AP CHEMISTRY MIDTERM REVIEWTOPIC: GASES

Savannah SnyderMichael Kleber

4th Period

Converting Pressures with Different Units of Measurement

1 atm = 760 mmHg = 760 torr (The unit in which you are converting FROM must

cancel out)

Example: Convert 30 atm to torr.30 atm X 760 torr 1 atm

= 22800 torr

Converting Units of Temperature

ºC to K: Add 273K to ºC: Subtract 273

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Differentiating between Ideal Gases and Non-Ideal Gases

Basic points of the kinetic theory of gases (Ideal Gas)

1. The volume of gas molecules is negligible (not important) compared to the volume of space in which they move.

2. Gas molecules move in a straight line between frequent collisions with the walls of the container and themselves.

3. All collisions are elastic (no energy is lost).4. No intermolecular forces are present.5. The average kinetic energy of the gas molecules

is directly proportional to the temperature.

http://www.dynamicscience.com.au

Examples of Questions Pertaining to Ideal Gas.

-When does a gas behave like a real gas?

  Low Temperature and High Pressure.    

-When would a gas behave like an ideal gas? High Temperature and Low Pressure

The Ideal Gas LawAs we have learned from our previous year in Chemistry as well as reviewed this year,

PV=nRT R=.0821 (gas constant)

When solving for one specific component of the ideal gas law the equation can be algebraically manipulated to accommodate the equation’s

need.

Examples Using Ideal Gas Law:Algebraically manipulating the law:

To solve for the pressure (not given) we simply divide both sides by volume (V)

PV=nRT

Final manipulating equation:

V V

P = nRT

V

An 18 liter container holds 16.00 grams of oxygen gas (O2) at 45 °C. What is the pressure in the container?A.  11.0 atm B.  23.2 atm C. 1.45 atm D.0.725 atm

Answer:D

REVIEW QUESTION CHECKPOINT 1

Boyle’s LawThis law describes the relationship between

the volume and the pressure of a gas when the temperature and amount of moles are constant

This equation is helpful when determining what a new pressure would be if the volume is decreased/increased (same for pressure)

A balloon with a volume of 2.0 L is filled with a gas at 3 atmospheres. If the pressure is reduced to 0.5 atmospheres without a change in temperature, what would be the volume of the balloon?

REVIEW QUESTION CHECKPOINT 2

The volume of the balloon will expand to 12 L.

Charles’s Law

This law describes the volume and temperature relationship of a gas when the pressure and amount are constant.

A 600 mL sample of nitrogen is heated from 27 °C to 77 °C at constant pressure. What is the final volume?

REVIEW QUESTION CHECKPOINT 3

The final volume after heating will be 700 mL.

Gay-Lussac’s LawThis law describes the relationship

between the pressure of a gas and it’s temperature IF the volume and amount of moles are held constant.

-As a gas is heated, particles move with greater kinetic energy causing the pressure to increase.

A 20 L cylinder containing 6 atm of gas at 27 °C. What would the pressure of the gas be if the gas was heated to 77 °C?

REVIEW QUESTION CHECKPOINT 4

The pressure will increase to 7 atm after heating the gas from 27 °C to 77 °C.

Combined Gas Law

If the number of moles of gas are constant (No gas can get in or out) then we can combine the previous three gas laws.

A cylinder contain a gas of volume 30 L, at a pressure of 340 mmHg and a temperature of 420 K. Find the temperature of the gas which has a volume 40. L at a pressure of 410 mmHg.

REVIEW QUESTION CHECKPOINT 5

The temperature of the gas will be 670 K

Video of Gas Laws

Gas Laws Experiments (6:39)

Ideal Gas Law Lecture (8:45)

Graham’s Law of EffusionDefines the relationship of the speed of a

gas diffusion and the gases’ molecular mass.

The lighter the gas, the faster is its rate of effusion.

How much faster does hydrogen escape through a porous container than sulfur dioxide?(Lighter gases will travel faster)

REVIEW QUESTION CHECKPOINT 6

Hydrogen will escape 5.66 times faster.

Root Mean Square Speed

vrms is the root mean square of the speed Mm is the molar mass of the gas in grams per mole R is the molar gas constantT is the temperature in kelvin

What is the root mean square speed of the molecules in a sample of oxygen gas weighing 30 kG at 0 °C?

REVIEW QUESTION CHECKPOINT 7

The speed of the oxygen gas will be 1.497

Calculating Kinetic Energy:

The equation K.E. is used when expressing the kinetic energy per mol of gas.

The equation RT is used represent the average kinetic energy per molecule.

-Questions on this topic are used heavily to trick students who over think. I.E. Will two gasses kinetic energies be equal or not equal if the mass of one is double the other.

K.E.

Two samples of gases are placed in identical containers. Both gases are at STP. The first gas weighs 32 g/mol and the second weighs 40 g/mol. Which gas will have a higher kinetic energy?

REVIEW QUESTION CHECKPOINT 8

The kinetic energies are equal.

Dalton’s Law of Partial Pressures

Dalton’s Law states that in a mixture of gases the total pressure is simply the sum of the partial pressures.

Where….Calculating the mole fraction:XA= Moles A Total Moles

To calculate the actual partial pressure (In atm):

P1= Ptotal X (XA)

For a deep descent, a scuba diver uses a mixture of helium and oxygen with a total pressure of 8.00 atm. If the oxygen has a partial pressure of 1280 mmHg, what is the partial pressure of the helium?

REVIEW QUESTION CHECKPOINT 9

The partial pressure of helium is 4800 mmHg

Questions ?

Sources www.apcentral.collegeboard.com www.sciencegeek.net/APChemistry www.chemmybear.com 5 Steps to a 5 AP Chemistry, 2012-

2013 Edition (5 Steps to a 5 on the Advanced Placement Examinations Series)

Richard H. Langley (Author), John Moore (Author)

Self Study: Free Response Question One