Question

What is the equivalent mass of $A{l_2}{O_3}$in the following reaction?
$4Al + 3{O_2}\xrightarrow{\Delta }2A{l_2}{O_3}$
(A) 34
(B) 17
(C) 68
(D) 51

Answer 1

The equivalent weight of an element is the mass that combines with or displaces $1.008gm$ of hydrogen or $8.0gm$ of oxygen or $38.5gm$ of chloride.

Complete step by step answer:
Let us calculate the mass of the reactant and product in the following reaction.
$4Al + 3{O_2}\xrightarrow{\Delta }2A{l_2}{O_3}$
Since Atomic mass of $Al$is$27.$
Therefore, total atoms reacted in reaction is
$4 \times 27 = 108$
Molecular mass of oxygen is $32.$
Therefore, that molecules reacted in reaction$= 3 \times 32$
$= 96.$
Molecular mass of $A{l_2}{O_3} = 2 \times$Atomic mass of $Al + 3 \times$Atomic mass of $O.$
$= 2 \times 27 + 3 \times 16$
$= 54 + 48$
$= 102.$
Therefore, according to equation,
$108gm$of Aluminum reacts with $96gm$of oxygen and produces $204gm$of $A{l_2}{O_3}.$
$\therefore$According to the definition of equivalent weight.
Since $96gm$of oxygen produces $204gm$ $A{l_2}{O_3}$on reaction with Aluminum.
$\therefore 8gm$of oxygen produces$= \dfrac{{204 \times 8}}{{96}}$
$= 17gm.A{l_2}{O_3}.$
Therefore, from the above explanation the correct option is (B)$17.$
When the amount of substance is represented in grams, equivalent weight is equal to the gram equivalent.
Equivalent mass is used to compare the chemically different elements.
We know that Atoms are always present in definite proportion by mass. Therefore, this property is used to calculate the combining capacity of different molecules.
The equivalent weight can also be calculated by dividing molecular weight by the valence
Equivalent weight=Molecular weight/Valency
Example: Equivalent weight of $NaCl.$
Molecular weight of $NaCl = Na + Cl$
$= 23 + 35.5$
$= 58.5$
Valance$= 1$
$\therefore$equivalent weight$= \dfrac{{58.5}}{1}$
$= 58.5$

Note:
Equivalent weight can be termed as equivalent, equivalency.
Same chemical compound can have a wide variety of equivalent weights because some reactants participating in different reactions have different stoichiometry.