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4.- Mole.

posted Mar 25, 2016, 11:46 AM by Upali Salpadoru   [ updated Aug 29, 2019, 8:26 PM ]
  Mole
  

What is a mole?


The ‘mole’ is the SI unit for the measurement of large numbers of minute particles.

Number of particles in 1 mol is

6.02 x 1023     …….

Symbol =AN.

= 602,000,000,000,000,000,000,000.

This is named Avagadro number.



Fig.1 Mass of a mole.

 Avagadoro number:- Avogadro number is equal to the number of atoms in 12 grams of the isotope  Carbon-12


A simple way to look at it.

If you take the atomic mass of any element measured in grams it can be considered as

1 mole of atoms..

Or

If you take the molecular mass of a chemical in grams, it will have 1 mole of molecules.

Similarly you can take one mole of cat-ions or an-ions and even a mole of photons.

  •   The symbol for mole is - 'mol'.

Table 1.

This table gives the mass of 1 mole of some elements and the number of particles in each..

 Atom C O S Cl Pb 
 Mass 12 g 16 g 32 g 35.5 g 207 
 No. of Atoms. AN AN AN AN AN 
AN = 6.02 x 1023

Table 2.

This table gives the number of molecules in 1 mole.

 Molecule CO2 O2 SO2 Cl2 PbO 
 Mass 44 g 32 g 64 g 71 g  223g 
 # Molecules AN AN AN AN AN 

Table 3.

This table gives the number of radicals in 1 mole.

 Radical H+ OH- SO4-- Mass of electrons is negligible
 Mass 1 g 17 g. 96 g. 
 # Radicals AN AN AN 


If you are fond of formulae:-

 
 

Molarity...."M'

This is a unit to show the strength or the concentration of solutions. This is usually given as the number of solute moles dissolved in a litre of a solution.

If one ltre of a salt NaCl , solution contains 1 mole of salt; (That is 58.5 g), its molarity will be 1 mol/L ( L for litre)


A few questions to practice.

1.0

 Question Answer
 1.How many Carbon dioxide molecules are there in 22 g. of CO2? AN ÷  2 molecules.
 2. What is the mass of 2 moles of PbO ? 446 g.
 3.  What is the mass of 2 moles of Sulphur? 64 g.
 4. What is the (real) mass of 1 Molecule of chlorine    Cl2   71 ÷  AN   g.
 5  What is the mass of one Atom of chlorine.  Cl ? 35.5 ÷  AN    g.
Highlight to get the answers.

2.0

       Let us tabulate some of these quantities for oxygen,  water and salt.

   

Oxygen  O2

Water=H2O

Salt =  NaCl

Molar mass.

32 g.

18 g.

58.5g

No.of moles in the given mass.

1

1

1

No. of molecules in the mass.

AN …=…. 6.02x1023  

6.02x1023  

6.02x1023  

No. of atoms in the mass.

2x 6.02x1023  

3x6.02x1023  

2x6.02x1023  

Mass of 1 molecule

32/AN   g..

18/AN   g..

56.5/AN  g.

   Highlight to get the answers.
    
  

  3.0
  What is the molecular mole of Sulphur? 
   Sulhur
 This is a difficult problem.  Sulphur has many types of molecules. The diagram gives a molecule with 8 atoms.


Normally  32 g /mol. is used for calculations.



Example 1

Find the no. of moles and the no. of molecules in    800g.. of anhydrous Copper sulphate.  

Answer:

Molar mass of CuSO4    =   64+ 32+ 64   =  160g./mol.

160 g.…………………………..1  mol.

800g …………………………   800/160   mol   =  5 mol.

1mol has……………………….AN molecules.

5 mol…………………………...5AN   molecules.

Highlight to get the answers.

 Example 2

If you have to weigh the no. of moles in column 1 of the chemicals in row 1 how many grams would you   weigh?


No. of moles required.

