gaseous state

Question 1

State Avogadro’s Law.

Answer 1

Equal volumes of all gases, under the same temperature and pressure, contain the same
number of particles (atoms or molecules).
One mole contains exactly 6.02 × 1023 (or Avogadro constant, L) elementary entities.

Question 2

What is relative molecular mass?

Answer 2

The average mass of one molecule of a substance over 1/12 of the mass of one atom of 12C
*Mr has no units as it is a ratio of two masses

Question 3

Define gas pressure.

Answer 3

Gas pressure is a gauge of the frequency and force of collisions between gas particles
and the walls of the container that hold them.

Question 4

What is the ideal gas equation and what units is each variable in?

Answer 4

PV = nRT
P in Pa (1 atm = 101325 Pa, 1 bar = 10^5 Pa)
V in m3 (1cm3 = 10^-6 m3, 1dm3 = 10^-3 m3)
T in K (T in C +273)
n in mol
R = 8.31 J/Kmol

Question 5

What are the equations for partial pressures?

Answer 5

Ptotal= Pa + Pb + Pc…
Ptotal= ntotal(RT/V)
Pa = (na/ntotal)(Ptotal)

Question 5

What is the combined gas equation and when is it used?

Answer 5

(𝑃1𝑉1)/𝑇1 = (𝑃2𝑉2)/𝑇2
Used when there are changes to gas systems and the amount of gas, n, is kept constant. Temperatures must only be in K for expression to be valid.

Question 6

State the three assumptions of ideal gases.

Answer 6

1. The gas particles have negligible volume compared to the volume of the container.
2. The intermolecular forces of attraction between gas particles are negligible.
3. Collisions between gas particles, and their collisions with the walls of the container, are perfectly
   elastic; i.e. there is no net loss or gain of kinetic energy during collision.

Question 7

What are the conditions for real gases to behave ideally?

Answer 7

● At low pressures, the gaseous molecules are relatively far apart. The volume of the molecules
themselves is negligible compared to the volume of the container. Thus, real gas molecules at low
pressure can be approximated to have negligible volume. Also, intermolecular forces are negligible as the particles are far apart. Hence their behaviour at low pressures would approach that of ideal gases.
● At high temperatures, gas particles have enough kinetic energy to overcome intermolecular forces, which can thus be considered insignificant. As such, the behavior of real gases approach ideal gas behaviour at high temperatures.

Question 8

Explain deviations from ideal gas behaviour.

Answer 8

● The gas particles can no longer be considered to have negligible volume compared to the volume of
the container.
At high pressures, the volume of the container decreases. The molecules are pushed closely together and
take up a significant portion of the container volume, resulting in less space in which the molecules can
move. Thus, it is no longer valid to assume that its volume is negligible compared to the container. (In fact, the total volume occupied by a real gas is actually greater than the volume predicted by the ideal gas equation.)
Also, since the gas particles are close together, they tend to interact with one another, hence intermolecular attractions are not negligible.
● The intermolecular forces between gas particles become significant.
As temperature is lowered, the kinetic energy of the gas particles decreases, causing them to move more slowly and intermolecular forces to become more significant. This also causes collisions to become inelastic. Eventually, it reaches a point where the particles can no longer overcome the intermolecular forces, at which point real gases liquefy when cooled to below its boiling point.