2.Molar mass
2.This no. represents the Chapter no. of the book "Step by step Chemical Calculations" This lesson is a revised translation of that.
This is really the mass of a very large number of molecules taken in grams. So how can this be determined?
It is much easier than you think.
Chemists have adopted a very important number , Avagadro’s number named in honour of the Italian lawyer Amedeo Avagadro (1776 to 1856)
Avagadro’s number (Symbol NA = 6.02 x 10 23 )
We use this number to count particles such as electrons, protons , atoms , molecules or ions.
This is also called a Mole- symbol ‘mol’.
Avagadro’s number is the number of particles in the molecular mass of any compound. The mass of this quantity is the Molar mass.
The unit for molar mass is grams per mol. (g/mol)
Example 1.
Let’s find the molar mass of water.
A water molecule has two atoms of hydrogen and one atom of oxygen.
The molecular mass of Oxygen is:
= H + H + O
= 1.01+ 1.01 + 16.0 = 18.02 amu.
Therefore Molar mass = 18.02 g/mol.
Atomic mass units of some important atoms..
|
Ag
|
Al
|
Ar
|
Au
|
B
|
Ba
|
Be
|
Br
|
C
|
Ca
|
Cd
|
Cl
|
Cu
|
Cr
|
Fe
|
H
|
He
|
Hg
|
|
108
|
27.0
|
40.0
|
197
|
10.8
|
137
|
9.0
|
79.9
|
12.0
|
40.1
|
112
|
35.5
|
63.6
|
52.0
|
55.9
|
1.01
|
4.0
|
201
|
|
I
|
K
|
Li
|
Mg
|
Mn
|
N
|
Na
|
O
|
P
|
Pb
|
S
|
Sn
|
Si
|
Zn
|
|
127
|
39.1
|
6.9
|
24.3
|
54.0
|
14.0
|
23.0
|
16.0
|
31.0
|
207
|
32.1
|
119
|
26.1
|
119
|
Example 2.
Find the Molecular mass of Calcium hydroxide.
The formula is Ca (OH)2.
Molecular mass =40.1 + 2(16+ 1.01)
=40.1 + 2X17.1 = 74.12g/mol.
Example 3
Find the real mass of 1 molecule of Calcium oxide. (Use Avagadro’s number as NA.
Molar mass of CaO = 40 + 16 = 56g/mol
1 mol of Cao has a mass of………………………. 56g
Therefore 1 molecule of CaO has a mass of = 56 / NA grams. Or 56/ 6.02x1023
Find the Molar mass of the following compounds by adding the atomic masses.
| Name |
Formula |
Workimg |
Molecular
mass |
Molae
mass |
Example 1. Hydrogen
|
H2 |
H + H = 1.01 + 1.01 = 2.02 |
2.02 |
2.02 g/mol |
|
Example 2.
Ammonium sulphate.
|
(NH4)2 SO4 |
2( 14 +4.04) + 32 + (4x16)
= 36.08 + 32 + 64 = 132.08
|
132.08 |
132.08 g/mol |
| |
|
|
|
|
1. Oxygen
(di atomic)
|
O2
|
O + O
=…….. +……..=……….
|
32
|
32.g/mol
|
|
2. Hydrogen peroxide
|
H2O2
|
2H + 2.O
2.02 + 32
|
………
|
………g/mol
|
|
3. Potassium sulphate.
|
K2SO4
|
2K +S + 4O
= 2x……… + 32.1 4x16 = ……..
|
……….
|
…….. g/mol
|
|
4. Phosphorous pentoxide
|
P2O5
|
P + P + 5x O
= ……………………………
|
……….
|
………g/mol
|
|
5. Acetic acid
|
|
= …………………….
|
|
………
|
|
6. Glucose
|
C6H12O6
|
=…………………….
|
……….
|
……. g/mol
|
|
7. Sodium carbonate decahydrate.
|
Na2CO3.10H2O
|
=……………………….
|
………
|
…… g/mol
|
|
8. Potash Alum.
|
KAl(SO4)2.
|
=
|
………
|
……g/mol
|
9. Using the Avogadro’s number as NA find the real mass of one molecule of chlorine.
10. Find the real mass of 1 atom of normal chlorine.
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