Gas Laws Flashcards
Boyles Law
PV=K
or
V ~ 1/P
At a constant temperature
P= Pressure
V= Volume
K = constant
Nil units.
Describe Boyles law
PV=K
V inversely proportional to P
Describes one of the characteristics of an ideal gas.
If the temp of the gas is held constant, then pressure and volume are inversely proportional.
An ideal gas is a theoretical gas that obeys the universal gas equation
What is an ideal gas
An ideal gas is a theoretical gas that obeys the universal gas equation
PV = nRT
P - pressure
V - volume
n- no of moles of the gas
R - universal gas constant (8.31 J/K/mol)
T = temp
You are asked to transfer a patient that requires 15l/min of O2 and there is one full E-cylinger of oxygen available. How long will this last?
Describe a clinical application of Boyles law
Body Plethysmography
- used to determine FRC.
P1V1 = P2V2
P1 = initial p in box
V1 = initial vol of box
P2 = p of box at end of chest expansion.
Calculate for V2
V2 -V1 = change in volume of the chest. (V3)
THEN
P1V1 = P2V2 (solving for V1)
- V1 = initial v of chest (FRC)
P1 = initial pressure at mouth
V2 = V1+V3
P2 = Pressure at mouth during respiratory effort
Solve for V1 = FRC
Charles’ Law
V/T = K
or
V=T at a constant pressure
V= Vol
T = Temp
K = Constant
Nil units
Describe Charles’ law
Describes one of the characteristics of an ideal gas.
If the pressure of a fixed mass of gas is held constant, then the Vol and Temp are proportional.
Clinical Example of Charles law.
Spirometry
During PFTs a patient will exhale gas at body temp into a spirometer at room temp.
Therefore according to Charles law, as the temp drops the volume of the gas decreases to maintain a constant.
Therefore we have the terms:
- BTPS: Body temp and pressure, saturated w water vapour
- ATPS: ambient temp and pressure, saturated w water vapour.
The volume in a spirometer can be corrected from ATPS to BTPS.
- Heat Loss
During anaesthesia, the air around the body is heated by convection. AS this happens, according to Charles law, the volume of the mass of gas increases, and therefore rises away from the patient.
Gay-Lussacs Law (3rd Gass Law)
P=T
or
P/T =K
At constant volume
No units
Describe Gay - Lussac’s Law
Describes one of the characteristics of an ideal gas
If the volume of a fixed mass of gas is held constant, then the P and T are proportional.
Describe the Filling Ratio in relation to Nitrous oxide cylinders.
Clinical application of Gay Lussac’s law
Filling Ratio =
Weight of the fluid in the cylinder /
weight of the water required to fill the cylinder.
Within a cylinder of gas, according to the 3rd gas law, as the ambient temp rises, the pressure inside the cylinder will also rise.
This is important in the storage of nitrous oxide w its low critical temp. At room temp it is stored in a cylinder as a liquid, w vapour on top. As the Temp rises, the pressure exerted by the vapour (the saturated vapour pressure), also rises. IF this exceeds the pressure capacity of the cylinder, then it could explode as the volume is constant.
For this reason, the filling ratio for nitrous oxide is 0.75 in temperate regions, and 0.67 in warmer regions.
Compressed air is allowed to re-expand. As it does so it loses heat energy, as per Gay Lussac’s law, and liquefies.
Apply the 3rd Gas law to the hydrogen thermometer
When a constant volume of hydrogen, inside the thermometer, is heated, its pressure increases.
The measured pressure change is directly proportional to the change in temp.
Avogadro’s Equation
V/n = K
V= volume of gas
n= amount of substance of the gas
K = proportionality constant
No units
Explain Avogadro’s equation
The equation states that equal volumes of gases, at the same temp and pressure, contains the same number of molecules, regardless of their chemical nature and physical properties.
This number = Avogadro’s number = 6x10^23. = 1 mole.
1 mole= quantity of a substance containing the same number of particles as there are atoms in 12g of carbon 12 = 6x10^23 = 1 mole.
The mass of gases is different, but the concept of number of molecules or moles, enables comparison between them.
One mole of any gas at STP occupies 22.4 L
Standard Temp
273.15 K = 0 C
Standard pressure
101.325KPa
Avogadros Number
6x10^23 = 1 mole
1 MOLE =
1 mole = quantity of a substance containing the same number of particles as there are atoms in 12g of Carbon 12.
= 6x10^23
1mole of any gas at STP = 22.4 L
What does one mole of any gas at STP occupy?
22.4L
Use Avogadro’s Law to explain how to calibrate a sevoflurane vaporiser
Sevo MW = 200.
V/n= K
Number of moles = (mass of substance/atomic mass)
Sevo MW = 200
Therefore 200g of sevoflurane = 1 mole and would occupy 22.4L at STP.
If a vaporiser contains 20ml of sevo, this is equivalent to 0.1mole because the density of secondary is 1g/ml.
If 1 mol occupies 22.4L at STP then 0.1mole will occupy 2.24L at STP.
IF this volume of sevo is fully vaporised into 224L of O2, the resulting concentration will be
V/n = K
2.24/224 = 0.01 or 1%
How much liquid agent does a vaporiser use per hour.
