# Equivalent mass calculation

## 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?