2. 2
Unit FiveUnit Five
• The Mole
• Formula Mass
• Molar Mass
• Relationship among these terms
• Avogadro's number
• Calculations
• Stoichiometric calculations in chemical
equations:
• Mole-mole
• Mole-mass
• Mass-mass
3. 3
THE MOLETHE MOLE
In chemistry, particles such as atoms, molecules, and ions are
counted by the mole.
A mole: is a unit that contains 6.022 x 1023
items (PARTICLES)
A mole is the amount of substance that chemists often use to
measure chemicals.
One mole of an element is its relative atomic mass in
grams.
example: C = 12, so one mole of carbon atoms weighs 12g.
The number of particles in one mole of a substance is 6.022 x 1023
(Avogadro's number)
1 dozen = 12 objects
1 mole = 6.022 x 1023
objects
C atom + O atom CO molecule
l mole C + l mole O 1mole CO
(6.022 x 1023
atoms C) + (6.022 x 1023
atoms O) (6.022 x 1023
molecule CO)
4. 4
Atomic or molecular mass (weight) of a substance
expressed in grams.
of substances are
Moles
in grams is the
Molar mass
which converts
Moles to
grams
Grams to
moles
Chemical
quantities
5. 5
Carbon Iron
FeC
6.022 X 1023
atoms
1 mole of Fe atoms
55.78 g of Fe atoms
6.022 X 1023
atoms
1 mole of C atoms
12.01 g of C atoms
Avogadro’s Number (N) = 6.022 x 1023
one mole of any element always contains Avogadro’s
number of atoms
1 mole of an element is 6.022 X 1023
atoms of that element
6. 6
Avogadro’s Number, (N)
Avogadro’s Number in 1 Mole = 6.022 x 1023
particles
Number of Particles in One-Mole Samples
Substance Number and Types of Particles
1 mole sulphur 6.02 x 1023
sulphur atoms
1 mole Hydrogen 6.02 x 1023
Hydrogen atoms
1 mole water (H2O) 6.02 x 1023
H2O molecules
1 mole NaCl 6.02 x 1023
NaCl formula units
1 mole vitamin C, (C6H8O6) 6.02 x 1023
vitamin C molecules
1 mole of a compound is 6.022 X 1023
of molecules or ions
of that compound
7. 7
Moles of Elements in Compounds
How many moles of each elements are present
in 1 mole of caffeine ( C8H10N4O2)?
Answer: (C=8, H=10, N=4, O=2)
Moles of Elements in a Formula
CC1212HH2222OO1111Sugar
Carbon Hydrogen Oxygen
22 atoms H
22 moles H
11 atoms O
11 moles O
12 atoms C
12 moles C
8. 8
Mole Calculations
a. How many moles of chloride ions are in 3
moles of AlCl3?
Answer:
1 mole of AlCl3 contains = 3 moles of Cl-
ion
3 mole of AlCl3 contains = 3 x 3 = 9 moles of Cl-
particles or ion
b.. Find out number of moles of each element in two
mole of Vitamin C (Ascorbic acid C6H8O6)
Answer: C= 12 ; H= 16 ; O = 12 .
c. How many moles of carbon ,Hydrogen and Nitrogen
are in 5 moles of quinine (C20H24N2O2 )?
Answer: C = 100; H = 120; N = 10; O = 10 ;
9. 9
Conversion from Moles to gramsConversion from Moles to grams
Amount of substance (m) = no of moles (n) x molar mass (M)
m = n x M
(6.02 x 1023
particles = 1 mole)
n =
m
M
or m = n M
10. 10
Molar Mass
The mass of one mole of pure substance is molar
mass of that substance which is taken in grams..
Number of moles x Atomic mass =Total mass
or molar mass
Molar mass of SO3:
1 mole S x 32.1g = 32.1g S
3 mole O x 16.0g =
48.0g O --------
Molar Mass of SO3 =80.1g
Q: Calculate the molar mass of fructose, C6H12O6.
