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Relation between equivalent weight, valency and atomic weight

Consider an element X of atomic weight A and valency n.

Suppose the element X combines with hydrogen to give the compound hydride, XHn.

X + nH ?XHn

1 atom of X combines with n atoms of hydrogen.

1 gram-atom of X combines with n gram-atoms of hydrogen.

Atomic weight of X = A

Atomic weight of hydrogen = 1.008

n × 1.008g of hydrogen combines with = A gram of X

A

1.008 grams of hydrogen combines with = ------------- × 1.008

n × 1.008

A

= ------

n

By definition, this gives the equivalent weight of the element X.

Atomic weight

Equivalent weight = -------------------

Valency

In the case of univalent elements, the atomic weight itself gives the equivalent weight. Elements having varying valency have different equivalent weight.

Equivalent weight of copper in cuprous oxide Cu2O is 63.5. (here, valency of Cu is one). Equivalent weight of copper in cupricoxide,

63.5

CuO is ------ = 31.75 (here, valency of Cu is 2).

2

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