Gas LawsGas LawsChemistry ClassChemistry Class

ObjectivesObjectives

Define Define absolute zeroabsolute zero Convert °C to KConvert °C to K Solve problems involving Solve problems involving

temperature, pressure, and temperature, pressure, and volume changesvolume changes

The gas lawsThe gas laws

As you saw in yesterday’s As you saw in yesterday’s activity on the computers there activity on the computers there is a relationship between is a relationship between pressure, temperature and pressure, temperature and number of particles.number of particles.

We’ll look at this more today.We’ll look at this more today. Let’s review what we saw Let’s review what we saw

yesterday..yesterday..

Absolute zeroAbsolute zero

Absolute zero is the lowest possible Absolute zero is the lowest possible temperature that a material can temperature that a material can reach.reach.

How does molecular motion relate How does molecular motion relate temperature?temperature?

What molecular motion would you What molecular motion would you predict at absolute zero?predict at absolute zero?

Absolute zeroAbsolute zero

At absolute zero there is no At absolute zero there is no molecular motion.molecular motion.

No substance can ever reach No substance can ever reach absolute zero.absolute zero.

Why?Why?

Absolute Zero

-273.15 °C

Degrees KelvinDegrees Kelvin

Absolute zero is defined as 0 Kelvin Absolute zero is defined as 0 Kelvin ((K).K).

To convert from K to °C use the To convert from K to °C use the conversion factor belowconversion factor below

K = °C + 273K = °C + 273

Sample conversionSample conversion

1.1. Convert 0 Convert 0 °C to K°C to K

2.2. Convert 84 °C to KConvert 84 °C to K

3.3. Convert -13 °C to KConvert -13 °C to K

4.4. Convert 84 K to °CConvert 84 K to °C

The combined gas lawThe combined gas law

We discussed the combined gas law We discussed the combined gas law earlier in this lecture. Now we’ll earlier in this lecture. Now we’ll learn the RULES for the equation.learn the RULES for the equation.

The combined gas law is presented The combined gas law is presented belowbelow

The rules!!!The rules!!!

1.1. Whenever you solve a combined Whenever you solve a combined gas law problem you MUST be sure gas law problem you MUST be sure that the units of pressure and that the units of pressure and volume are the volume are the samesame on on bothboth sides of the equals sign.sides of the equals sign.

2.2. TEMPERATURE MUST ALWAYS TEMPERATURE MUST ALWAYS

BE IN UNITS OF BE IN UNITS OF K!!!K!!!

Sample problemSample problem

A sample of oxygen gas has a volume of 300.0 mL A sample of oxygen gas has a volume of 300.0 mL when its pressure is 0.947 atm. What will the when its pressure is 0.947 atm. What will the volume of the gas be at a pressure of 0.987 atm if volume of the gas be at a pressure of 0.987 atm if the temperature remains constant?the temperature remains constant?

Sample problemSample problem A sample of oxygen gas has a volume of 300.0 mL A sample of oxygen gas has a volume of 300.0 mL

when its pressure is 0.947 atm and its temperature when its pressure is 0.947 atm and its temperature is 50 °Cis 50 °C . What will the temperature of the gas be . What will the temperature of the gas be at a pressure of 0.987 atm if the volume is at a pressure of 0.987 atm if the volume is decreased to 200.0 mL?decreased to 200.0 mL?

Break into your Break into your groups and solve groups and solve some problemssome problems

HAND STUFF BACKHAND STUFF BACK

Avogadro’s lawAvogadro’s law

Avagadro found that for a gas at constant Avagadro found that for a gas at constant temperature and pressure, the volume is temperature and pressure, the volume is directly proportional to the number of directly proportional to the number of moles of gas.moles of gas.

This is written asThis is written as

2

2

1

1 VV ORK

V OR KV

nnnn

Where V is volume and n is the number Where V is volume and n is the number of moles, and a is a constant.of moles, and a is a constant.

RamificationsRamifications

It may seem strange but all gasses It may seem strange but all gasses occupy the same volume if they are occupy the same volume if they are under the same pressure and under the same pressure and temperature.temperature.

UFUF66 has a molar mass of over 300 has a molar mass of over 300 g/mol yet a mole of it occupies the g/mol yet a mole of it occupies the same volume as a mole of Hsame volume as a mole of H22 which which has a molar mass of just over 2 g/mol. has a molar mass of just over 2 g/mol. We’ll get into how later. We’ll get into how later.

The ideal gas lawThe ideal gas law

We’ll look at the laws that we’ve studied We’ll look at the laws that we’ve studied so far:so far:

Kn

V :Law sAvogadro'

KT

V :Law sCharle'

KPV :Law sBoyle'

nRTPV

orK nT

PV

:law gas Ideal

R is the gas R is the gas constantconstant

The ideal gas law RULESThe ideal gas law RULES

1.1. Volume must be in LITERSVolume must be in LITERS

2.2. Pressure must be in ATMPressure must be in ATM

3.3. n must be in MOLn must be in MOL

4.4. R = 0.0821R = 0.0821

5.5. Temperature must be in KelvinTemperature must be in Kelvin