Anesthesia Department
S.I. units
Gases and Gas Laws
By
Ahmed Ibrahim ; M.D.
Prof.of Anesthesia
Ain Shams University
Intended Learning Outcomes
By the end of this lecture , the student will be able to:
1..know different units used for clinical
measurements .
2..understand different laws controlling ideal
gas behavior.
3..interpret the applications of different gas
laws in anesthetic practice.
©
Basic 7 S.I units
Meter
m
length
Kilogram
Kg
mass
Second
s
time
Ampere
A
electric current
Kelvin
oK
temperature
Mole
mol
amount of substance
Candela
cd
luminous intensity
Fractions
&
Multiples
10-18,
10-15,
10-12,
10-9,
10-6,
10-3,
10-2,
10-1
10
102
103
106
109
1012
1015
1018
atto
femto
pico
nano
micro
milli
centi
deci
deca
hecto
kilo
mega
giga
tera
penta
exa
a
f
p
n
µ
m
c
d
Da
h
K
M
G
T
P
E
•area
m2
•volume
m3
•velocity
displacement / time
ms-1
•acceleration
velocity / time
ms-2
•force
mass x acceleration
Kgms-2
•work (energy)
force x distance
Kgm2s-2,
•power
work / time
Kgm2s-3,
•pressure
force / area
•frequency
•electric charge
Newton
N
Nm
Joule
J
Js-1
Watt
W
Nm-2
Pascal
Pa
cycles / second
Hertz
electric current x time
Coulomb
Hz
C
Volt
V
Ohm
Ω
•electric capacitance
Farad
F
•magnetic flux
Weber
Wb
•potential difference
(EMF)
•electric resistance
potential difference / current
SOLID
LIQUID
GAS/VAPOUR
+++
+
±
++
++
++
+
+++
+++
SHAPE OF CONTAINER
NO
YES
YES
MISSIBILITY
NO
YES
YES
I.M.FORCE
I.M.SPACE
MOLECULAR MOTION
VAPOUR
gaseous state of the substance which is present in the liquid form at RTP
GAS
a substance which cannot exist in the liquid form at RTP
Kinetic Theory of Gases
GAS MOLECULES are:
•Widely separated (negligible molecular volume)
•In constant motion (diffusion)
•In constant striking (pressure)
IDEAL (PERFECT) GAS, obeys the 3 assumptions
VOLUME
GAS
TEMP
PRESSURE
Robert Boyle (1627-1691). , Ireland
Jacques Charles (1746-1823)
Critical Temperature
“ the temperature above which a gas cannot be liquefied whatever pressure is applied ”
Critical Pressure ” the pressure needed to liquefy a gas at its critical temperature “
N.B.
in T
in P needed
CO2
N2O
O2
31oC
36.5 oC
-116oC
(C.P = 74 atm)
Joseph Louis Gay-Lussac (1778-1850)
for a given mass of a gas, at a constant:
TEMPERATURE
PRESSURE
VOLUME
V  T(oA)
V  1/P
P x V =Const.
V=T x Const. (K)
P1xV1= P2xV2
V/T = K
Boyle’s law
Charles' law
P T
Gay-Lussac’s law
a Perfect
Gas ,is the one
that always obey Boyle’s and Charles’ laws.
 in which the intermolecular forces and molecular
volume are too small ( negligible ).

a Real Gas behaves approximately as a perfect gas especially at low
pressures and high temperatures.
Equation of state of a perfect gas:
V

1/P
V
V

T/P
V = T/P x const
PV/T = const (R)
P1V1 / T1 = P2V2 / T2

T
Avogadro’s hypothesis
Amadeo Avogadro (1776-1856) Italy
for perfect gases at EQUAL
1 mole of any gas at ,
Volumes
Temp
Pressure
STP
Room temp
Body temp
contain equal number of molecules
22.4 L
24.1 L
25.4 L

