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Chemistry departmentChemistry department

CHEM-111CHEM-111General ChemistryGeneral Chemistry

Unit five (a)Unit five (a)THE MoleTHE Mole

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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

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THE MOLETHE MOLEIn 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)

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Atomic or molecular mass (weight) of a substance expressed in grams.

of substances are

Moles

in grams is the

Molar mass

which converts

Moles tograms

Grams tomoles

Chemicalquantities

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Carbon IronFeC

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

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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

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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 Oxygen22 atoms H

22 moles H

11 atoms O

11 moles O

12 atoms C

12 moles C

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Mole Calculationsa. 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 ;

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Conversion from Moles to Conversion from Moles to gramsgrams

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

Mor m = n M

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Molar MassThe 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 massMolar 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 )

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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

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40.16.02 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

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Solve the following Solve the following questionsquestions

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

Mor m = n M

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% by weight = weight of part

weight of wholex 100%

% C = x 100%12.01119.37 = 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

SimilarlyH = 0.844%, Cl = 89.09% Total = 99.99%

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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 Formula1) 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.

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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

Molecular Formula = 2(NO2) = N2O4 (dinitrogen tetra oxide)

= 92/46 = 2

2) Molecular Formula

Calculations

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CHEMICAL FORMULAE FOR SIMPLE CHEMICAL FORMULAE FOR SIMPLE SUBSTANCESSUBSTANCES

Covalent Formula & Valency The power of an element to combine

with other element is valency. In covalent compounds the number of In covalent compounds the number of

unpaired electrons in an atom is its unpaired electrons in an atom is its valency.valency.

H2 Cl2 O2 , N2

H , .... Cl....

, . ......O .. .., N

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Ionic Formula & Valency The number of electrons lost or gained by an The number of electrons lost or gained by an

atom of element is its valencyatom of element 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

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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)

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Mole Relationships in Chemical Mole Relationships in Chemical EquationsEquations

How many moles of CO2 will be formed when 6 moles of propane burns?C3H8+ 5O2 3CO2+4H2O

1 3C3H8 CO2

6 ?

= 3 x 6= 18

(Ans. =18 moles of CO2)

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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 Clmole mole 2 6

6.4 ?

=6 x6.4

2= 19. 2 moles of chlorine

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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

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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.

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X 4.87

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Calculations involving GramsHow 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

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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

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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

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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

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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: