• Anything that occupies space and has mass
THREE STATES OF MATTER
●All substances/matter can exist in 3 states : solid, liquid and gas.
●Solid - molecules are held close together in an orderly fashion with little
freedom of motion.
●Liquid - molecules are close together but are not held so rigidly in
position and can move past one another.
●Gas - molecules are separated by distances that are large compared with
the size of the molecules.
The three statesof matter are interconvertable.
The physical properties of a substance depends on the state
of the substance. When a substance undergoes a change in
state, many of its physical properties change.
THE GASEOUS STATE
Under certain condition of pressure & temp., most substances can
exist in any one of the three state of matter.
E.g. water - solid :ice ; liquid : water ; gaseous : steam / water vapor
In gases - molecular motion is totally random, forces of interaction
between molecules are so small, each molecules moves freely and
essentially independently of other molecules.
SUBSTANCES THAT EXIST AS GASES
Under normal condition of pressure & temp. (1 atm , 25oC), elements
that exist as gases are:
The noble gases (Group 8A elements) : He, Ne, Ar, Kr, Xe, Rn -
Hydrogen, nitrogen, oxygen, fluorine, and chlorine - exist as diatomic
molecules : H2, N2, O2, F2, Cl2.
Allotrope of oxygen, Ozone (O3).
Compounds that exist as gases are HCl, CO, CO2,
NH3, N2O, NO, NO2, SO2, H2S, HCN, CH4.
Only O2 essential for our survival.
H2S, HCN - deadly poisons.
CO, NO2, SO2, O3 - less toxic.
He, Ne, Ar - chemically inert.
Most gases are colorless except F2, Cl2, NO2
General Properties of gas ;
Have low density (about 2 kg per m3)
Diffuses quickly (mix thoroughly)
Fills up a container uniformly
Exert pressure uniformly on all sides of a container
independently of the height or depth.
PRESSURE OF A GAS
Gas exert pressure on any surface with which they come into contact - gas
molecules are constantly in motion and collide with the surface.
Instrument to measure atmospheric pressure : barometer
- Long tube filled with mercury (Hg) is inverted into a dish of mercury.
-Atm. pressure pushing on the surface of the Hg
in the tube is proportional to atmosphericpressure.
- Height of Hg (h) atm pressure.
A second type of barometer, a manometer has
two arms, one opened
to the atmosphere and one closed or connected to a container filled
with gas. The pressure exerted by the atmosphere or by gas in a
container is proportional to the difference in the mercury levels (h).
The std atmospheric pressure (1 atm) is equal to
the pressure that supports a column of mercury
exactly 760 mm (76 cm) high at 0oC at sealevel.
1 atm = 760 mmHg (mmHg represents pressure
exerted by a column of mercury 1 mmhigh)
SI Units - Pascals, Pa : Pressure =force/area
1 Pa = 1 N/m2
1 atm =101,325 Pa =1.01325 x105 Pa =1.01325 x102kPa.
THE GAS LAWS
Important generalizations regarding the macroscopic behavior of gaseous
1. The Pressure -Volume Relationship : Boyle’s Law
-studied by Robert Boyle in 17th century.
Volume of a fixed amount of gas maintained at constant temperature is
inversely proportional to the gas pressure
V 1/P, : proportional to
or V = k1 x 1/P k1: proportionality constant
PV = k1
For a given sample of gas under two diff sets of conditions at constant
temp. : P1V1 = k1 = P2V2
P versus V graph at constant
temp.: Volume of gas doubles
as the pressure is halved.
P versus 1/V
The Temperature-Volume Relationship :
At constant pressure, the volume of a gas sample expands when
heated and contracts when cooled.
Study on the temp. – vol. relationship at various pressure showed that :
At any given pressure, the plot of vol. vs temp. yields a straight
• Extending the line to zero vol., the intercept on temp. axis is
-273.15 oC (absolute temperature)
Lord Kelvin identified the temp. -273.15 oC as
theoretically the lowest attainable temp., called
• absolute temp. scale, now called Kelvin temp.
Absolute zero : 0 K = -273.15oC
273.15 K = 0oC
373.15 K =100oC
Relationship between oC and K:
T (K) = t(oC) +273.15oC
Charles’ Law states that : the vol. of a fixed amounts of a gas maintainedat
Under 2 diff. sets of conditions for a given sample of gas at constant
V1 / T1 = k2 = V2 / T2
V1 / T1 = V2 / T2
V1, V2 are volumes of the gases at temp. T1 , T2 (both in kelvins).
constant pressure is directly proportional to the absolute temp. of the gas.
V = k2T or V/T = k2
k2 is proportionality constant.
The Volume – Amount Relationship : Avogadro,s Law
Gay-Lussac. He published a hypothesis
that stated :
At the same temp. and pressure, equal volumes of different gases
contain the same number of molecules (or atoms if the gas is
The volume of any given gas must be proportional to the number
of molecules present;
Amedeo Avogadro – complemented the studies of Boyle, Charles and
V = k3n where n represents the number of moles and
k3 is the proportionalityconstant.
Avogadro’s Law – at constant pressure and
temp., the vol. of a gas is
directly proportional to the number of
moles of the gas present.
The Ideal Gas
Boyle,s Law :
Charles’ Law :
V 1/P (at constant n and T)
V T (at constant n and P)
Avogadro’s Law: V n (at contant P and T)
Combine all three expressions :
V nT / P
Or PV = nRT …………….ideal gas equation
R, the proportionality constant is called the gas
Ideal gas – a hypothetical gas whose pressure- volume-temp. behavior can be
completely accounted for by the ideal gas equation. The molecules of an ideal
gas do not attract or repel one another, and their vol. is negligible compared
with the volume of the container.
To apply the ideal gas equation to a real system, we must evaluate the
gas constant, R.
At 0oC (273.15K) and 1 atm pressure, many real gas behave like an
ideal gas. Exp. show that under these conditions, 1 mole of an ideal
gas occupies 22.414L.
The conditions 0oC (273.15K) and 1 atm pressure are called standard
temp. and pressure (STP).
From PV = nRT
R = PV/nT
=( 1 atm)(22.414L) = 0.082057 L.atm/K.mol
For most calculations, use R=0.0821 L.atm/K.mol and
the molar volume of a gas at STP as 22.4L.
When pressure. volume, temp. and amount change, a modified
form of equation must be employed, which involves initial and
R = P1V1 / n1T1, (before change)
R = P2V2 / n2T2, (after change)
P1V1 / n1T1 = P2V2 / n2T2
If n1 = n2 as is usually the case because the amount of gas
normally does not change,
P1V1 / T1 = P2V2 / T2
From the ideal gas equation ,
PV = nRT we can calculate the density of gas.
The number of moles of the gas, n, is given by :
n = m /M , m = mass of gas (grams) and M is its molarmass
Since density is mass per unit volume,
n/V = P / RT
m/VM =P / RT
We can write d = m/V = PM
Unit of gas density are usually grams per liter (g/L), rather than grams per
mL (g/mL) : density of gases is very low at atmospheric condition.
Deriving Quantity of Density
The Molar Mass of a Gaseous Substance
Molar Mass of a substance is found by examining its formula and
summing the molar masses of its component atoms (if the actual
formula of the substance is known).
For an unknown gaseous substance, an experiment is needed to
determine the density value (or mass and volume data) at a known
temp. and pressure.
From d = m/V = PM , we get
M = dRT