1. Gas Laws: Quick Tips for Written Boards
Dr Ogunnake
2016

2. What is Gas? (Chemistry 101)
A set of randomly moving particles or molecules between which there is no type of interaction
Simplistically:
A gas completely fills a closed container
A liquid conforms completely to the bottom of the container
A solid retains its shape despite the container

3. Main Gas Laws Boyle’s Law Charles’ Law Gay-Lussac’s Law
(Avogadro’s Law)
Dalton’s Law
Ideal Gas Law

4. Boyle’s Law
At constant temperature, the volume of a given quantity of gas is inversely proportional to its pressure
V ∝ 1/P, or
PV = constant, or
P1V1 = P2V2

5. Charles’ Law
At constant pressure, the volume of a given quantity of gas is directly proportional to the absolute temperature (KELVIN)
V ∝ T, or
V/T = constant, or
V1/T1 = V2/T2

6. Gay-Lussac’s Law (aka Amonton’s Law)
At constant volume, the pressure of a given quantity of gas is directly proportional to the absolute temperature (KELVIN)
P ∝ T, or
P/T = constant, or
P1V1 = P2V2

7. Avogadro’s Law
1 mole of any gas has the same volume as one mole of any other gas, as long as the temperature and pressure are the same
V/n = k
Avogadro’s number (number of molecules) = × 10
1 mole of an ideal gas occupies 22.4 L at STP

8. Dalton’s Law
The total pressure for a mixture of gases is the sum of the (partial) pressure of each of the gases; i.e., the pressure each gas would exert if it were the only gas
Ptotal = P1 + P Pn
Works if gases are ideal (move randomly; no intermolecular forces)

9. Atmospheric Pressure
Atmospheric pressure is the sum of the partial pressures of all of the gases in the atmosphere
Varies according to sea level
Nitrogen 78%, oxygen 21%; also argon carbon dioxide, neon, krypton, xenon, and radon

10. Standard Temperature and Pressure (STP)
Standard temperature is 273 Kelvin, or zero degrees Celsius, or 32 degrees Fahrenheit
Standard pressure is 101 kPa, or 1 atm, or 760 mmHg (Torr), or 14.7 psi

11. What is an Ideal Gas? An Ideal Gas obeys all of the Gas Laws exactly
Anesthetic agents are considered ideal gases

12. Ideal Gas Law PV = nRT P = pressure (in kPa) V = volume (in liters)
n = moles of gas
R = gas constant (next slide)
T = temperature (MUST BE IN KELVIN!)

13. Universal Gas Constant (R)
Depends on the units of pressure:
If P is in kPa, R = kPa • L/ K • mol
If P is in atm, R = atm • L/ K • mol
(JUST MEMORIZE ONE, THEN CONVERT atm ↔ kPa --EASIER!)

14. Additional Laws/Facts
Depends on the units of pressure:
If P is in kPa, R = kPa • L/ K • mol
If P is in atm, R = atm • L/ K • mol
(JUST MEMORIZE ONE, THEN CONVERT atm ↔ kPa --EASIER!)

15. Vaporizer Output Depends on the units of pressure:
If P is in kPa, R = kPa • L/ K • mol
If P is in atm, R = atm • L/ K • mol
(JUST MEMORIZE ONE, THEN CONVERT atm ↔ kPa --EASIER!)

16. Henry’s Law (Diving Question)
Depends on the units of pressure:
If P is in kPa, R = kPa • L/ K • mol
If P is in atm, R = atm • L/ K • mol
(JUST MEMORIZE ONE, THEN CONVERT atm ↔ kPa – EASIER!)

17. Henry’s Law Nitrous oxide tanks
The pressure gauge measures only the vapor pressure
As the vapor is used up, it is replenished by the liquid source; therefore, the pressure remains the same until almost all of the liquid is gone

18. Adiabatic Cooling
When a substance changes states (solid ↔ liquid ↔ gas), the immediate environment changes temperature
Desflurane vaporizer
Nitrous oxide tank

19. Heliox Mixture of helium and oxygen
Various concentrations, commonly 21% oxygen and 79% helium
Helium is less dense than oxygen

20. Heliox R=flow, r=radius, p=pressure, μ=viscosity, l=length
Uses the principle of the Hagen-Poiseuille Law to facilitate airflow (laminar vs. turbulent, less resistance) past a narrowed orifice
R = 8 L μ
∏ r4
R=flow, r=radius, p=pressure, μ=viscosity, l=length
(ANOTHER MAJOR POINT: SMALL CHANGES IN THE INTERNAL DIAMETER OF A LUMEN CAN MAKE A BIG DIFFERENCE IN THE RATE OF FLOW)

21. Heliox Reynolds number (Re) Predicts turbulent vs. laminar flow
Re = ρ V d μ
ρ = density
V = velocity
d = diameter of tube
μ = viscosity
Flow in a branching tube (airways): < approx 2000 = laminar, > approx 3000 = turbulent

22. Fluid and Gas Flow Principles
Pharmacology2000.com. Anesthesia Fundamentals

23. Entonox 50/50 mixture of oxygen and nitrous oxide
Contraindications are that for nitrous oxide
Used for situations in which pain is predictable and of a short duration
Burn dressing change
UK- labor and delivery
Fracture reduction
Venipuncture

24. References www.ganfyd.org www.pharmacology2000.org
state.edu/betha/nealGasLas/
dex.shtml
bp/ch4/gaslaws3.html

25. References www.chemfiesta.com www.cavalcadepublishing.com
law.ppt
APPLET:
P.htm

26. Handouts
Martin and Karel, Use of Heliox during Spontaneous Ventilation: Model Study, World Academy of Science, Engineering and Technology,
Polaner, The Use of Heliox and the Laryngeal Mask Airway in a Child with an Anterior Mediastinal Mass, Anesth Analg 1996;82:208-10
Sample questions/answers