8. The Gas Phase

Question 1

(8.1) Name some characteristics that make the gas phase unique. (x3)

Answer 1

1. Gases are compressible fluids with rapid molecular motion
2. large intermolecular distances
3. weak intermolecular forces.

Question 2

(8.1) A mercury barometer is primarily affected by atmospheric pressure. What would happen to the level of the mercury in the column if:

1. The barometer was moved to the top of the mountain
2. The Barometer was moved 10 meters under water

Answer 2

top of the mountain- it would decrease, because there’s less pressure pushing downwards, so more mercury in the tube can flow out.

under water: it would increase, because there would be a lot of pressure exerting force.

Question 3

(8.1) What are the conditions for STP and standard conditions?

Answer 3

STP: 273 K and 1 ATM
standard Conditions: 298 K, 1 ATM, 1 M

Note, the only difference is that in standard conditions, the temperature is 25 degrees higher.

Question 4

(8.1) With gas, what’s the equivalent between atm, mmhg, toor, and kPa?

__atm= ___mmHg= ___torr= ___ kPa

Answer 4

1 atm= 760 mmHg= 760 torr= 101.325 kPa

Question 5

How do real gasses deviate from the laws of ideal gasses? (x2)

Answer 5

Ideal gasses assume there are no intermolecular forces and occupy no volume.

However, gas molecules do have intermolecular force and do volume.

Question 6

At STP (273 K, 1 ATM), how much space does one mole of an ideal gas occupy?

Answer 6

22.4 L

Question 7

What is the ideal gas law?

What is the ideal gas constant?

Answer 7

PV=nRT

P= Pressure (atm)
V= volume (L)
n= moles
T= temperature (K)

r= ideal gas constant= 8.21x 10^-2(L *atm)/(mol *K)

Remember, if it assumes standard pressure, volume = 22.4 L

Question 8

What is Avogadro’s principle?

Answer 8

there is a direct relationship between moles and volume if pressure and temperature are held at a constant.

This is just a modified version of the ideal gas law PV=nRT

Question 9

What is Dalton’s law of partial pressure?

Answer 9

Ptotal= P_{gas A}+ P_{gas B} + P_{gas A} + P_gas… ……

The partial pressure of a gas is related to its mole fraction and can be determined using the following equation:

P_A= X_AP_T
X_A= (moles of gas A)/(total moles of gas)
P_T= total pressure

Question 10

A container with 4 moles of a gas at a pressure of 8 atm has a volume of 12 liters. What is its temperature? (note: r= 8.21x 10-2(L *atm)/(mol *K) )

Answer 10

To solve, use the equation:
PV=nRT

answer= 300k. (actual 292.3K)

Question 11

What is the density of argon gas at 4 atm and 127°C?

Answer 11

Density = g/L

PV=nRT –> P/(RT)= n/L

then convert moles to G to get G/L

answer 5 g/L (actual 4.85 g/L)

Question 12

A 20 L sample at 300 ° C and 5 atm of pressure contains 2 moles of a gas. If an additional 3 moles of gas at the same pressure and temperature are added, what is the final total volume of the gas?

Answer 12

PV=nRT
n1/L1= n2/L2
2mol/20L= 5mol/x

2 moles + 3 moles = 5 mole
Solve the proportion.

Answer: 50 L

Question 13

What would be the volume of a 2 L sample of neon if its pressure is changed from 1 atm to 40 atm under isothermal conditions?

Answer 13

PV= nRT

P1V1=P2V2

(1)(2)= (40)(x)

=2/40=1/20=.05

Answer= .05 L

Question 14

If the temperature of 6 L of gas at constant pressure is changed from 27°C to 127 °C, what would be its final volume?

Answer 14

PV=nRT
V1/T1=V2/T2

COVERT TO KELVIN

6/300=V/400

V=8

Answer= 8 L

Question 15

If the pressure of a sample of gas with a temperature of 227 °C is changed from 5 atm to 2 atm during cooling, what would be the final temperature?

Answer 15

Answer: 200K

PV=nRT
P/T= P/T

Remember, Temp needs to be converted to Kalvins

Question 16

How can the concentration of carbon dioxide in sodas or other carbonated beverages be so much higher than that of atmospheric carbon dioxide?

Answer 16

High pressures of carbon dioxide gas are forced on top of the liquid in the sodas, increasing the concentration in the liquid.

Question 17

A vessel contains 8 mol O2, 3 mol of CH4, and 1 mol of CO2 at a total pressure of 240 atm. What is the partial pressure of each gas?

Answer 17

Oxygen 160 atm
CH4= 60 atm
CO2= 20 atm

Question 18

What is the kinetic molecular theory and what does it attempt to do?

Hint: involves gases.

What are its assumptions?

Answer 18

kinetic molecular theory attempts to explain the behavior of gas particles (as opposed to describing them). It makes a number of assumptions about the gas particles. Regardless of the type of gas or molecule, in general,

This law assumes:
-Gas particles have negligible volume.
-Gas particles do not have intermolecular attractions or repulsions (because they are already so far apart).
-Gas particles undergo random collisions with each other and the walls of the container
-Collisions between gas particles (and with the walls of the container) are elastic.
-The average kinetic energy of the gas particles is directly proportional to temperature (as temperature increases, kinetic energy increases, and they go faster).

Question 19

What does Gram’s law describe? (regarding gases)

Hint: diffusion and effusion

Answer 19

describes the behavior of gas diffusion or effusion, stating that gases with lower molar masses will diffuse or effuse faster than gases with higher molar masses at the same temperature

Question 20

When thinking about gasses, what’s the difference between diffusion and effusion?

Answer 20

Diffusion- spreading out of particles from high to low concentrations (gasses mix with one and other)

Effusion: is the movement of gas from one compartment to another through a small opening under pressure.

Question 21

What are the assumptions made by the kinetic molecular theory?

Answer 21

1. Negligible volume of gas particles
2. No intermolecular forces
3. random motion
4. Elastic collisions
5. Proportionality between absolute temperatures and energy

Question 22

Hydrogen sulfide (H₂S) has a very strong rotten egg odor. Methyl salicylate (C₈H₈O) has a winter green odor, and benzaldehyde (C₇H₆O) has a pleasant almond odor. If the vapors for these three substances were released at the same time from across the room, which odor would one smell the odors?

Answer 22

Hydrogen sulfide first because it travels the quickest because it has the smallest molar mass, then benzaldehyde, then methyl salicylate last because it is the largest.

Question 23

What is the average speed of helium atoms at -173°C?

Answer 23

This is very difficult.
790 m/s

Question 24

If neon gas travels at 400 m/s at a given temperature, calculate the average speed of krypton at the same temperature

Answer 24

200 m/s

Question 25

In what ways do real gases differ from ideal gasses?

In what circumstances will this be evident?

Answer 25

Real gas molecules have non-negligible volume and attractive forces.

Real gases deviate from ideal gases at high pressure (low volume) and low temperature.