1. Mole Concept

7/30/2019 1. Mole Concept

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10/19/12 MATTER 1

1.3 MOLE CONCEPT


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10/19/12 MATTER 2

Ojectives:

(a) Define mole in terms of mass of

carbon-12 and Avogadro constant, NA.

(b) Interconvert between moles, mass, number

of particles, molar volume of gas at s.t.p.

and room temperature.

(c) Determine empirical and molecular formulae

from mass composition or combustion data.


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10/19/12 MATTER 3

(d) Define and perform calculation for each of

the following concentration measurements :

i) amount concentration (molarity), c

ii) molality, miii) mole fraction, X

iv) percentage by mass, % w/w

v) percentage by volume, %V/V


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IntroductionThe atoms, molecules or formula units are

the entities that react with one another, sowe would like to know the numbers of

them that we mix together.

But how can we possibly count entities that areso small ?

To do this, chemists have devised a unit called the

MOLE to count chemical entities by weighing them.


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10/19/12 MATTER 5

1.3 Mole Concept

A mole is defined as the amount ofsubstance which contains equal number of

particles (atoms / molecules / ions) as

there are atoms in exactly 12.000g ofcarbon-12.


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10/19/12 MATTER 6

One mole of carbon-12 atom has a massof exactly 12.000 grams and contains 6.02

x 1023 atoms.

The value 6.02 x 1023 is known as

Avogadro Constant.

NA = 6.02 x 1023

mol-1

symbol: NA unit: mol-1


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The term mole, just like a pair or dozen, refers to aparticular number of things.

one pair = 2onedozen = 12one mole = 6.02 x 1023

Avogadroconstant (NA)

The number of atoms in a 12 gram sample of carbon-12

(1 mole) is called Avogadro constant (symbol NA)


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Mole

1 mol of sodium metal (Na) contains :

6.02 1023 Na atoms

1 mol of chlorine gas (Cl2

) contains :

6.02 1023 chlorine molecules

2 x (6.02 1023 ) chlorine atoms

1 mol of calcium bromide (CaBr2) contains :

6.02 1023 CaBr2 formula units

6.02 1023 Ca2+ions2 (6.02 1023) Br- ions.


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Counting the number of atoms/molecules

Mole

Questions:

a)How many atoms are there in 0.8 mole of copper?

b)How many moles of chlorine molecules in 8 1022

molecules of chlorine gas?

Mole = number of atoms or molecules

Avogadro constant


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Mole = number of atoms or molecules

Avogadro constant

Mole

Solution a) :

How many atoms are there in 0.8 mole of copper?a)

Number of copper atoms = 0.8 6.02 1023

= 4.82 1023 atoms

123mol106.02

atomsofnumbermol0.8

Cu


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b) Moles of chlorine molecules123

22

m106.02

mol108

=1.3 10-1 mol

or1 mole of Cl26.02 x 10

23 molecules of chlorine

8 x 1022 molecules of chlorine ? mole of Cl2

1 mole of Cl2X=

6.02 x 1023 molecules of chlorine

1 mole of Cl2

= 1.3 10

-1

mol

8 x 1022 molecules of chlorine? mole of Cl2


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1. Number of atoms in 0.500 mole of Al

a) 500 Al atoms

b) 6.02 x 1023 Al atoms

c) 3.01 x 1023 Alatoms

2.Number of moles of S in 1.8 x 1024 S atoms

a) 1.0 mole S atomsb) 3.0 mole S atoms

c) 1.1 x 1048 mole S atoms

Learning Check


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Molar mass

The molar mass of a substance is the mass of one

mole of the substance in grams


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The mass of 1 mole (in grams)

Equal to the numerical value of the average atomic

mass (Z, nucleon number = no. of proton + no. ofneutron)

1 mole of C atoms = 12.0 g = 6.02 1023 atoms C

1 mole of Mg atoms = 24.0 g = 6.02 1023 atoms Mg

1 mole of Cu atoms = 63.5 g = 6.02 1023 atoms Cu

Molar Mass


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Counting atoms/molecules by weighing,

Molar mass

Moles represent number of atoms/molecules,

since atoms are small it is impossible to count

them.By weighing the substances we are able to

work with a known number of particles.

