Gases & Properties and Phase Diagram

Which of the following has the greatest amount of movement at a given temperature?

• Gas

• Liquid

• Solid

• All matters have the same amount of movement

Answer explanation

Gas are not tightly packed. They have the space to move around.

How are pressure and volume related?

• Indirectly

• Directly

• They are not related

• None of the all the answers

Answer explanation

Remember the formula PV=nRT;

by rearranging P = nRT / V

They are inversely proportional.

You go camping and roast marshmallows over a campfire. What happens to the volume of the air bubbles inside the marshmallows? 

• It decreased

• It increased

• It stay the same

• It melted

Answer explanation

Remember the formula of PV = nRT

P and T is proportional to each other. If temperature increases, volume will also increases.

By referring to the diagram, what actually happen?

• Gas molecules cannot be compressed. There is no volume.

• Gas molecules can be compressed at low pressure. There is volume.

• Gas molecules can be compressed at high pressure. There is volume.

• Gas molecules can be compressed at high pressure. There is no volume.

Observe the diagram. At which conditions favors the liquification of methane (CH4)?

• Pressure of 400 psi and compressibility factor of 0.9

• Pressure of 200 psi and compressibility factor of 1.4

• Pressure of 200 psi and compressibility factor of 0.85

• Pressure of 300 psi and compressibility factor of 0.6

Answer explanation

at pressure of 200 psi and compressibility factor of 0.85, there is an attractive intermolecular forces between the molecules.

If compressibility factor above 1.0, there is repulsion between the molecules causing instability.

A mixture containing 4.0 mol He (g), 2.5 mol N₂ (g) and 3.5 mol Ar (g) exerts a total pressure of 1.2 atm. What is the partial pressure of He (g)? (Assume the volume and temperature is constant for all)

• 0.30 atm

• 0.48 atm

• 0.72 atm

• 1.2 atm

Answer explanation

apply this formula

Ptotal = P1 + P2 + P3

Determine the number of moles in a container of gas at STP with a volume of 99.2 L. 

• 222.08 moles

• 22.4 moles

• 4.43 moles

• 76.8 moles

Answer explanation

L = dm³

1 mol = 22.4 dm³ at STP

X mol = 99.2 dm³

Apply cross multiply to get X mol

A sealed jar whose volume is exactly 1 liter contains exactly 1 mole of air at 20.00 °C. Assuming the air behaves as an ideal gas, what is the pressure (Pa) inside the jar?

• 2.437 × 10⁶ Pa

• 24.37 × 10⁶ Pa

• 243.7 × 10⁶ Pa

• 24370 Pa

Answer explanation

Solve using the ideal gas equation, PV = nRT.

(a) Rearrange the equation to show P as the subject:

P = nRT / V

(b) Write down the known values with the correct SI units:

n = 1

R = 8.314 J K^-1 mol^-1

T = 20 °C = (20 + 273.15) K = 293.15 K

V = 1 liter = 0.001 m³

(c) Put the values into the equation P = nRT / V:

     P  =  (1 × 8.314 × 293.15) / 0.001

⇒  P  =  2,437,249

⇒  P  =  2.437 × 10⁶ Pa

Which of the following is correct for real gases? There are two answers.

• At high pressure, volume is larger than predicted by ideal gas law

• volume is small than predicted by ideal gas law

• At high pressure and low temperature, particles are much closer together and develop intermolecular attractions between the molecules.

• At low pressure and high temperature, particles are much closer together and develop intermolecular attractions between the molecules.

Calculate the pressure in mmHg, given 50 atm.

• 38,000 mmHg

• 3800 mmHg

• 380 mmHg

• 38 mmHg

Answer explanation

Remember the conversion unit:

atm -> mmHg - multiply by 760

atm -> kPa - multiply by 101.32

psi -> kPa - multiply by 6.895