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?
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=112.2g |
2x120.3=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 |