NB. a mole of volatile liquid (e.g. 197.4 gm Halothane)

PV=RT
22.4 L vapour at STP
R (molar gas constant) = 1 x 22.4 / 273=0.082 litre.atm /degree oA/ mole
=1.987 joule / degree / mole (S.I)

A mole of any gas contains the same number of molecules = 6.02 x 1023…………
……………….” Avogadro’s number”
John Dalton 1766-1844
DALTON’S law of Partial Pressure
“ the pressure exerted by a mixture of gases (and/ or vapours) – enclosed in a given space –
is equal to the sum of pressures which each gas (or vapour) would exert if it alone occupied
the same space”
Partial Pressure (P.P) = total pressure x volume %
Volume % = (P.P / total P ) x 100
Saturated Vapor Pressure (SVP)
•nature of liquid
•its temperature
mmHg (20oC)
-Desflurane
670
-Diethyl ether
440
-Halothane
243
-Isoflurane
239
-Enflurane
175
-Sevoflurane
160
-Trilene
60
-Methoxyflurane
23
varies as a function of:
Summary of important points
•Basic S.I units
- length = Meter m
- mass = Kilogram Kg
- time = Second s
- electric current = Ampere A
- temperature = Kelvin oK
- amount of substance = Mole mol
- luminous intensity = Candela cd
•Derived units
- area = m2
- volume = m3
- velocity =ms-1
- acceleration = ms-2
- force =Kgms-2 = Newton N
- work (energy) = Kgm2s-2 = Nm = Joule J
- power = Kgm2s-3 = Js-1 = Watt W
- pressure = Nm-2 = Pascal Pa
- frequency = Hertz Hz
- electric charge = Coulomb C
- potential difference (EMF) =Volt V
- electric resistance = Ohm Ω
- electric capacitance = Farad F
- magnetic flux = Weber Wb
•VAPOUR is the gaseous state of the substance which is present in the
liquid form at RTP
•GAS is a substance which cannot exist in the liquid form at RTP
•Boyle’s law :
V  1/P
at constant temperature
•Charles' law :
V  T (oA)
at constant pressure
•Gay-Lussac’s law :
P T
at constant volume
•Critical Temperature : “ the temperature above which a gas cannot be
liquefied whatever pressure is applied ”
•Avogadro’s hypothesis : “EQUAL volumes of perfect gases at same
temperature and pressure contain same number of molecules”
•Avogadro’s number : A mole of any gas contains the same number of
molecules = 6.02 x 1023
•DALTON’S law of Partial Pressure
Partial Pressure (P.P) = total pressure x volume %
Examples of questions to assess the ILOs
1. Which of the following units are BASIC SI units
of measurement?
a) Kilometre.
b) Candela.
c) Watt.
d) Kilogram.
2.The following are correct S.I. Units
a) The unit of energy is the Newton
b) The unit of power is the Watt
c) The unit of frequency is the Hertz
d) The unit of mass is the gram
a) The unit of length is the metre
3.The critical temperature is:
a) 273 K
b) the temperature above which a gas cannot be liquified by pressure
alone
c) the temperature below which a gas does not vaporise
d) 36.5 degrees C for nitrous oxide
e) different if a substance is in a mixture rather than on its own
4.Concerning the gas laws
a) Boyle's law refers to the relationship between temperature and
pressure of a gas
b) Temperature is measured on the absolute temperature scale
c) Temperature is a constant in Charles' law
d) Boyle's law states that at a constant volume pressure varies
with temperature
e) The gas laws are only true for air
5.One mole of a gas
a) Occupies 22.4 Litres at room temperature
b) Has the same volume for any gas
c) Contains Avogadro's number of molecules
d) May be liquefied by compression if above critical temperature
e) Is one gram molecular weight
Answers
1. false, true, false, true
2. false, true ,true, false, true
3. false, true, false, true, true
4. false, true, false, false, false
5. false, true, true, false, true
Thank You