Chapter 5
Chapter 5
Properties of Gases
Properties of Gases
Three phases of matter
– solid
Definite shape and volume
– liquid
Definite volume, shape of
container
– gas
Shape and volume of
container
Chapter 5
Properties of Gases
A gas is a collection of molecules that are very far apart
on average.
Gases exert pressure on the objects in their
surroundings.
Pressure is caused by collisions between the gas
molecules and objects with which they are in contact.
Pressure: the force exerted on a unit area
P = F
A
Atmospheric pressure: the pressure exerted by gas
molecules in the air on all objects exposed to the
atmosphere
Chapter 5
Pressure
Many different units used to report pressure.
atmosphere (atm)
torr (torr)
millimeters of Hg (mm Hg)
inches of Hg (in. Hg)
pascal (Pa) = SI base unit
kilopascal (kPa)
pounds per square inch (psi)
Chapter 5
Chapter 5
Pressure
Relationships between different pressure units:
1 atm = 760 mm Hg
= 760 torr
= 101.325 kPa
= 29.92 in. Hg
= 14.7 psi
= 1.01325 x 105 Pa
Chapter 5
Pressure
Convert the pressure 669 torr into pressure
in atm?
Given: 669 torr
Find: P (atm)
Conversion factor:
1 atm = 760 torr
P (atm) = 669 torr x 1 atm = 0.880 atm
760 torr
Chapter 5
Gas Laws
Four variables are needed to define the physical
condition or state of any gas:
– Temperature (T)
– Pressure (P)
– Volume (V)
– Amount of gas (moles: n)
Equations relating these variables are known as
the gas laws.
Gas Laws
Assume gas molecules are widely separated
and non-interacting, thus the equations are
independent of the identity of the gas
Size, shape, mass of gas molecules don’t
matter
Chapter 5
Chapter 5
P and V Relationships
Boyle’s Law:
Relates pressure (P) and volume (V) of a fixed
amount (i.e. constant number of moles) of gas at
constant temperature (T)
V and T Relationships
Charles’s Law:
Relates Volume (V) and temperature (T) of a fixed
amount of (i.e. constant number of moles) gas at
constant pressure (P)
Pressure of a gas is inversely proportional to its
volume at constant temperature and for constant
number of moles
Volume of a gas is inversely proportional to its
temperature at constant pressure and constant
number of moles
P∝ 1
V
V∝ T
V = k 2T
P = k1
V
Chapter 5
Behavior at Standard Condition
Standard Temperature and Pressure (STP) :
0°C (273.15 K) and 1 atm
T= 0°C = 273.15 K; P = 1atm = 760 torr
Under these condition the volume of 1 mole of
an ideal gas is called standard molar volume
and is equal to 22.4 L
Chapter 5
Behavior at Standard Condition
Chapter 5
Chapter 5
P, T and V Relationships
Boyle’s Law and Charle’s Law can be merged to give
the combined gas law (for a fixed amount of gas):
PV
=k
T
P, T and V Relationships
A sample of gas has a volume of 500. mL at 27oC and
800. torr. We wish to compress the sample to 250. mL
at 900. torr. What temperature is required?
P1V1 P2 V2
=
T1
T2
If the amount of gas remains constant, we can use this
combined gas law to calculate how a gas will respond
to changes in T, P and V
Chapter 5
P, T and V Relationships
A sample of Ar gas is confined to a 10.0 L container at
P = 1.90 atm and T = 30 oC. What would be the volume
at 1.00 atm and –10 oC?
Chapter 5
Avogadro’s Law
The combined gas law only works if the number of gas
molecules remains constant
What if we wish to compare two gases with different
numbers of gas molecules?
Avogadro’s Law states that:
Equal volumes of gases measured at the same T
and P contain equal numbers of molecules
V ∝n or V/n = k (a constant)
V1 V2
=
n1 n 2
The law holds because gas molecules don’t interact
appreciably
Chapter 5
Chapter 5
Gas Laws
Boyle’s Law
V = k1 x 1
P
Gas Laws
Three equation can be combined to make a
more general gas law:
PV = nRT
Charles’ Law
V = k2 x T
Avogadro’s Law
V = k3 x n
Chapter 5
Gas Laws
The value of the gas constant (R) depends on the units of
P, V, n, and T.
T must always be in Kelvin
n is usually in moles
If P (atm) and V (L),
then R = 0.08206 atm.L
mol.K
If P (torr) and V (L),
then R = 62.36 L.torr
mol.K
where P = pressure
V = volume
n = moles
T = temperature (K)
R = gas constant
Chapter 5
Ideal Gas Laws
A 2.15 mol sample of SO2 occupies a volume
of 12.6 L at 30 oC. What is the pressure of the
gas in atm?
Chapter 5
Chapter 5
Ideal Gas Laws
A 1.25 g sample of CO2 is contained in a 750. ml
flask at 22.4 °C. What is the pressure of the gas in
torr?
Chapter 5
Ideal Gas Laws
The reaction of lithium carbonate and nitric
acid generates lithium nitrate, CO2 and water?
If 3.00 g of lithium carbonate reacts with 100.0
mL of 0.500 M nitric acid, what volume of CO2
will form?
Chapter 5
Ideal Gas Laws
A 1.007g sample of an unknown gas exerts a
pressure of 715 mm Hg in a 452-ml container at
23 °C. What is the molar mass of the gas?
The Ideal Gas Law
A 6.00 g sample of a Group 13 metal reacts
with excess HCl to form hydrogen gas. The
gas is collected in a 2.00 L flask at 25.0 oC,
giving a final pressure of 1200. torr. What
metal was used?