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.
