Chapter 9. Gases. Gases and Gas Pressure. Gases – constituent atoms and molecules that have little attraction for one another Free to move in available volume Some properties of gases Mixtures are always homogenous Very weak attraction between gas molecules
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Pressure – force exerted per unit area
SI unit equals Pascal (Pa)
1 Pa = 1 N/m2 (1 N = 1 (kg•m)/s2)
Millimeters of mercury (mmHg)
1.0 atm = 760 mmHg = 101, 325 Pa
1.0 atm = 760 torr
nnumber of moles
PV = k
(constant n and T)
A sample of helium gas has a pressure of 3.54 atm in a container with a volume of 23.1 L. This sample is transferred to a new container and the pressure is measured to be 1.87 atm. Assume constant temperature.
(constant n and P)
(constant T and P)
PinitialVinitial = PfinalVfinal
R = 0.082058
R = 0.082058
Ideal Gas Law:
PV = nRT
R is the gas constant and is the same for all gases.
T = 0 °C (273.15 K)
Standard Temperature and Pressure (STP) for Gases
P = 1 atm
What is the volume of 1 mol of gas at STP?
= 22.414 L
2Na(s) + 3N2(g)
P4(s) + 6 H2(g) 4H3(g)
What is the amount of P4 is required to react with 5.39 L of hydrogen gas at 27.0oC and 1.25 atm?
can predict P, V or T when condition is changed
Moles of component
Total moles in mixture
where P1, P2, ….refer to the pressure of the individual gases in the mixture
Mole Fraction (X) =
1.Total pressure depends on the total molar amount of gas present
Ptotal = ntotal (RT/V)
A.Model that can explain the behavior of gases
1.A gas consists of particles in constant random motion
2.Most of the volume of a gas is empty spaces
3.The attractive and repulsive forces between molecules of gases are negligible
4.The total kinetic energy of the gas particles is constant at constant T
5.Average Ek α T
Effusion: The escape of a gas through a pinhole into a vacuum without molecular collisions.
The volume of a real gas is larger than predicted by the ideal gas law.
Attractive forces between particles become more important at higher pressures.
van der Waals equation
Correction for intermolecular attractions.
V - n
Correction for molecular volume.
Assume that you have 0.500 mol of N2 in a volume of 0.600L at 300K. Calculate the pressure in the atmosphere using both the ideal gas law and the van der Waals equation. For N2, a = 1.35 (L2·atm)mol2, and b = 0.0387 L/mol