Textbook Ch.5: Gases

Question 1

What units can volumes of gases be expressed in?

Answer 1

L, cm³, mL³

Question 2

How can the number of moles be related to mass in grams and molar mass?

Answer 2

n = number of moles
m = mass in grams 
MM = molar mass
n = m/MM

Question 3

How is the temperature of a gas ordinarily measured using a thermometer?

Answer 3

in degrees celsius °C

Question 4

How must you express temperatures of gases when you are doing any calculations involving the physical behavior of gases?

Answer 4

using the Kelvin scale

Question 5

What relation can be used to convert between °C and K?

Answer 5

Tₖ = T꜀ + 273

Celsius –> kelvin (take your value and add 273)

Kelvin –> celsius (take your value and subtract 273)

Question 6

Pressure

Answer 6

force per unit of area

Question 7

Pai

Answer 7

a unit of pressure “pounds for square inch”

Question 8

Barometers and manometers

Answer 8

devices commonly used to measure atmospheric pressure using mercury

Question 9

Millimeters of mercury (mm Hg)

Answer 9

a unit for measuring pressure

Question 10

Atmosphere (atm)

Answer 10

a unit for measuring pressure

Question 11

Pascal (Pa)

Answer 11

a standard unit of pressure

Question 12

Bar

Answer 12

10⁵ pascals

Question 13

Relate bars, to atm, to mm Hg, to psi, to Pa

Answer 13

1.013 bar = 1 atm = 760 mmHg = 14.7 psi = 100 kPa

Question 14

Relate temperature, volume, number of particles/amount, and pressure of gases

Answer 14

• Volume is directly proportional to amount
• Volume is directly proportional to absolute temperature (Kelvin scale!)
• Volume is inversely proportional to pressure

Question 15

Ideal gas law

Answer 15

PV = nRT
P = pressure
V = volume
n = number of moles
T = temperature on the Kelvin scale 
R = a constant

Question 16

What is the value of R in gas law problems?

Answer 16

0.0821 latm / molk

Question 17

What is the value of R in equations involving energy in joules?

Answer 17

8.31 J / mol*k

Question 18

What is the value of R in the calculation of molecular speed?

Answer 18

8.31 * 10³ gm^2 / s^2mol*K

Question 19

Standard temperature and pressure (STP)

Answer 19

0°C and 1 atm

Question 20

What is the relationship between final and initial state for pressure and temperature?

Answer 20

P2/P1 = T2/T1

Question 21

The density of a gas is dependent on:

Answer 21

• pressure
• temperature
• molar mass

Question 22

The ___ ratio of any two gases in a reaction at a constant temperature and pressure is the same as the ___ ratio

Answer 22

volume ratio is the same as the mole ratio

Question 23

Partial pressures

Answer 23

Ptot = nA*(RT/V) + nB

pressures certain gases would exert if they were alone

Question 24

Dalton’s law

Answer 24

the total pressure of a gas mixture is the sum of the partial pressures of the components of the mixture

Question 25

Vapor pressure

Answer 25

a measure of the tendency of a material to change into the gaseous or vapor state

Question 26

Mole fraction

Answer 26

nA / ntot

Question 27

Express partial pressure in terms of mole fraction and total pressure

Answer 27

Xₐ = mole fraction
Pₐ= Xₐ*Ptot

Question 28

What does a simple molecular model of gas assume?

Answer 28

• Gases are mostly empty space. The total volume of the molecules is negligibly small compared with that of the container to which they are confined
• Gas molecules are in constant, chaotic motion. They collide frequently with one another and the container walls –> their velocities are constantly changing
• Collisions are elastic. There are no attractive forces that would tend to make molecules “stick” to one another or the container walls
• Gas pressure is caused by collisions of molecules with the walls of the container –> pressure increases with the energy and frequency of these collisions

Question 29

Formula for: the pressure (P) exerted by a gas in a container of volume V

Answer 29

P = Nmv² / 3V
N = number of molecules
m = mass of a molecule
u = average speed

Question 30

What does the ratio N/V express?

Answer 30

the concentration of gas molecules in the container. The more molecules there are in a given volume, the greater the collision frequency and so the greater the pressure

Question 31

What does the product mu² measure?

Answer 31

the energy of the collision

Question 32

Expression of the average translational kinetic energy of a gas molecule:

Answer 32

E = 3RT / 2N

• at a given temperature, molecules of different gases must all have the same average kinetic energy of translational motion
• the average translational kinetic energy of a gas molecule is directly proportional to the Kelvin temperature, T

Question 33

Expression for the average atomic speed

Answer 33

u = (3RT / MM)^1/2

• average speed (u) is directly proportional to the square root of the absolute temperature

Question 34

Effusion

Answer 34

the flow of gas molecules at low pressures through tiny pores or pinholes

Question 35

Graham’s law

Answer 35

at a given temperature and pressure, the rate of effusion of a gas, in moles per unit time, is inversely proportional to the square
root of its molar mass

Question 36

In general, the closer a gas is to the ____ state, the more it will deviate from the ideal gas law

Answer 36

liquid

Question 37

Deviations from the ideal gas law arise because it neglects two factors:

Answer 37

1. Attractive forces between gas particles

2. The finite volume of gas particles