Gases, vapours and liquids

Question 1

What is Avogadro’s hypothesis?

Answer 1

At standard temperature and pressure, an equal volume of two gases will contain the same number of particles

Question 2

What is standard temperature and pressure?

Answer 2

Temperature = 0 degrees C / 273.15K
Pressure = 1 atm / 101.325 kPa

1 mole of a gas at STP will occupy 22.4L

Question 3

What is the definition of temperature?

Answer 3

The average kinetic energy of the molecules within a substance

Question 4

What is Dalton’s law?

Answer 4

The total pressure of a mixture of gases is the sum of all the partial pressures of gases present

Question 5

What is Boyle’s law?
(1st ideal gas law)

Answer 5

At a constant temperature, the volume of a gas is inversely proportional to its pressure
P ∝ 1/V

At a constant temperature, the product of pressure and volume are constant
PV = constant (K₁)
Therefore halving the volume of a gas will double its pressure

Mneomic = Water ‘Boyles’ at a constant temperature

Question 6

A partly used ‘E’ oxygen cylinder has a pressure of 100 bar with an internal volume of 4.5L. If oxygen was required at a flow of 2.5L.min⁻¹, roughly how long would the cylinder last?

Answer 6

Using Boyle’s law / combined gas law:
P₁V₁ / T₁ = P₂V₂ / T₂
As temperature is constant: P₁V₁ = P₂V₂
In this case: Pcyl Vcyl = Patm Vatm
This questions requires finding the volume the oxygen would occupy at atmospheric pressure

1) Convert bar to kPa (SI unit)
1bar = 100kPa
100bar = 10,000kPa

Atmospheric pressure = 101.3kPa - in this case can use 100kPa as questions asks for an estimate

2) 10,000 x 4.5 = 100 x Vatm
Therefore 10,000 x 4.5 / 100 = Vatm
Vatm = 450L
HOWEVER the ‘empty’ cylinder will still contain 4.5L of gas at 1atm, and so the volume of oxygen which can actually be delivered is 445.5L is the question required complete accuracy

3) 450L / 2.5L.min⁻¹ = 180mins

Question 7

How do respiratory mechanics relate to an ideal gas law?

Answer 7

Boyle’s law
Temperature is constant

During inspiration the diaphragm flattens in order to increase the intrathoracic volume. The increased volume results in a fall in pressure, which generates negative pressure and therefore the pressure gradient which results of movement of air into the lungs

Question 8

How does a Bourdon gauge work?

Answer 8

A Bourdon gauge is a type of pressure gauge which can be seen on gas cylinders and anaesthetic machines. Boyle’s law can be applied - as temperature is constant

The gauge has a C-shaped hollow tube which changes shape due to expansion of the gas contained within when exposed to higher pressure. The straightening of this tube moves a needle around the gauge to give an indication of pressure

NOTE: Different to a Bourdon thermometer, which may use Charles’ law???

Question 9

What is Charles’ law?
(2nd ideal gas law)

Answer 9

At a constant pressure, the volume of a gas is directly proportional to its temperature
V ∝ T

Therefore V / T = constant (k₂)

Mnemonic = King Charles was under constant pressure

Question 10

What is Gay-Lussac’s law?
(3rd ideal gas law)

Answer 10

At a constant volume, the pressure of a gas is directly proportional to its temperature
P ∝ T

Therefore P / T = constant (k₃)

Question 11

What is the combined gas law?

Answer 11

PV / T = constant

And for the same gas under different conditions:
P₁V₁ / T₁ = P₂V₂ / T₂

Question 12

A bubble is discharged from the tank of a diver at 40m depth in the sea, at a water temperature of 10 degrees C. The bubble is initially 20ml in volume; what size will the bubble be on reaching the surface where the water temperature is 20 degrees centigrade?
* Assume that for every 10m in depth the pressure increases by the equivalent of the atmospheric pressure at the surface
* The temperature must be in kelvin and the volume in m³

Answer 12

Uses the combined gas law:
P₁V₁ / T₁ = P₂V₂ / T₂
For this question 1=diving, 2=surface

1) Conversions:
- Pressure: 1atm (101.3kPa) at surface, further 1atm for every 10m dived so underwater pressure = 5atm (506.5kPa)
- Volume: 1m³ = 1000L, 1cm³ = 1ml. Therefore 20mls = 20cm³
- Temperature: Degrees C + 273 = kelvin. Therefore 10 degrees C = 283K, 20 degrees C = 293K

2) 5 x 20 / 283 = 1 x V₂ / 293
5 x 20 x 293 / 283 = V₂ = 103.5mls

Question 13

What is the universal gas law?

Answer 13

PV = nRT

n = number of moles
R = universal gas constant (8.31J.K⁻¹.mol⁻¹)

(Found by re-arranging combined gas equation - PV / T = constant)

Question 14

How much volume would 32g of oxygen occupy at STP?

