Name ______
GASES
Properties of Gases
Molecules have lots of space between them
Low density
Compressible – why are gases compressible but solids & liquids are not?
Effectively there is no attractive force between gas molecules, this allows them to move freely/independently from one another.
Kinetic Molecular Theory
Gas molecules very small in relation to distance between them, so volume of gas molecules is negligible.
No attractive force between molecules this allows gases expand to fill the space of a container
Molecules in constant random motion, ie. they move independently of each other & collisions are perfectly elastic so no energy is lost
Energy of a gas is proportional to its temperature
Diffusion:mixing of gases to make a homogeneous mixture.
Units used for describing gases
Volume – L
Temperature – Kelvin
273K = 0oC
All temperatures must be in K. Add 273 to change oC to K.
Pressure – Pascals, mmHg, torr, atm
1 torr = 1 mmHg
760 torr = 1 atm
101.3 kPa = 1 atm
Practice if needed: Zumdahl Ch. 5 # 35, 36c
Average Ek vs velocity
If the Hulk & I run into you with the same amount of energy (force) what has to be true?
KE = ½ mv2
Temperature is a measure of the kinetic energy of molecules.
If 2 gases have the same temp they have the same KE.
Which has a larger velocity at the same temp, H2 or Xe?
Zumdahl Ch. 5 #99-102
Ideal Gas Law
PV = nRTR = 0.08206 L*atm/K*mol
Eg. 8 A balloon with 0.81 mol of gas at 300K has a pressure of 1.02 atm. What is the volume of the balloon?
Eg. 9 A sample of oxygen at 24.0oC and 745 torr was found to have a volume of 455mL. How many grams of O2 were in the sample?
Additional practice: Z Ch5 42-46, 49-51, 59,61,
STP for gases
Standard temperature & pressure
For gases STP means
T = 273K of 0oC
P = 1 atm
Avogadro’s Law: At STP 1 mol of any gas has a volume of 22.4 L.
At STP 1 mol = 22.4L
Gas stoichiometry
What mass of He is required to fill a 2.5 L balloon?
At STP, how many liters of oxygen at are needed to combine with exactly 1.50L of hydrogen to produce water?
A student uses the decomposition of NaCO3 to prepare CO2. How large should the flask be in order to hold all the CO2 if 1.25g of NaCO3 is used. The pressure of the CO2 will be 740 torr & final temperature will be 25.0oC.
Na2CO3 + 2HCl 2NaCl + CO2 + H2O
Practice: Z Ch. 5 63-65, 68
Calculating Molar Mass of Gases
A student collected a sample of gas in a 0.220 L gas collection tube until its pressure reached 0.757 atm at a temp of 25.0oC. The sample had a mass of 0.299 g. Calculate the molecular mass of the gas.
Density, MM & PV=nRT
A gaseous compound of phosphorus and fluorine was found to have a density of 3.50g/L at a temp of 25.0oC and pressure of 740 torr. Determine the molar mass of the gas.
Z Ch. 5 #75-77
Partial pressure of gases
The pressure of an individual gas does NOT change w/ the addition of other gases.
If the pressure of each gas in a container is known, then the sum of these pressures is the total pressure. Each gas adds a part to the pressure i.e. has a partial pressure.
Analogy: People pounding on a wall. How do we determine the partial pressure of my pounding on the desk?
Suppose you want to fill a pressurized tank having a volume of 4.00L with oxygen enriched air for use in diving, and you want the tank to contain 50.0 g of O2 and 150g of N2. What will the total pressure have to be at 25.0oC?
Now compare their pressure ratios.
PO2 = 9.54atmPN2 = 32.8atm
A mixture of gases has a PAr of 0.725 atm, PO2 of 0.210atm.
What is the mole fraction of each gas?
If the flask has a volume of 2.0L container & is at 32oC, then calculate the mass of each gas.
Practice: Z Ch.5 #79-82, 85, 86, 88
More possible practice: Zumdahl 5th ed. Ch. 5 handout #95,97-99,102,103