Question

Equivalent weight of $Al(OH{)}_3$ is:
A. 78
B. 39
C. 26
D. 35

Answer 1

Hint: The equivalent weight is calculated by dividing the atomic weight or molecular weight of the compound with the value of its basicity or valency or acidity. Valency is the combining capacity of the atom. The valency of aluminium is 3.

Complete answer:Given,
The equivalent weight is defined as the mass in grams which is chemical equal to the eight grams of oxygen or one gram of hydrogen. The equivalent weight is equal to the atomic weight or molecular weight divided by the valency or acidity.
The acidity of the base is defined as the number of replaceable hydroxide ions present in the compound.
The formula for calculating the equivalent weight is shown below.
$E.W={\displaystyle \frac{M}{A}}$
Where,
E.W is the equivalent weight.
M is the molecular mass of the compound.
A is the acidity of base.
The given compound is aluminium hydroxide $Al(OH{)}_3$.
The molecular mass of aluminium hydroxide is calculated as the sum of atomic masses of individual atoms multiplied with the number of atoms.
The atomic number of aluminium is 26.98, the atomic number of oxygen is 15.99 and atomic weight of hydrogen is 1.
The molecular mass of aluminium hydroxide is calculated as shown below.
$M=26.98+(15.99+1.00)\times 3$
$M=78.003g/mol$
Thus, the molecular mass of aluminium hydroxide is 78.003 g/mol.
In $Al(OH{)}_3$, three replaceable hydroxide atoms are present. Therefore the value of acidity is 3.
To calculate the equivalent weight of aluminium hydroxide, substitute the value of molecular mass and acidity in the equation.
$\Rightarrow E.W={\displaystyle \frac{78g/mol}{3}}$
$\Rightarrow E.W=26$

Thus, the equivalent weight of aluminium hydroxide $Al(OH{)}_3$ is 26.
Therefore, the correct option is C.

Note:The basicity is used for the acidic compound which denotes the number of replaceable hydrogen ions present in the compound. Basicity, acidity and valency are different terms but the values are similar.