Answer 14

Uses the universal gas law:
PV = nRT

1)Work out values and convert to SI units
* Pressure: Standard pressure is 1atm = 101.3kPa = 101300Pa
* Volume: The variable we are trying to calculate. SI unit for volume is m³, so to convert from litres need to divide by 1000 (1m³ = 1000L)
* Moles: Calculated by mass of gas / molar mass. Molar mass of oxygen is 32 (2 x 16), therefore in this case 32 / 32 = 1
* Universal gas constant: 8.31J.K⁻¹.mol⁻¹
* Temperature: Standard temperature is 0 degrees C = 273K

2) V = 1 x 8.31 x 273 / 101300
= 0.0224m³
x 1000 = 22.4L

Question 15

How does altitude affect the % of oxygen in air, and the partial pressure of oxygen in air

Answer 15

The % of oxygen in air stays the same

The partial pressure of oxygen in air will reduce, as the total air pressure falls. This will reduce the concentration gradient of oxygen across the alveolar membrane, impairing gas exchange

Question 16

What ideal gas law applies to compressed gas cylinders?

Answer 16

Boyle’s law
Temperature is constant, volume and pressure are inversely proportional
Gas is forced into a reduced volume, and therefore pressure must increase

Question 17

Which gases exist as liquids within a compressed gas cylinder?

Answer 17

Carbon dioxide (CO₂) and nitrous oxide (N₂O)

Question 18

What is the pressure within medical gas cylinders? (excluding CO₂ and N₂O)

Answer 18

137 bar
=13700 kPa

Question 19

What is the critical temperature of a gas?

Answer 19

The temperature above which it is not possible to compress a gaseous substance into its liquid or solid phase
The critical temperature of oxygen is -119 degrees C
The critical temperature of nitrous oxide is 36.5 degrees C

Question 20

What is the critical pressure of a gas?

Answer 20

The pressure required to completely liquefy a vapour at its critical temperature

Question 21

What is the saturated vapour pressure?

Answer 21

The vapour pressure at a particular temperature when an equilibrium has been reached between the liquid and gaseous phases of a volatile substance in a container
SVP will increase as temperature increases as more of the substance enters the gaseous phase, therefore exerting more pressure

SVP is often quoted at standard temperature and pressure to allow comparison between substances

Question 22

What is the definition of a vapour?

Answer 22

A gas below its critical temperature, which therefore is possible to compress into its liquid or solid forms

Question 23

Describe how changes in volume, pressure and temperature affect the phase of nitrous oxide

Answer 23

Critical temperature = 36.6 degrees C

At 40 degrees C nitrous oxide is above its critical temperature, and therefore will be a gas regardless of compression

At 36.6 degrees C nitrous oxide exists as a vapour. As it is compressed, the pressure will steadily increase until it reaches critical pressure (72 bar) and is then liquefied. Note that as as liquids are less compressible, pressure then increases significantly with any further compression

At 20 degrees C nitrous oxide exists as a vapour. As it is compressed, the pressure will steadily increase until it reaches the saturated vapour pressure (52 bar at 20 degrees C), when an equilibrium is reached between the liquid and vapour states. As further compression occurs, more of the nitrous oxide becomes liquid with no change in pressure, until it is completely liquefied and further compression causes a significant increase in pressure

Question 24

What is the filling ratio?

Answer 24

The ratio of the mass of a gas in a cylinder to the mass of water which would fill the cylinder

= Mass of cylinder contents / mass of water to fill cylinder

In the UK a maximum filling ratio of 0.75 is used, where as in hot countries it may be 0.67

Cylinders are partially filled to prevent dramatic increases in pressure which may cause the cylinder to rupture / explode

Question 25

Why can a pressure gauge not be used on a cylinder on nitrous oxide?

Answer 25

As nitrous oxide is stored below its critical temperature, there is both liquid and vapour within the cylinder

Whilst there is still liquid in the cylinder, a pressure gauge would only show the saturated vapour pressure at that temperature. When the cylinder is open, it is the vapour which will leave the cylinder - and then will be replenished by evaporation from the liquid so that pressure remains the same

When all the liquid has evaporated, the pressure would then fall as the remaining vapour leaves the cylinder

Question 26

What is entonox?
What are the important points about its storage?

Answer 26

Entonox is a mixture of 50% oxygen and 50% nitrous oxide

The mixture has a ‘pseudo-critical temperature’ of -5.5 degrees C when stored in a cylinder at 137 bar. Below this temperature ‘lamination’ occurs whereby the mixture separates into a liquid containing nitrous oxide with ~20% oxygen, and high concentration of oxygen gas above it. Therefore the patient would receive initially a gas with no analgesic benefit, and then a hypoxic mixture of gas

Entonox cylinders may have a ‘dip-tube’ to mitigate some of the effects of lamination by delivering some liquid from the bottom of the cylinder

Entonox is stored in a white cylinder with blue/white quartered shoulders

Piped entonox has a pseudo-critical temperature of -30 degrees C as it is delivered at 4 bar, and therefore may be considered safer

Question 27

Why might a cylinder of nitrous oxide become cold during its use?

Answer 27

Latent heat of vaporization
As vapour leaves the cylinder, it is replenished by evaporation of liquid nitrous oxide. This requires energy in order to break bonds, which is taken from the contents of the cylinder - which therefore cools

Question 28

What is the triple point of water?

Answer 28

The temperature at which water can exist in equilibirium in all three phases (solid, liquid, gas)
= 0.01 degrees C @ 611.7Pa