The Gas Laws and Stoichiometry

Honors Option for Chemistry

Kinetic Model of Gases

The Big Variables are P, V and TP is created by molecules colliding with the walls of the container

V is the space allowed for the gas: remember gases are compressible

T is proportional to the average kinetic energy

Boyle’s Law

At a certain, fixed temperature the volume of a gas is proportional to the inverse of the pressure.

Charles’ Law

Under constant pressure, as the temperature of a gas increases so does the volume

Gay-Lussac’s Law

Under constant volume, the pressure and temperature are proportional

Avogadro’s LawEqual volumes of gases at the same temperature and pressure have the same number of molecules

Which Law is It?

Combined Gas Law

So, why struggle to remember all those people and laws whenWe can summarize themWith the combined gas law.

This law can be used to solveProblems with ANY of the Boyle-Charles-Gay-Lussac Conditions. Just leave out the Variable that is constant

And only 1 law to remember!

Using the Ideal Gas Law

What is an ideal gas? How do gases deviate from the ideal?

No gas is really ideal. However, this law gives good approximation of gas behavior. It does not work well when the temperature is low (near boiling point) or when the Pressure is high. Both of those conditions mean that the particles are very close Together and will experience forces between them. Molecules with high molar masses Also tent away from the ideal.

PV =nRT

The ideal gas law!This law uses the relationships from the combined law and add molar ratio

This is often used to find molar mass or densityRemember that molar mass is g/mole so g/nAnd density is mass/volumen/V x g/mole (molar mass)

Which Law to Use?P1V1 = P2V2

T1 T2

Conditions are changing

Example:

A piston (volume changes as it moves in and out)

A balloon moving up in the atmosphere (the outside P decreases)

PV = nRTConditions are constantBut one variable is unknown

Finding the number of moles of a gas at known temperature, volume and pressure (STP for one)

Finding any variable for a given chemical reaction

Stoichiometry Combined with Gas Laws

Example: For the reaction: 2H2 + O2 2H2O

How many liters of oxygen are needed to react with 2.56 moles of hydrogen at STP?

1. Find the moles of O2 needed. Use the molar ratio.

2.56 moles 1 mole = 1.28 moles O2

2 moles2. Change the moles to liters Sub into PV = nRT V = nRT/PV = (1.28 moles)(.08205 L atm/Kmol)(273K)/(1atm)V = 28.67 L; with sig figs 28.7 L

An unknown gas has a density of 0.89 g/ml at a pressure of 0.900 atm and temperature of 25 C.

What is the molar mass?1. Use PV = nRT molar mass = g/mol2. n/V = P/RT sub in3. n/V = (0.900 atm)/(0.08205 Latm/Kmol)(25 +

273 K)n/V = 1.0 mol/L4. We want g/mol for molar mass0.89 g x 1000 ml x 1 L = 890 g/mol ml 1 L 1.0 mol