LOADING AWESOME

**GAS LAWS**

PART TWO

**www.propofology.com**

Dr. David Lyness

@Gas_Craic

**BOYLE'S LAW -TEMPERATURE & PRESSURE - P1V1 = P2V2**

**For a constant amount of gas at a constant temperature, the product of the pressure and volume of the gas is a constant.**

**The volume of gas DECREASES as the PRESSURE is raised.**

**If you put your thumb over the end of a bicycle pump and push the plunger down, it gets harder to hold your thumb on.... Pressure is INVERSELY proportional to volume.**

**CHARLES'S LAW - VOLUME & TEMPERATURE - V1/T1 = V2/T2.**

**For a constant amount of gas at a constant pressure, the volume of the gas is directly proportional to the absolute temperature. Volume is directly proportional to temperature. If you heat a gas up - it will expand.**

**This law was refined later by Gay-Lussac.**

**PRESSURE LAW - CONSTANT VOLUME**

**“For a fixed mass of gas at a constant volume, the pressure is directly proportional to the temperature”**

**UNIVERSAL GAS LAW - COMBINATION OF THE ABOVE THREE LAWS**

**PV=nRT where n is the number of moles of the gas and R is the**

**constant 8.31J/K**

**This equation would be used, say, to calculate how many litres of oxygen are in a medical oxygen cylinder.**

**The physical volume of the cylinder is around 5L but, we have 652L available to us due to pressure.**

**DALTON'S LAW OF PARTIAL PRESSURE**

**S - Mixture = P1 + P2 + P3 etc**

**Total pressure of a gas mixture was the sum of the pressures of each of the gases if they were to exist on their own.**

**If a cylinder of gas mixture at 400 kPa contains 21% oxygen and 79% helium, then if the oxygen existed on its own it would exert a partial pressure of 21% of 400 kPa = 88 kPa. The helium therefore exerts a partial pressure of 400 – 88 = 312 kPa**

**HENRY'S LAW - GAS DISSOLVING IN A LIQUID**

**For a gas-liquid interface the amount of the gas that dissolves in the liquid is proportional to its partial pressure.**

**This helps to predict how much gas will be dissolved in the liquid.**

**The actual amount also depends on the solubility of the gas as well as its partial pressure.**

**It is directly proportional to the pressure of the gas above the substance.**

**GRAHAM'S LAW OF EFFUSION - LARGER MOLECULES DIFFUSE MORE SLOWLY**

Rate of diffusion was inversely proportional to the square root of the molecular mass of the gas.

So the larger the molecule the slower it diffuses.

Direction and rate of change of diffusion is important - consider the

**alveolus to plasma.**

__This explains the second gas effect when using nitrous and a volatile in Oxygen__**Rate of effusion of A/Rate of effusion of B = Square root of molecular mass of B/Square root of molecular mass of A**

So if B is more massive than A, A will effuse out of the alveolus quicker than B, leaving behind more of B and so raising its conc. Halothane is larger than Nitrous = nitrous will diffuse quicker and so raise the concentration of the halothane in the alveolus.

**VAPORIZER** According to Henry’s law, the partial pressure of the anaesthetic agent in the blood is proportional to the partial pressure of the volatile in the alveoli.