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Law of Isomorphism

When two or more compound forms similar type of crystals or able to form mixed crystals, they are known as isomorphs. For examples:MgSO4.7H2OZnSO4.7H2O and FeSO4.7H2Oare isomorphs of each other as their crystals posses same shape.
According to Mitscherlich [year 1819]. The valency of elements that are similarly placed to that of other elements in their isomorphs are always same.
In the above example Fe, Zn and Mg have same valency [2] and equal ratio of water molecule in each isomorphs.
If equivalent mass of one element is known then atomic mass can be calculated by knowing the valency of other isomorphs key element.
ILLUSTRATIVE EXAMPLE(4): Which pair of the following substances is said to be isomorphous?
(A) White vitriol and blue vitriol      
(B) Epsom salt and Glauber's salt
(C) Blue vitriol and Glauber's salt      
(D) White vitriol and Epsom salt
SOLUTION:   
Epsom salt (MgSO4.7H2O) and White vitriol
(ZnSO4.7H2O) contains divalent cation 
Mg2+ and Zn2+ and same number of 
water molecules as water of crystallization 
which hold criteria for isomorphism. Hence (D) is correct.

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