRY

3. Volume to mass problems: Ammonia is synthesize from hydrogen and nitrogen. If 5.0 L of nitrogen react completely with hydrogen at a pressure of 3.00 atm and temperature of 298 K, how much ammonia, in grams, is produced?

___N2 (g) + ___H2 (g) ___NH3 (g)

GAS STOICHIOMETRY

4. Volume to mass problem: When iron rusts, it undergoes a reaction with oxygen to form iron (III) oxide. Calculate the volume of oxygen gas at STP that is required to completely react with 55.9 grams of iron.

___Fe (s) + ___O2 (g) ___Fe2O3 (s)