5.1.4: Ideal gases

Question 1

Avogadro’s constant

Answer 1

6.02x10^23

Question 2

Ideal gas assumptions

Answer 2

• Large number of molecules in rapid random motion

Question 3

Ideal gas assumptions

Answer 3

• Collisions are instantaneous* (Time during collisions is negligible when compared to time between collisions)

Question 4

Ideal gas assumptions

Answer 4

• Collisions are perfectly elastic (Kinetic energy is conserved)

Question 5

Ideal gas assumptions

Answer 5

• Volume of the particles is negligible when compared to the volume of the gas

Question 6

Ideal gas assumptions

Answer 6

• Negligible forces between molecules except during collisions (As there is no potential energy between molecules - as electrostatic attraction is ignored - there is no potential energy in the gas therefore the internal energy (sum of Ke and Pe) is just kinetic energy)

Question 7

Boyle’s Law

Answer 7

P ∝ 1/V
PV = constant
At a constant temperature and mass, the pressure of an ideal gas is inversely proportional to volume

Question 8

Investigating Boyle’s Law

Answer 8

• Glass tube with Mercury to seal it connected to a pump with a pressure gauge attached. Measure length L of the glass tube between its end and the Hg. SLOWLY pump the air and increase the pressure. As L is proportional to V for constant cross sectional area A, graph of P and V is a downward curve like a slide. Graph of P against 1/V is a straight line. Therefore, P ∝ 1/V

Question 9

Approximating absolute zero

Answer 9

The graph of pressure P against temperature T is a y=mx+c graph. If you extrapolate it backwards to lower and lower temperatures, it eventually hits the x axis at absolute zero. Kinetic energy is zero so pressure is also zero. Both the mass and the volume of the gas need to be kept constant in order for pressure to be proportional to the absolute temperature of an ideal gas.
P ∝ T
P/T = Constant
KELVIN NOT CELCIUS