1-gmasmolagmasmole


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Case 1: mole mass

i) 1 mol copper, Cu = ____ g

Arfor

1 mol monatomic element = Arg

ii) 1 mol silver ion, Ag+ = ____ g

0.25 mol silver, Ag = 0.25 x ____ g = g

a) Monatomic element

Ag = Cu =108 63.5

0.1 mol copper(II) ion,Cu2+ = 0.1 x ____ g = g

63.563.5

6.35

108108

27


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Mass in grams of 1 mole equal numerically tothe sum of the relative atomic masses

1 mole of CaCl2 = 111.1 g/mol

That is

1 mole Ca x 40.1 g/mol

+ 2 moles Cl x 35.5 g/mol = 111.1 g/mol CaCl2

Molar Mass of Molecules andCompounds


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Example:What is the mass (g ) of CO2 in 0.25 moles

of CO2? (MrCO2 = 44)

Solution:

mass (g) of CO2:

Molar Mass

Mass 1 mole CO2

Mass 0.25 mole CO2

44 g44 g x 0.25 = 11 g


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A. Molar Mass of K2O = ?grams/mole

B. MolarMass of antacid Al(OH)3 = ?

grams/mole

Learning Check!

(Ans 78)

(Ans 94)

[Ar ; K,39; O,16; Al,27; H,1 ]


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Prozac, C17H18F3NO, is a widely used

antidepressant that inhibits the uptake of

serotonin by the brain. Find its molarmass.

Learning Check

Ans: 309

[Ar ; C,12; O,16; F,19; H,1; N,14 ]


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Aluminum is often used for the structureof light-weight bicycle frames. Howmany grams of Al are in 3.00 moles ofAl? [Ar ; Al, 27 ]

3.00 moles Al ? g Al

Converting Moles and Grams

Ans: 81


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Gases

Many of the chemicals we deal with, aregases.

They are difficult to weigh.

Two factors affect the volume of gas:i) temperature (T)

ii) pressure (P)


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At the same temperature and pressure,equal volumes of gases contain equal number

of atoms/molecules.

Molar volume

At s.t.p (T = 0o C , P = 1 atm)1 mole of any gas occupies 22.4 dm3 (22.4 L)

22.4 dm3

mol-1

is known as the molar volume.

At room temperature,( T= 25oC, P= 1 atm)Volume of 1 mole of gas = 24.0 dm3 (24.0 L)

Avogadro s law states that :


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How many litres 60 g of O2 will occupy at s.t.p?

Solution:

Molar volume

Question:

1 mole of gas has a volume of 22.4 dm3 at s.t.p

Thus, volume of O2 at s.t.p=1.875 mol 22.4 dm3 mol-1

=42.0 dm3

Moles of O2 = = 1.875 mol132

60

gmol

g

=42.0 L


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Homework1. How many moles of atoms are contained in

the following?a) 22.5 g Zn

Solution

a) Mass moles

65 g Zn 1 mol22.5 g Zn 22.5 g x 1 mol

65 g

= 0.346 mol

b) 8.5 x 10 24 molecules N2


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Molecules moles

6.02 x 1023 molecules 1mol

8.5 x 1024 molecules N2

8.5 x 1024 x 1 mol = 14.12 mol N2

6.02 x 1023

1 mol molecule N2 2 mol atom N

mol atom N = 14.12 x 2 = 28.24

b) 8.5 x 10 24 molecules N2


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2. Calculate the mass of the following substances

in gram:

a) 4.25 X 10-4 mol H2SO4

b) 1.5 X 1016 atom S

( Ans:0.0417g H2SO4 ; 8.0 X 10-7 g S)


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3. Calculate the number of moles O2 gas ats.t.p if the volume is 67.2 L.

(Ans: 3 moles)


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Exercise Calculate how many moles in:

a) 1.85x1021 atoms Al

b) 3.6 x 1020 molecules (NH4)3PO4

c) 4.56g of PCl3

d) 407.56g iron(II)phosphate, Fe3(PO4)2

e) 608g of C10H14N2

f) 0.123dm3 N2 at s.t.p

g) 3.21 L H2SO4 at room temperature

[Ar ; Al,27; H,1; N, 14; O.16; P,31; Cl,35.5; S,32; Fe,56 ]

S


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1 mol = 6.02 x 1023particles(Avogadro constant = 6.02 x 1023mol1 )

Weight 1 mol = Aror Mr g Volume 1 mol of any gas

= 22.4 L or 22.4 dm3at s.t.p and

=24 L at room temp

Summary

Mole concept


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Vol. of gas at

STPMole

No. of particles

Vol. of gas at

room temperature

Mass in g

NA

Aror Mr

24 L 22.4 L

Units for volume -

L @ dm

-3

NA

24 L

22.4 L

Aror Mr


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Points to remember

1 mole 1 molecule

Units:

the atomic mass = Aramuthe molecular mass = Mramu

the molar mass M= Aror Mrg mol1

the mass of 1 mol of X = Aror Mrgthe relative atomic mass Ar, or the relativemolecular mass Mraredimensionless

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1. Mole Concept