[C=12, H=1.0, O=16] ( Answer = 180 g )
11. 11
Conversion from Moles to gramsConversion from Moles to grams
Calculate the grams of Ag (silver) in 4.65
moles of Ag. (Ag =107.9)
Answer:
Amount of Ag = 4.65 x 107.9 = 501.73g
Amount of sub = no of moles x molar mass
m = n x M
or mass of Ag
12. 12
40.1
6.02 x 1023
= 6.66 x 10-23
g Ca1 atom of Ca has the mass =
What is mass of one atom of Ca?
(6.02 x 1023
atoms = 1 mol)
1 mol Ca =40.1 g Ca ( atomic mass)
or
6.02 x 1023
atoms of Ca have mass = 40.1g
Avogadro’s Number & Mole
Answer
13. 13
Solve the following questionsSolve the following questions
1. How many grams does 0.500 moles of NaCl
weigh? : [Na=23, Cl=35.5]
Ans.(29.3g)
2. How many moles of Na2O are in its
231g? [ Na =23]
Ans.(3.725 moles)
n =
m
M
or m = n M
14. 14
% by weight =
weight of part
weight of whole
x 100%
% C = x 100%
12.01
119.37 = 10.06 % C
PERCENTAGE COMPOSITION
of
ELEMENTS IN COMPOUNDS
What is % composition of chloroform CHCl3 (an
anesthetic)?
Mol. Mass of CHCl3: 12.01 + 1.008 + 3 x 35.5 =
119.37
Similarly
H = 0.844%, Cl = 89.09% Total = 99.99%
15. 15
Chemical FormulaChemical Formula
The symbolic representation of a chemical
compound, which shows the number & kind
of atoms.
H2O, CH4, NaCl, MgO, C2H6 ,,C6H12O6, N2O4
Types of Chemical Formula
1) Empirical (experimental) Formula
A formula that gives the simplest whole
number ratio of atoms in a formula unit.
H2O, CH4, NaCl, MgO, CH2, CH2O, NO2.
16. 16
A formula that describes the actual number of atoms in a
molecule is molecular formula.
H2, H2O, CH4, C2H4, C6H12O6, N2O4.
Question
Empirical Formula of a colorless liquid is NO2, and has
Molecular mass of 92. What is its Molecular formula?
Formula mass of NO2 = 14 + 32 = 46,
[Atomic weight: N = 14, O = 16]
Given Molecular mass = 92
Number of times empirical formula occurs in compound:
Molecular Formula = Number of times x Empirical Formula
= 92/46 = 2
2) Molecular Formula
Calculations
17. 17
CHEMICAL FORMULAE FOR SIMPLECHEMICAL FORMULAE FOR SIMPLE
SUBSTANCESSUBSTANCES
Covalent Formula & Valency
The power of an element to combine with
other element is valency.
In covalent compounds the number ofIn covalent compounds the number of
unpaired electrons in an atom is its valency.unpaired electrons in an atom is its valency.
H2 Cl2 O2 , N2
H , .... Cl..
..
, . ....
..
O ....
, N
18. 18
Ionic Formula & Valency
The number of electrons lost or gained by an atom ofThe number of electrons lost or gained by an atom of
element is its valencyelement is its valency..
i) 3Li 2Li+1
+ 1e -1
ii) 20Ca 18Ca+2
+ 2e -
Lithium & calcium lose electrons & have +1 & +2
valencies respectively.
iii) F + 1e- F-1
iv) O + 2e- O-2
Fluorine & oxygen gain electron & possess
valency -1 and -2.
Loss of one
electron
Loss of Two
electrons
Gain of 1 electron
Gain of 2 electrons
19. 19
Molecular MassMolecular Mass
(Mostly for Covalent-compounds)
The sum of atomic masses of all atoms present
in a molecule is its molecular mass
e.g. Mol. Mass of NH3 = 1 x 14 + 3 x1 = 17
Formula Mass
The sum of atomic masses of elements present
in a formula unit ( group of ions).
