What is the combined gas law? What do each of the variables stand for? What units do the variables need to be in? HINT: Check in your notes from Friday!

Warm-up

• What is the combined gas law?• What do each of the variables stand for?• What units do the variables need to be in?•HINT: Check in your notes from Friday!

The Combined Gas LawUnit 8, Day 6Kimrey2 December 2012

Gas Law

• P = Pressure• V = Volume• T = Temperature

• 1 shows the initial conditions• 2 shows the final conditions

Units• Temperature must be in Kelvin•Degrees Celsius plus 273 = Temperature in Kelvin• You don’t even have to remember this! The reference

tables show that 0 °C is the same as 273K• Pressure can be in whichever unit you want, as

long as you keep it in that unit (this means the unit of P1 must be the same as the unit of P2)!• Volume must be the same on each side. • MAKE SURE YOU’RE PAYING ATTENTION TO THE

UNITS!

Example• A gas at 285 K, 5 atm, and 25 mL is cooled to 225 k and a

volume of 15 mL. What is the new pressure?

• 6.579 atm

The three laws• If you hold one of the three variables constant, then we can

see how a change of one will effect another.

• You don’t have to remember these three, but it is important to understand the relationships between the variables!

Volume is held constant

• Pressure and temperature are directly proportional.• This means that, if volume is held constant and

pressure is increased, temperature will increase as well.

•

Temperature is held constant

• Volume and pressure are inversely related. • This means that, if temperature is held

constant and pressure goes up, volume goes down.

• P1V1=P2V2

Pressure is held constant

• Volume and temperature are directly proportional.• This means that, when pressure is held

constant, increasing volume will also increase temperature

•

Example

• A gas at 335 K and 1 atm is cooled to 285 K. What is the new pressure?

Example

• A gas at 4.00 atm and in a 20 mL container is moved to a container with a volume of 30 mL. What is the new pressure?

Example

• A gas at 305 K and 35 mL is heated to 400 k. What is its new volume?

Warm Up

• I have an unknown volume of gas held at a temperature of 115 K in a container with a pressure of 60 atm. If by increasing the temperature to 225 K and decreasing the pressure to 30 atm causes the volume of the gas to be 29 liters, how many liters of gas did I start with?

The Ideal Gas LawUnit 8, Day 6Kimrey2 December 2012

A Bit More About Gases

• Remember gases are really tiny particles bouncing off of everything. • These collisions are perfectly elastic. Meaning

that no energy is lost as they collide. • It’s because of this that the combined gas law

works (P1V1/T1 = P2V2/T2).

Adding more gas

• So what happens if we add more of a gas?• This can be accounted for by the ideal gas law.

Ideal Gas Law

• PV=nRT• P= Pressure• V= Volume• n= moles• R= gas constant• T= Temperature

Units

• Temperature must be in Kelvin.• Volume must be in liters (Remember: King Henry

Died by drinking chocolate milk)• Pressure can be in atm, mmHg, or kPa BUT you

must use the appropriate R for whatever units of pressure you have. • R is a constant and can be: (three options are

found on the reference tables)

Converting to Kelvin• Temperature can also be measured in a unit

known as Kelvin. •Degrees Celsius plus 273 = Temperature in Kelvin• You don’t have to remember this! The reference

tables show that 0 °C is the same as 273K

• 45°C is what in Kelvin?• 5°C is what in Kelvin?

Example

• How many moles of a gas at 100°C does it take to fill a 1.00L flask to a pressure of 1.50 atm?

Example

• If I have 4 moles of a gas at a pressure of 5.6 atm and a volume of 12 liters, what is the temperature?

Example

• If I contain 3 moles of gas in a container with a volume of 60 liters and at a temperature of 400 K, how many atmospheres of pressure are inside the container?

Example

• If I have an unknown quantity of N2 held at a temperature of 1195 K in a container with a volume of 25 liters and a pressure of 560 atm, how many grams of N2 do I have?