Question

The Equivalent weight of \[Mg{\left( {OH} \right)_2}\] is equal to:
A. \[\dfrac{{Formula{\kern 1pt} {\kern 1pt} weight}}{1}\]
B. \[\dfrac{{Formula{\kern 1pt} {\kern 1pt} weight}}{2}\]
C.\[\dfrac{{Formula{\kern 1pt} {\kern 1pt} weight}}{3}\]
D.\[\dfrac{{Formula{\kern 1pt} {\kern 1pt} weight}}{4}\]

Answer 1

Hint: To calculate Equivalent weight of any substance, we need to divide n-factor by its formula weight or molecular weight. n-factor of bases is the same as the no of replaceable \[O{H^ - }\]ions.

Step by step answer: Equivalent weight can be defined as the exact amount of substance required to combine with or react with another substance’s fixed amount.
To calculate the equivalent weight of any substance we need to divide the molecular weight/formula weight of that substance by its valency factor or n-factor.
\[Equivalent{\kern 1pt} {\kern 1pt} weight{\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} = {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} \dfrac{{Molecular{\kern 1pt} {\kern 1pt} weight{\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} or{\kern 1pt} {\kern 1pt} formula{\kern 1pt} {\kern 1pt} weight}}{{n - factor}}\]
Now n-factor of acid and bases can be calculated by counting the no. of replaceable \[{H^ + }\]ions &\[O{H^ - }\]ions respectively.
Replaceable ions can be defined as those ions which can be replaced by metal or other element during any chemical change. In the case of\[Mg{\left( {OH} \right)_2}\], 2 \[O{H^ - }\]ions can be replaced by other elements as you can see in the following reaction.
\[Mg{(OH)_2} \to {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} M{g^{ + 2}}{\kern 1pt} {\kern 1pt} {\kern 1pt} + {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} 2O{H^ - }\]
As we can see in the reaction 2 ions can be replaced so the n-factor for \[Mg{\left( {OH} \right)_2}\] will be 2.
So according to question the correct answer will be \[\dfrac{{Formula{\kern 1pt} {\kern 1pt} weight}}{2}\] as we have calculated n-factor of \[Mg{\left( {OH} \right)_2}\] is 2.

Hence the correct option is B.

Uses:
\[Mg{\left( {OH} \right)_2}\] Popularly known as milk of magnesia. It is widely used as antacids, which can cure stomach acidity by neutralising excess gastric acid. Generally weak bases are used to treat excess acids and milk of magnesia is considered as a weak base which does not harm.

Note: While calculating n-factor of acids and bases, we need to count only replaceable ions as the actual number of ions can be different. Equivalent weight will always be lesser than molecular weight or formula weight.