Gas Laws

Question 1

3 states of matter

Answer 1

-Solids
- Liquid
- Gases

Question 2

Describe motion of particles in a solid

Answer 2

Particles cannot move, they can only vibrate about fixed positions

Question 3

Motion of a Liquid

Answer 3

Particles have more energy and can break away and slide over each other.

Question 4

Motion of a Gas

Answer 4

Particles have most energy, and are widely spaced moving at great speed

Question 5

Demonstrating diffusion in gases

Answer 5

• Piece of cotton wool soaked in NH3 in one end of a large glass tubing and cotton wool soaked in HCL on the other end.
• When two gases meet a white ring of ammonium chloride forms (NH4Cl).
• Forms closer to HCL proving that NH3 diffuses quicker

Question 6

Changing C into K

Answer 6

add 273 = K
Minus 273 = C

Question 7

Pressure

Answer 7

The force a gas exerts on a unit area

Question 8

Pa into kPa

Answer 8

Pa —> kPa = ÷ 1000
kPa —> Pa = x 1000

Question 9

Volume

Answer 9

Amount of space a gas takes up

Question 10

cm3 into m3
L into m3

Answer 10

cm3 into m3 = x 10 ^-6

litres into m3 = x 10^-3

Question 11

Boyles Law

Answer 11

states that for a definite mass of gas at constant temperature, volume is inversely proportional to pressure

p₁ × V₁ = p₂ × V₂

Question 12

What is Charles’ Law?

Answer 12

states that for a definite mass of gas at constant pressure, volume is directly proportional to temperature

V₁ / T₁ = V₂ / T₂,

Question 13

Combines gas Law

Answer 13

p₁ × V₁ = p₂ × V₂
——— ———-
T₁ T₂

NOTE: temp must be in kelvins

Question 14

What is Gay-Lussac’s law of combining volumes?

Answer 14

states that in a reaction between gases, there is a simple whole number ratio of volumes of reactants and products
at the same conditions of temperature and pressure

EG : 2H2 (g) + O2 (g) —-> 2H2O (g)
ratio = 2 : 1 : 2

Question 15

Avogadro’s law?

Answer 15

• states that equal volumes of gases at the same conditions of temperature and pressure have equal numbers of molecules and therefore equal numbers of moles

Question 16

Outline the assumptions of the kinetic theory of gases

Answer 16

(1) Gases are made up of particles in continuous rapid random straight line motion, colliding with each other and with the walls of their container

(2) There are no attractive or repulsive forces between the molecules of a gas

(3) The volume of the molecules is negligible compared to the space they occupy

(4) Collisions between molecules are perfectly elastic - no loss of kinetic energy

(5) The average kinetic energy of molecules is directly proportional to temperature in Kelvin

Question 17

What is an ideal gas?

Answer 17

An ideal gas is one that perfectly obeys all the gas laws under all conditions of temperature and pressure

Question 18

Outline why real gases deviate from ideal gas behaviour/What are limitations of the kinetic theory of gases?

Answer 18

Real gases:
1. Have attractive or repulsive forces between their molecules – intermolecular forces
2. The volume of molecules is not always negligible compared to the space they occupy

Note: This is especially the case at high pressures and low temperatures

Question 19

At what conditions do real gases come closest to ideal gas behaviour?

Answer 19

a) At low pressures- molecules are widely spaced

b) At high temperatures – molecules have enough energy to overcome attractive
or repulsive forces

Question 20

The equation of state for an ideal gas / Ideal gas equation/Universal gas equation

Answer 20

pV = nRT

Pg: 64 of log tables

Question 21

What is meant by a volatile liquid? Name a volatile liquid

Answer 21

• A volatile liquid is a liquid with a low boiling point
• Propanone is a volatile liquid suitable for this experiment

Question 22

explain why the pressure of the vapour at the end of the heating stage of the experiment was known to be equal to the atmospheric pressure
at your location,
& how the volume of the vapour was measured.

Answer 22

• Plunger of larger gas syringe moved back and became steady
• subtract initial and final readings on large gas syringe scale

Question 23

How is the volatile liquid vapourised and how is the temperature of the vapour obtained?

Answer 23

Steam flows through insulate box , heats box and goes out the outlet to sink.
thermometer inside insulated box

Question 24

How is the pressure of the vapour in the gas syringe measured.

Answer 24

• A barometer is used to measure atmospheric pressure.
• The pressure of the vapour in the gas syringe will be the same as atmospheric pressure

Question 25

Why is this method unsuitable for non-volatile liquids?

Answer 25

• The boiling points of non-volatile liquids are too high – steam will not vapourise them

Question 25

What modern instrumental technique can be used as a more accurate method to measure the relative molecular mass of volatile and non-volatile liquids as well as of solid and gaseous substances?

Answer 25

Mass spectrometry

Question 26

Give three errors in this experiment that may lead to inaccurate results

Answer 26

1. Accuracy of the measuring instruments i.e. the balance, the graduated gas syringe.
2. Air bubbles in the hypodermic syringe.
3. There is no such thing as an ideal gas and we use the ideal gas equation to calculate the molecular mass of the volatile liquid.

Question 27

To measure the relative molecular mass of a volatile liquid (using a gas syringe)
Procedure:

Answer 27

1.) About 5cm3 of air was drawn into the graduated gas syringe.The rubber cap was placed on the graduated syringe.
2.) Steam was then passed through the insulated box until the volume of the air and temperature reading became steady, These values were noted
3.) Some propane was drawn into the hypodermic syringe and the mass was noted using an electronic balance.
4.) The hypodermic syringe was then pushed through the rubber cap and some of the propanone was injected into the graduated syringe.
5.) Note the plunger of the graduated syringe moved out due to the propanone vaporising. Note the new volume reading.
6.) The hypodermic syringe was withdrawn and reweighed and the new mass noted.
7.) Atmospheric pressure was noted.

Formula =
Initial mass of hypodermic syringe and liquid
-
Final mass of hypodermic syringe and liquid
= Mass of liquid

Initial volume of air in graduated gas syringe
+
Final volume of air and vapour in graduated gas syringe
= volume of vapour

n= PV/RT

Question 28

describe how the mass of the vapour in the container
at the end of the heating stage was found.

Answer 28

-Subtract mass of hypodermic (small) syringe after injection from the mass of hypodermic (small) syringe mass and liquid contents before injection.

Question 29

Bromine (Br2) (Mr = 160) has a significantly greater relative molecular mass than water (Mr = 18). Account for the volatility of bromine (boiling point 58.8 °C)
compared to that of water (boiling point 100 °C).

Answer 29

hydrogen (H) bonds in water /
dipole-dipole forces (bonds, interactions) in water (H2O) /
van der Waals (London, dispersion, temporary intermolecular, induced dipole) forces (bonds, interactions) in bromine (Br2) /

intermolecular forces (bonds, interactions) in water (H2O) stronger / intermolecular forces (bonds, interactions) in bromine (Br2) weaker /
bromine (Br2) non-polar (pure covalent) / water (H2O) is polar