Gases Part 2

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# Gases Part 2 - PowerPoint PPT Presentation

Gases Part 2. Standard Temp and Pressure, STP. For gases, chemists have defined a standard set of conditions: standard temperature and pressure or STP. STP is defined as 1.00 atm pressure and 0°C or 273.15K.

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### Gases Part 2

Standard Temp and Pressure, STP
• For gases, chemists have defined a standard set of conditions: standard temperature and pressure or STP.
• STP is defined as 1.00 atm pressure and 0°C or 273.15K.
• If a 1.00 mol sample of ANY gas is at STP, then the volume which this sample occupies is 22.414L.
• This means that at STP, we have another conversion factor: 1mol = 22.414L
• Problem: A 12.37L sample of gas is at STP. How many moles are in this sample?
Using PV=nRT to find density
• Density of gases is given as mass/volume or g/L.
• But in PV = nRT, this is very close to n/V or mol/volume.
• Then we have:
• But we really want g/L, not mol/L.
• How can we get from n in mol to g?
Using PV=nRT to find density
• We know that:

n = g/MW where MW is the molar mass (really mol. mass)

• Now we substitute in n = g/MW in the above to get:
• Problem: Find the density of helium at STP and at 30°C and 1.00atm.
Using PV=nRT to find MW
• This is similar to the above:
Dalton’s Law of Partial Pressures
• When we have a mixture of 2 or more gases, they act essentially independently of each other.
• This means that they each exert their own pressure, or partial pressure.
• Therefore, the total pressure of the gas mixture is equal to the sum of the partial pressures of the individual gases in the mixture.
• This is stated thus:
Dalton’s Law of Partial Pressures
• We also use mol fractions, Xi:
• If the partial pressure of water vapor is 23.756 torr at 25°C, what is the mol fraction of water if atmospheric pressure is 765 torr?
RMS Speed of Gas Particles
• For gas particles, we talk about the root-mean square speed (RMS speed) of particles instead of the average speed:
RMS Speed of Gas Particles
• What does this mean?
• That heavy gases move slower than light ones!
Graham’s Law of Effusion
• Effusion is when gas particles escape through pinholes.
• Diffusion is when gas particles mix throughout a container.
• The speed or rate of effusion is related to the molar mass or molecular weight as seen above.
Graham’s Law of Effusion
• More importantly, we can compare the rates of 2 gases:
Graham’s Law of Effusion
• Again, this tells us what we already knew (or would have guessed intuitively): lighter gases effuse faster.
• However, this difference in the rate of effusion is actually used to separate gases with different molecular weights.
Gas Stoichiometry
• We can use PV = nRT or the fact that at STP 1 mol = 22.414 L to solve gas stoichiometry problems.
• Let’s look at the rxn of hydrogen gas with oxygen gas to produce liquid water:
• How many g of water can be produced from 5.72 L of hydrogen gas at STP?
• If 17.9 g of water is produced at 25°C and 1.00 atm, how many liters of oxygen were consumed?
Deviations from the Ideal Gas Law
• The Ideal Gas Law, PV = nRT is based on some assumptions.

1) Gases have negligible volume!

• Wrong! Particularly for high pressures, the volume of gas particles may take up as much as 10% of the total volume of the container.
• This means that gas particles exert a greater pressure, or Preal > Pideal
Deviations from the Ideal Gas Law
• Next wrong assumption:

2) Gas particles have no interactions!

• Wrong! They do interact to some extent.
• When they do interact, the pressure decreases, or Preal < Pideal
• So these 2 factors tend to cancel each other out, and for pressures below around 4 atm, they pretty much do!
Deviations from the Ideal Gas Law
• So if the pressure is below 4 atm, we ignore deviations from ideal gas behavior.
• For intermediate pressures the gas particle interactions are more important, so Preal < Pideal.
• For high pressures the particles are closer and closer together, so the volume effect is much greater, or Preal > Pideal.
Deviations from the Ideal Gas Law
• These 2 deviation may be seen in the Van der Waals equation (don’t need to memorize or use):