4: Gases

Question 1

What is pressure and what is its unit?

Answer 1

Pressure measures the normal (perpendicular) force exerted on a surface per unit area. It is measured in pascal (Pa).

Question 2

What is the formula for pressure?

Answer 2

Pressure=normal force on a surface/area of the surface=F/A

Question 3

1Pa=?N

Answer 3

1Pa=1N M^-2 (newton per square meter)

Question 4

What is atmostpheric pressure?

Answer 4

Atmospheric pressure is the force from the atmosphere around us, which acts on all surfaces exposed to the atmosphere, including our bodies.

Question 5

What is the unit for atmospheric pressure and what is its relationship with Pa?

Answer 5

atm, 1atm=100 kPa=atmospheric pressure at sea level

Question 6

What tool is used to measure gas pressure and how does it work?

Answer 6

Bourdon gauge. It has a curved metal tube which uncoils when pressure is applied, which moves the pointer around a dial.

Question 7

What is Boyle’s law?

Answer 7

Boyles’ Law states that for any gas with a fixed mass and temperature, its pressure is inversely proportional tro its volume

Question 8

If the pressure of a gas becomes k times the original pressure, and the temperature stays constant, what is the new volume of the gas in relation to the original volume?

Answer 8

The volume will become 1/k of the original volume.

Question 9

What is the shape of a graph plotted against the pressure and temperature of a gas? Where does the graph cut the x-axis (temperature axis)?

Answer 9

It is a linear graph with x-intercept at -273.15°C (absolute zero).

Question 10

What is the kelvin temperature scale in relation with the celcius temperature scale?

Answer 10

273 kelvin=0°C, 0 kelvin=-273°C

Question 11

What is another name for the kelvin temperature scale? Why?

Answer 11

It is also called absolute temperature scale, because 0 kelvin=absolute zero

Question 12

What is the pressure law?

Answer 12

For a gas with fixed volume and mass, its pressure is directly proportional to its kelvin temperature.

Question 13

What is the shape of a graph plotted against the volume and temperature of a gas? Where does the graph cut the x-axis (temperature axis)?

Answer 13

It is a linear graph with x-intercept at -273.15°C (absolute zero)

Question 14

What is Charles’ law?

Answer 14

Charles’ law shows that for a gas with fixed mass and pressure, its volume is directly proportional to its kelvin temperature.

Question 15

What can be inferred from Boyle’s law, the pressure law, and Charles’ law?

Answer 15

The product of the pressure of a gas and its volume is directly proportional to its temperature, or pV/T=constant.

Question 16

How many molecules are there in a mole?

Answer 16

6.02*10^23 (Avogadro’s number)

Question 17

What is the general gas law?

Answer 17

pV=nRT, where n is the number of moles of molecules in a gas

Question 18

What is R in the general gas law?

Answer 18

R is called the universal gas constant, which is found to be 8.31 J mol^-1 K^-1

Question 19

What is an ideal gas and how can a real gas behave like an ideal gas?

Answer 19

An ideal gas is a gas that oberys the general gas law. A real gas behaves like an ideal gas at low pressures and high temperatures.

Question 20

What does the kinetic gas theory state?

Answer 20

Gas molecules are in random motion all the time.

Question 21

What are the assumptions of molecules in an ideal gas?

Answer 21

1. All molecules have the same mass and are identical.
2. All molecules are in continuous random motion.
3. There is a large number of molecules in the container.
4. The size of the molecules are negligible compared with the separation between them.
5. The duration of collisions are negligible compared with the time between collisions.
6. The collisions of the molecules themselves/with the container are perfectly elastic.
7. Intermolecular forces are negligible.

Question 22

In which conditions do the gas satisfy the assumptions of the ideal gases?

Answer 22

When the temperature is high (well above its boiling point) and pressure is low.

Question 23

What is the change in gas pressure when there is a change in momentum?

Answer 23

When there is a higher momentum, the gas pressure increases.

Question 24

What is the relationship between pressure, volume, mass, and velocity in an ideal gas?

Answer 24

pressurevolume=1/3mass*mean square value of velocities
pV=1/3Nm(msv.c^2)

Question 25

What is the total kinetic energy in one mole of gas?

Answer 25

3/2RT

Question 26

What is the average kinetic energy of a molecule?

Answer 26

3RT/2Na(where Na denotes the number of molecules in one mole of gas)

Question 27

Why can we infer the amount of total KE of gas molecules from their temperature?

Answer 27

Shown from the equation KEavg=3RT/2Na, temperature (in kelvin) is directly proportional to the average KE.

Question 28

What is the molecular potential energy for ideal gases? What does this tell about the total internal energy of an ideal gas?

Answer 28

There is no potential energy for an ideal gas from the assumptions. Therefore, the total internal energy of an ideal gas is given by its total kinetic energy=3/2nRT.

Question 29

What is the root mean square speed of gas molecules and how can it be caluclated?

Answer 29

The root-mean-square speed (rms) is the typical speed of molecules, calculated by taking the square root of (msv.c^2). rms.c=sqrt(3pV/Nm)=sqrt(3RT/mNa)

Question 30

What is Nm and mNa in calculating the rms.c?

Answer 30

Nm: total mass of the gas
mNa: molar mass of the gas

Question 31

How can a mechanical simulator simulate Boyle’s law?

Answer 31

When the voltage is unchanged, and when the weight of the piston is increased, the height of the piston decreases, which simulates what happens when the temperature is constant but the pressure increases. This results in a decrease in volume, simulating Boyle’s law.

Question 32

How can a mechanical simulator simulate the pressure law?

Answer 32

When the voltage is increased, and the height of the piston is unchanged, the weight of the piston increases, which simulates what happens when the temperature rises and the volume stays constant. This results in an increase in pressure, simulating the pressure law.

Question 33

How can a mechanical simulator simulate Charles’ law?

Answer 33

When the voltage is increased, and the weight of the piston is unchanged, the height of the piston increases, which simulates what happens when the temperature rises and the pressure stays constant. This results in an increase in volume, simulating the Charles’ law.

Question 34

How can a mechanical simulator simulate the relationship between volume and the number of molecules?

Answer 34

When the voltage and weight of the piston ar eunchanged, but more ball bearings are added to the simulator, the height of the piston increases, which simulates what happens when the temperature and pressure is constant but more gas molecules are added. This results in an increase in volume, simulating the relationship between volume and the number of molecules.