eLFH - Gas Laws

Question 1

Boyle’s law

Answer 1

At a constant temperature, the volume of a given mass of gas is inversely proportional to the absolute pressure

P x V = constant
Aka: P1 x V1 = P2 x V2

Question 2

How to calculate volume of gas that can be delivered from cylinder from gauge pressure

Answer 2

Use Boyle’s Law
P1 x V1 = P2 x V2
Therefore V2 = (P1 x V1) / P2

Convert units to kPa
P1 = Gauge pressure + atmospheric pressure (to give absolute pressure)
V1 = cylinder volume
P2 = atmospheric pressure
V2 therefore is volume delivered (but remember volume of cannister will remain inside once pressures equalise so minus that from total volume to be delivered)

Eg. 10 L cylinder with gauge pressure 137 bar
P1 = 137 bar + 1 bar = 138 bar = 13,800 kPa
P2 = 1 bar = 100 kPa
V1 = 10 L

V2 = (P1 x V1) / P2
= (13,800 x 10) / 100
= 1380 L
(Total volume delivered = 1380L - 10L which will remain in cylinder)
Total gas delivery = 1370 L

Question 3

Charles’ Law

Answer 3

At a constant pressure, the volume of a given mass of gas is directly proportional with the absolute temperature

V / T = constant
V1 / T1 = V2 / T2

Question 4

Gay-Lussac’s Law

Answer 4

At a constant volume, the absolute pressure of a given mass of gas is directly proportional to the absolute temperature

P / T = constant
P1 / T1 = P2 / T2

Question 5

Combined gas law

Answer 5

Combining all 3 gas laws gives following equation:
PV / T = C

Therefore:
(P1 x V1) / T1 = (P2 x V2) / T2

Temperature must be in Kelvin, but pressure and volume can be in any units as long as they are consistent on both sides

Question 6

Avogadro’s Principle

Answer 6

Equal volumes of all gases, at the same temperature and pressure, have the same number of molecules

Question 7

Mole definition

Answer 7

A mole is the quantity of substance containing the same number of particles as there are atoms in it

I.e 12 g of Carbon 12 will contain 1 mole (6.022 x 10^23 particles)
and 16 g od Oxygen 16 will also contain 1 mole

This number of particles is known as Avogadro’s constant - 6.022 x 10^23

Question 8

Avogadro’s constant

Answer 8

6.022 x 10^23

Number of particles in 1 mole

Question 9

Volume of 1 mole of any gas at standard temperature and pressure

Answer 9

22.4 Litres

Question 10

Standard temperature

Answer 10

273.15 Kelvin

Question 11

Standard pressure

Answer 11

1 atmosphere

Question 12

Ideal Gas Law
(Combined gas law with Avogadro’s law)

Answer 12

PV / T = C

Constant will vary according to number of moles present (n)

Therefore C = nR
Where R is the universal gas constant and n is number of moles

Therefore PV / T = nR
Usually written as PV = nRT

Question 13

Universal gas constant (R) value

Answer 13

8.31 J / K / mol

Same R is in Nernst Equation

Question 14

Dalton’s Law of partial pressures

Answer 14

In a mixture of gases, the pressure exerted by each gas is the same as that which it would exert if it were alone in the container

Total pressure within a container = sum of individual pressures exerted by each gas
P tot = P1 + P2 + P3 +…

I.e
PV = (n1 + n2 +…) RT

Question 15

Critical temperature

Answer 15

The temperature above which a gas cannot be liquified no matter how much pressure is applied

Different temperature for different gases

Question 16

Critical pressure

Answer 16

Saturated vapour pressure of a substance at its critical temperature

Question 17

Gas vs Vapour

Answer 17

Gas is a substance above its critical temperature

Vapour is a substance below its critical temperature

Question 18

Critical temperature of oxygen

Answer 18

• 119 degrees Celsius

Question 19

Factors which make real gases deviate further from the ideal gas

Answer 19

Decrease in temperature
Increase in pressure

Question 20

Volume that one mole of a gas will occupy at standard pressure and temperature

Answer 20

22.4 L

Question 21

Henry’s Law

Answer 21

At a constant temperature, the amount of gas dissolved in a solvent is proportional to its partial pressure above the solvent