3xFGF(l/min)xVolume%=ml.
Universal Gas Equation
PV=nRT
P- pressure
V- volume
n - no of moles of the gas
R - the universal gas constant (8.31J/K/mol)
T - temp
Universal Gas Constant (R)
- P x V = K1
- V/T = K2
- P/T = K3
Therefore
PV/T = K
For 1 mole of gas, PV/T = R
R = 8.31J/K/mol.
Explain the universal gas equation
Describes the behaviour of an ideal gas. It’s a combination of Avogadro’s law, Boyles law and Charles law.
Calculate the contents of an oxygen cylinder
Using universal gas equation.
PV=nRT
T is constant room temp
V is constant as the cylinder has a fixed volume
R = constant 8.31
Therefore remove these from the eqn
P is directly proportional to n
P= gauge pressure which can thus be used to measure the amount of oxygen (n) remaining in a cylinder.
Daltons Law of partial Pressures
Ptotal = Pgasa + Pgasb
P total = total pressure
P Gas a = Partial pressure of gas A
P Gas B = partial pressure of gas B
Explain Daltons Law
Ptot = Pgasa + PgasB
States that in a mixture of gases the total pressure is always equal to the sum of the individual partial pressures of the gases present.
The pressure of each gas is determined by both the no of molecules present and the total volume occupied and is independent of the presence of any other gases in a mixture.
Calculate the Alveolar Partial Pressure of O2 (PAO2) given:
- FiO2 100%
- Temp = 37
- Atmos press = 100kPa
- PACO2 = 4 kPa
PiO2 (partial pressure of inspired O2) = FiO2 x atoms P
= 0.21x100 = 21
however the air in the lungs is saturated w water vapour and mixed alv CO2
At 37c in normal circumstances the SVP of water = 6.3 KPA
Therefore using daltons law
PAO2 = PiO2 - (PACO2 + PAH2O)
= 21 - (4+6.3)
= 10.7 KPA
What is the partial pressure of O2 at the top of Everest
Ptot = Pgasa + Pgasb
Ptot = atmospheric pressure.
Atmospheric pressure at sea level = 101.3KPa
At top of Everest = 33.7 KPa.
Concentration of oxygen = 21%
Therefore using daltons law & assuming all other gases are constant:
At sea level:
Ptot = Po2 + P other gases
Po2 = Ptot - Pother gases
PO2 = 101.3-80.1
PO2 = 21.2 KPA
Everest
Ptot = Po2 + P other gases
PO2 = Ptot - Pother gases
= 33.7 - 26.6
= 7.1
P other gases determined by:
- O2 = 21%
- other gases = 79%
- 79% of P total = 26.6 (when p total is 33.7)
Henrys Law
At a constant t, the mass of a gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium w that liquid.
Therefore mass of gas = PP
Partial Pressure
The pressure which the gas would have if it alone occupied the volume of the container.
Explain Henrys Law
When a liquid is placed into a closed container, with time equilibrium will be reached between the VAPOUR pressure of the gas above the liquid, AND the LIQUID itself.
IE The solubility of a gas in a liquid is directly proportional to the Partial pressure of the gas above that liquid.
Discuss the use of Henrys Law in Volatile Anaesthetic Agents
Partial Pressure of the anaesthetic agent in the blood = the partial pressure of the volatile in the alveoli.
Therefore if the inspired concentration of the volatile is increased, then the concentration in the blood will also increase.
At altitude this is still the case as Henrys law also says that the only factors that affect the partial pressure of an agent in the blood are:
- SVP of the volatile
- Concentration in the alveolus
- ambient temp
Therefore the standard vaporisers dont need to be altered for altitude, except tec6 which is heated and pressurised.
Describe hyperbaric oxygen therapy using Henrys law
HOT is used in conditions where we need to increase O2 delivery eg decompression sickness, CO poisoning, refractory OM or necrotising wounds.
At sea level, atmospheric pressure and breathing room air, the amount of oxygen stored in the blood is very small. About 0.3ml/dl.
Under hyperbaric conditions, the partial pressure of O2 can be significantly increased and thus according to Henrys law, so too will the concentration of Oxygen in the dissolved blood.
At 3 atomspheres, breathing 100% O2, the o2 conc in blood reaches 5.6ml/dl, independent of any other factor, including HB
Grahams law of diffusion
Rate of Diffusion = 1/ Sqroot MW
Units = Mass/time
Explain grahams law of diffusion
The r ate of diffusion of a gas is inversely proportional to the square root of its molecular weight.
Therefore the large the molecule, the slower it diffuses across the membrane
Describe the Second Gas Effect.
If two gases are present in the lungs, the gas with the smaller molecular weight will diffuse into the blood more rapidly, leaving a higher concentration of the larger molecule behind in the lung.
EG N2O + isoflurane
- MW of N2o = 44.012g/mol
- MW of iso = 184.5g/mol.
According to grahams law, N2O will diffuse across the alv membrane and into the blood much quicker, therefore leaving behind an increased concentration of iso in the alveolus.
According to henrys law, increasing the concentration of Iso in the alveolus will also increase its partial pressure.