NaCl, CaCl2, MgO, CaCO3
e.g. Mass of NaCl=23 + 35.5 = 58.5
(Mostly for Ionic-compounds)
20. 20
Mole Relationships in ChemicalMole Relationships in Chemical
EquationsEquations
How many moles of CO2 will be formed
when 6 moles of propane burns?
C3H8+ 5O2 → 3CO2+4H2O
1 3
C3H8 CO2
6 ?
= 3 x 6= 18
(Ans. =18 moles of CO2)
21. 21
MOLE CONCEPT-CALCULATIONS
How many moles of chlorine atoms are needed
to combine with 6.4 mole of carbon to form
C2Cl6. (Hexa-chloro ethane)
2C + 6Cl → C2Cl6 [Mole Ratio: 2:6]
Calculation
C Cl
mole mole
2 6
6.4 ?
=
6 x6.4
2
= 19. 2 moles of chlorine
22. 22
MOLES CONCEPTMOLES CONCEPT
Calculate the moles of Mg required to react with
3.50 mol of Cl to produce MgCl2.
Mg(1mol) + 2Cl(2mol) → MgCl2(1mol)
2 moles of Cl combine with 1moles of Mg.
3.50 moles of Cl combine with ? moles of Mg.
Mg Cl
1 2
? 3.5
Mg=
1 x 3.5
2
=1.75
Answer: 1.75 moles of Mg
23. 23
Mole Concept-CalculationsMole Concept-Calculations
How many moles of carbon atoms are needed to
combine with 4.87 mol of chlorine to form C2Cl6.
(Hexa-chloro ethane)
2C + 6Cl C2Cl6 [Mole Ratio: 2:6]
Moles of C = = 1.62 moles.
Conversion from Grams to Moles
Calculate moles of S, in 40.5g of Sulfur. ( S=32)
Moles of S = 40.5/32 = 1.27 moles of S.
2
6
X 4.87
24. 24
Calculations involving Grams
How many grams of O2 are required to react
with 0.300mol of Al. ( O=16 ) ( AL=27)
4Al + 3O2 → 2Al2O3
Moles of O2 ( required to react with 0.300 mol of Al) =0.225 mol of O2
Amount of O2 = No of moles x Mol mass
or m = n M = 0.225 x 32 = 7.20g of O2.
Answer : 7.20g of O2
25. 25
Calculations involving GramsCalculations involving Grams
Calculate number of grams of Al2O3 when 12.5g
O2. reacts completely with Al( Al=27)
(Ans = 26.6g Al)
n =
m
M
4Al + 3O2 2Al2O3
26. 26
Atomic or molecular mass of a substance expressed in
grams is called:
a) Avogadro’s number b) Mole
c). Both a & b.
The number of particles in one mole of a substance is
6.022 x 1023
it is called:
a) Avogadro’s number b) Mole
c) Both a & b
One mole of a substance contains
6.022 x 1023
particles.
a) True b) False
Molar mass of SO3 is: [S=32.1, O=16]
a) 80.1 b) 48.1 c) 64
Exercise
27. 27
What is the Mole ratio of reactants in the given
equation? 2C + 6Cl → C2Cl6
a) 6 : 2 b) 2 : 6 c) 8 : 6
d) 6 : 8
A formula that describes the actual number of
atoms in a molecule is:
a.Molecular Formula b.Empirical Formula
c. Chemical Formula, d. Both a & b
A formula that gives the simplest whole number ratio
of atoms in a formula unit :
a. Molecular Formula b.Empirical Formula
c. Chemical Formula d. Both a & b
Empirical formula for glucose C6H12O6 is:
a. C3H6O3 , b. CH2O , c. C2H4O2 , d. C6H12O6
28. 28
Find out the mole ratio in oxygen & aluminum in the
reactants .
4Al + 3O2 → 2Al2O3 :
a. 3/4 b. 6/4 c. 4/6 d. 2/6
The sum of atomic masses of all atoms present in a
molecule is called:
a. Molecular mass b. Formula mass
c. Both a & b d. None of the above
1.How many moles are in 25.0 g of each of
the following compounds?
a. CO2 b. Al(OH)3 c. MgCl2
Short answer question:
