Question

Consider the following reaction, If the atomic mass of iron is 56, then its equivalent mass will be-
$3Fe + 4{H_2}O \to F{e_3}{O_4} + 4{H_2}$
A. 42
B. 21
C. 63
D. 84

Answer 1

All the elements combine with each other according to the law of chemical combination, and the number of parts by which an element combines with one part by mass of hydrogen, or 8 parts by mass of oxygen, or 35.5 parts by mass of chlorine, or one gram equivalent of any other element, is the value of equivalent mass of the element.

Complete step by step answer: atoms combine with each other and form a chemical compound, such that the elements are already present in definite proportions by mass and this property can be used to make chemically different molecules with different mass, equal. It is like balancing two different compounds which are chemically dissimilar. Just like if we say we have to add 1 mango to 4 bananas.
Consider this in hydrochloric acid; 1 gram of hydrogen reacts with 35.5 grams of chlorine on the other hand in silver chloride 108 gram of silver reacts with 35.5 grams of chlorine. And from these two equations we can conclude that one gram of hydrogen is equivalent to 108 gram of silver.
Or we can also see this as their combining capacity of those atoms with each other. And we can clearly see that valency of hydrogen is equal to the valency of silver.
So the equivalent mass can be calculate with the formula as-
Equivalent mass = Atomic mass/Valency
Iron on the right hand side of the reaction $3Fe + 4{H_2}O \to F{e_3}{O_4} + 4{H_2}$ is present as ferric oxide. And we have to calculate the equivalent mass of iron here. For that first we have to calculate the valency of it here.
Total charge over the molecule = 3(valency of iron) + 4( valency of oxygen)
0 = 3(valency of iron) + 4(-2)
3(valency of iron) = 8
Valency of iron = $\dfrac{8}{3}$
Now we will put the atomic mass value given in question and valency of iron in the equivalent mass formula-
Equivalent mass = $\dfrac{{56}}{{\dfrac{8}{3}}}$
On solving this algebraic expression we get the equivalent mass as 21

So the correct option will be B. 21.

Note: if we don’t have the valency, then we have three different solutions to find the equivalent mass of the atom. First is the hydrogen displacement method, used for acids generally. The second is the oxide method and the last one is the chloride method.