Sodium carbonate

Na2CO3

Sodium hydroxide.

NaOH.

Calcium oxide

CaO

Magnesium sulphate.

MgSO4

1 mol.

36+12+48

=  96 g,

23+16+1

=   40 g.

40.1 +16

= 56,1g

24.3 +32+64

=120.3

0.5 mol

92/2=48 g.40/2 = 20 g.28.1g,60.15 g.

2 mol.

2x96 =192 g.2x40= 80 g.2x56.1=112.2g2x120.3=240.6



   Atomic Masses in alphabetical order. (Approximate)

Atom

Mass

 

Atom

Mass

 

Atom

Mass

 

Atom

Mass

 

Atom

Mass

 

Atom

Mass

Ag

108

 

B

11

 

C

12.0

 

Fe

56

 

Li

7.0

 

P

31

Al

27

 

Ba

137

 

Ca

40

 

H

1

 

Mg

24

 

Pb

207

Ar

40

 

Be

9.0

 

Cd

112

 

He

4

 

Mn

54.

 

S

32

Au

197

 

Br

80

 

Cl

35.5

 

Hg

201

 

N

14

 

Sn

119

 

 

 

 

 

 

Cu

64

 

I

127

 

Na

23

 

Si

26

 

 

 

 

 

 

Cr

52

 

 

 

 

O

16

 

Zn

119

 

 

 

 

 

 

K

39

 

 

 

 

 

 

 

 

 


1.0

Select the most suitable word for these:-.

A  Mole,   B- Avogadro’s number. C – Molar mass    D -  Mass of one molecule 

E- Molarity ..

1.Digital number of atoms in 12 grams of C-12 isotope.



2. A unit name for the molecular mass of water taken in grams.


3. What you get if you divide the relative a molecular mass  in grams by 6.02x1023?


4. Three times the molar mass of a chemical, taken in grams is dissolved to make  a litre of a solution. What is three in this case?


5. If this taken in grams there will AN   molecules.

=5 marks

2..0

 Question Answer
1)  If   a solution of  MgSO4 has 120 g dissolved in 500 ml. what is the molarity?  
 2)  What mass of sodium carbonate is necessary to make 250 ml. of a 1M solution  
 3)  If 100 ml  of a solution of CuSO4  gave a residue of 8 g of residue on evaporation, what was the concentration of the original solution. (Answer in mol/L)  
 4)  3M solution was diluted by adding an equal volume of distilled water. What probably is the Molarity of the new solution.  
 
 5)  A molar solution was evaporated , preventing any chemical changes due to heat. The readings were as follows:-
   Original volume.........................100 ml
   Volume after evaporation ............80 ml
   What is the molarity of the  resulting solution?             
 
 2x5=10 marks.
3.0  Complete the blank spaces in this table.


Formula

Molar mass.g/mol.

No. of molecules in a mole.

Mass of 1 molecule in grams.

No. of atoms in a molecule.

Mass of an atom in grams.

Oxygen O2

32g.





Ozone.  O3


AN




Iodine  I2



254/ AN



White phosphorus P




1

31/AN

                                                                                                 15. marks.

 4.0


Chemical formula

Required mass.

Mol

No. of molecules.

O2

i.…………..

4 mol.

4AN

CO2

 88g

ii…………..

iii…………….

Ca(OH)2

iv…………………

0.5 mol.

AN/2

Na2CO3.10(H2O)

572 g

v……………...

vi…………….

C6H12O6

1.8g

vii…………….

viii……………..

Ag2Cr2O7

432g

ix………..


                                                            2x9 = 18marks

5.0

1000 ml of a sodium carbonate solution was made using 106g. of solute.

Answer the following:

1  How many moles of Na2CO3 are present in the solution?

 2. If you measure out 22 ml of solution into a beaker, what is the molarity of that?

3.  How many moles can be obtained by evaporating 250ml of solution?

4.  What would be the mass of the residue after evaporating the water?

5.  What volume of the solution will have 0.1 moles?

                                                           5x5= 25 marks

6.0.

