EQUIVALENT MASS
The study of the law of mass ratio
in which the various elements combined led to find out the equivalent masses, where
Dalton was the first one who calculate these masses, he assumed that the
element mass that combined with one atom of hydrogen's mass is the mass equivalent
to the element, because the hydrogen component failure in the formation of
compounds with the most other ele- ments, or to the fact that most elements do not
combine directly with hydrogen, and oxygen combine with it directly.
Oxygen has been adopted as a basis
for the calculation of equivalent masses, and it's combined mass considered
equal to «eight 8». This is the elements are not limited to the combination
with each other by equivalent quantities only, but replace each other in their
compounds with equivalent masses.
Definition of equivalent mass
Thus, the definition of equivalent
mass for an element is:-
"The mass of that element
which combine with eight mass parts of oxygen or pull these quantities from
their compounds"
The equivalence mass concept has enabled
drafting of the following law, called the law of equivalent masses:-
"The elements combine together
in quantities correspond with their equivalent masses."
When the equivalent mass estimated
in grams it is called then the (Gram Equi- valent), for example:
· The gram equivalent for
· Oxygen = 8 g
· Chlorine = 35.5 g
· Hydrogen = 1 g
· Silver = 107.9 g, and so on.
The equivalent masses can be estimated
from the data on the analysis of different compounds, or replace an element,
and it is not necessary to specify the equivalent masses to start from compounds
containing oxygen or with other equivalent mass element informa- tion depending
on the following:
(Mass of 1st
element/it's equivalent mass)=(mass of 2nd element/it's equivalent mass)
Example
3.5 g of iron combined with
sulphur to form 5.5 g of iron sulphide (II). Calculate the equivalent mass of iron
given that the equivalent mass of sulphur=16 g.
Solution:
Mass of S = 5.5 - 3.5 = 2 g
The Relationship between Atomic Mass, Equivalent Mass and Valence
While measuring atomic masses, O
atom was accepted as 16 units. While measuring equivalent masses, O atom was
accepted as 8 units. According to this, there is a mathematical relationship
between the two. In order to calculate equivalent mass of an element, mass of
the element is divided to the number of atoms which the element can be bonded
to or replace.
Here the common denominator is the
valence of the element. Equivalent mass (the result) is obtained by dividing
mass of element to its valence.
Equivalent mass
of element = atomic mass of element / valence
of element
Example
What is the valence of Al if its
atomic mass is 27 and equivalent mass is 9?
Solution:
Exercises
Exercise 1
1.31 g of Cu was obtained from reduction
of 1.64 g of copper oxide with hydrogen. Calculate equivalent mass of Cu.
(equivalent mass of O = 8g).
Exercise 2
What is the equivalent mass of an
element if its atomic mass is 55.85 and its valence is 3?
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