- Atomic weight
of an element is relative weight of elements expressed using 1/12th of
C- 12 atoms as reference element.
- Atomic
weight of an element is number of times an atom of the element is heavier than 1/12th
of an atom of C-12.
- The
unit for atomic weight is amu (Atomic Mass unit) and by definition.
- 1
amu =1/12 × weight
of an atom of C-12.
- The
absolute mass of 1 amu is 1.66 ×
10-24 grams.
- For
element having two or more isotopes, atomic weight for elements is average
weight of all possible isotopes. For eg. element chlorine has two isotopes
Cl-35 and Cl-37 with abundance 75% and 25% respectively. The atomic weight of
chlorine is
= 35 ×75
+ 37 ×
25
100
=
35.5 amu
Determination
of atomic weight of element :
1.
Dulong and Petit’s Law :
Dulong and Petit’s in 1819 started that
the product of atomic wt. and specific heat of a solid elements is
approximately equal to 6.4, called atomic heat of the element.
At. wt. × sp. Heat = 6.4 (approximately)
Following steps are involved in
determining atomic weight of a solid element by this method :
i) Determine
the eq. wt. of the the metal.
ii) Determine
the specific heat of the metal (generally given).
iii) Determine
the approximate atomic weight of the of the metal.
(approximate
atomic wt. =
6.4/(specific heat))
iv) Determine
the approximate valency of the metal.
(approximate
valency = approximate
at. wt./eq.wt.))
v) Convert
the approximate valency into whole number valency.
vi) Determine
the exact atomic atomic weight with whole number valency.
Correct
at. wt. = Eq. wt. × whole no. valency.
2. Vapour density of metal
chloride method:
When vapour density or molecular or
molecular formula weight of metal is known, the atomic weight of metal is
known, the atomic weight of metal can be determined as follows. Lets x be
valency of metal then of metal then the molecular formula of metal chloride
will be MClX.
Or, Molecular weight = X equivalent
weight of metal + X 35.5
∴valency of metal (x) =
Molecular wt.of metal chloride
Equivalent wt. of metal + 35.5
Therefore, from equivalent weight of
metal and its valency atomic weight can be dermined.
3. Cannizzaro’s method:
It is based on the fact that if an
element is present in a compound then at least one atom of the element is the
smallest mass of the element is the smallest mass of the element present in
different compounds of the same element. For eg, different compounds of carbon are
taken in the following example.
Compound
|
Molecular wt.
|
% of carbon by mass
|
Mass of carbon in 1 mole of compound
|
Methane
|
16
|
75
|
12
|
Ethane
|
30
|
80
|
24
|
Carbon monoxide
|
28
|
42.9
|
12
|
Carbondioxide
|
44
|
27.3
|
12
|
∴ atomic weight of carbon is 12 as least mass of carbon is 12.
4.
Law of Isomorphism:
Isorphism is the phenomenon by which substance form crystal having same
size and shape. Isomorphous substance have same number of atoms arranged
similarly.
Eg. K2SO4 and K2CrO4,ZnSO4.7H2O
and FeSO4.7H2O, K2SO4.Al2(SO4)3.24H2O
(Potash alum), K2SO4.Cr2(SO4)3.24H2O (Chrome alum).
(Potash alum), K2SO4.Cr2(SO4)3.24H2O (Chrome alum).
The valency of
the elements forming isomorphous compounds are same.
Equivalent Weight:
The term equivalent wt. was introduced by Henry Cavendish. It
represents parts by wt. of species on the basis of which it involved in
chemical reaction.
For element, equivalent wt. is parts by wt. of element the combines
with or displaces 1.008 parts by weight of hydrogen or 8 parts by weight of
oxygen or 35.5 parts by wt. of chlorine. It is obtained using the formula.
Equivalent weight =
atomic weight
Valency
For elements having variable
valency, the equivalent weight of element is also variable. Eg, Copper valency
1 and 2 so its equivalent weight is 63.5 in cuprous and 31.75 in cupric
compounds.
For ions, Equivalent weight is parts by weight of ion that carries a
positive or negative charge.
Eq. weight of ion =
Ionic weight
charge
Equivalent weight of compounds:
1.
Acid :Equivalent weight of acid is parts by weight of acid that give 1 H⁺- ion during
neutralization
Equivalent
weight of acid =
Molecular wt.
Basicity
Basicty of
acid is number of replaceable H⁺ or number of H⁺ replaced.
2.
Bases : equivalent weight of base is parts by weight of base that can neutralise
1 H⁺-ion or twice the number of O¯¯ - ion present in base. eg. NaOH-1, KOH-1, Ca(
-2, Mg(
-2,
-2, CaO-2, etc.
3.
Salt : Equivalent weight of salt that is obtained by replacing one H⁺-ion of acid by a cation during neutralisation
reaction.
4.
Oxidising agent and reducing agent : The equivalent weight of oxidant and reductant is parts by weight of
the reagent that gains or losses one electron during redox reaction.
OR, Equivalent
weight of oxidant and reductant is part by weight of the reagent that gives one
nascent oxygen during redox reaction.
Gram equivalent Weight : Equavalent
weight of a species expressed in terms of gram equivalent weight. The number of
gram equivalents of a species is given as, Number of gram equivalents =
Weight taken
Equivalent weight
Determination of Equivalent Weight :
1.
By hydrogen displacement method : Used for metal can displace hydrogen from non-oxidising acid. From
weight of metal and Hydrogen displaced by metal, is determined using the
formula. Equivalent wt. of metal =
Weight of metal ×1.008
Wt. of hydrogen displaced
= Weight of metal
Volume of H2 displaced at STP(incc)
2.
By oxide formation method : From the weight of an element and weight of oxygen combining with it,
equivalent weight of element is determined using the formula. Equivalent weight
of element =
Weight of element × 8
Weight of oxygen
Weight of element ×5600
Volm of oxygen at STP(incc)
3.
By double – displacement method : In this method, equivalent weight is calculated from the weight of
compounds using the relation,
Equivalent wt
of AB
= weight of AB
Equivalent wt.
of CD weight of
CD
0 comments:
Post a Comment