This question is regarding the Calcium chloride molecule. CaCl2.  Find the following.

4.1  Mass of 1 mol of the chemical compound?

4.2  The number of Ca++ ions in 1 mole of the compound.?

4.3  The number of Cl- ions in the  1 mole of the compound?

                                         4x3=12 marks


7.0

This is the structure of Auric chloride. (Gold (III) chloride.

Considering the structural formula given answer the following:

1   Find the molar mass of the compound.

2   Find the mass of gold in 1 mole of the compound.

3  How many Au)+++ ions can you get from 1 mol of the compound?

4.  How many Cl-  ions can you get from 1 mole of the compound.

5.  Write an equation to show how the molecule can break up into ions.

                                                 3x5=15 marks

               SOLUTIONS.                                        

1.0

A  Mole,   B- Avogadro’s number. C – Molar mass    D -  Mass of one molecule 

E- Molarity ..

1.Digital number of atoms in 12 grams of C-12 isotope.

B

2. A unit name for the molecular mass of water taken in grams.

A

3. What you get if you divide the relative a molecular mass  in grams by 6.02x1023?

D

4. Three times the molar mass of a chemical, taken in grams is dissolved to make  a litre of a solution. What is three in this case?

E

5. If this taken in grams there will AN   molecules.

C

2.0
                Question
 Answer
1)  If   a solution of  MgSOhas 120 g dissolved in 500 ml. what is the molarity? 
 24+32+64=120
500ml has120 g
1000ml has 240g
240g........2M.
 2)  What mass of sodium carbonate Na2CO3is necessary to make 250 ml. of a 1M solution 
46+ 12+48=106
For 1000ml.....106g
for 250ml...... 26.5g.
 3)  If 100 ml.  of a solution of CusO4  gave a residue of 8 g of residue on evaporation, what was the concentration of the original solution. (Answer in mol/L) 
 64+32+64=160
100ml gave...8g
1000 ml......80g
 M= 0.5mol/L.
 4)  A 3M solution was diluted by adding an equal volume of distilled water. What probably is the Molarity of the new solution. 
 When volume is doubled concentration should reduce to half.
1/2 0f 3 = 1.5mol/L
 
 5)  A molar solution was evaporated , preventing any chemical changes due to heat. The readings were as follows:-
       Original volume.........................100 ml
       Volume after evaporation ............80 ml
       What is the molarity of the original solution and the resulting solution..
No. of moles in 100ml. original solution  = 1/10 moles.
After evaporation 80 ml has =1/10 moles.
1000ml will
            have 0.1 x1000/80
1.25mol/L          or
m1V1  = m2 V2
1 x100 = m 80
m=  100/80  = 1.25 mol/L


  3.0  Complete the blank spaces in this table.


Formula

Molar mass.g/mol.

No. of molecules in a mole.

Mass of 1 molecule in grams.

No. of atoms in a molecule.

Mass of an atom in grams.

Oxygen O2

32g.

AN32/AN232/2AN

Ozone.  O3

48g.

AN

48/AN348/3AN

Iodine  I2

 254 g.AN

254/ AN

2254/2AN

White phosphorus P

31gAN31/AN

1

31/AN

 4.0

Chemical formula

Required mass.

Mol

No. of molecules.

O2

i).32x4= 128 g

4 mol.

4AN

CO2

 88g

ii) 88/44=2 mol.

iii) 2AN

Ca(OH)2

iv) 74 x0.5 = 37g

0.5 mol.

AN/2

Na2CO3.10(H2O)

572 g

v) 572/286= 2mol

vi) 2AN

C6H12O6

1.8g

vii) 1.8/180=0.01

viii) 0.01AN

Ag2Cr2O7

432g

ix)432/432=1 mol.

                     

                     


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