Question

In the reaction between phenol and bromine water, the equivalent weight of phenol is obtained by dividing the molecular weight of phenol by:
(A). $3$
(B). $4$
(C). $5$
(D). $6$

Answer 1

Equivalent weight is defined as the molecular mass of substance divided by its Acidity / Basicity / valency / No. of gram equivalents. The equivalent weight of an element is the mass which combines with or displaces $1.008$ gram of hydrogen or $8.0$ grams of oxygen or $35.5$ grams of chlorine.

Complete step by step answer:
First of all we have to understand the equivalent weight of an element. The weight of an element is the mass which displaces or reacts with $1$ gram of hydrogen, $8$ grams of oxygen and $35.5$ grams of chlorine. Equivalent weight and molecular weight both are different things. Molecular weight both are different things. Molecular mass of a substance is the average relative mass of its molecule as compared with an atom of carbon$- 12$ isotope. Molecular mass can be calculated by adding the atomic masses of all the atoms present in one molecule of the substance.
For e.g. molecular mass of ${H_2}S{O_4} = 2 \times$ Atomic mass of $S$
OR = $2 \times 1 + 32 + 4 \times 16 = 98.0\mu$
Whereas equivalent weights are calculated in various methods. Equivalent weight of acid can be calculated as molecular mass of acid divided by basicity. Here basicity is the number of replaceable ${H^ + }$ ions present in an acid.
Mathematically we can write equivalent weight of acid or $\dfrac{{MolecularWeight}}{{Basicity.}}$
Equivalent Weight of base can be calculated as Molecular Mass of base divided by acidity of base. Here acidity is the number of replaceable $O{H^ - }$ ions present in a base.
Mathematically we can write it as
$= \dfrac{{Molecular{\text{ }}weight}}{{Acidity.}}$
Equivalent weight of salt can be calculated as Molecular Mass of salt divided by valency or charge on the salt
Or mathematically it is $\dfrac{{Molecular{\text{ weight }}}}{{valency}}$
Now we have to calculate the equivalent weight of phenol.
The Reaction of phenol with bromine water takes place as follow:-

From the above equation, we got to know that one mole of phenol reacts with $3$ molecules of Bromine. This means that one mole of phenol reacts with $3 \times 2 = 6$ equivalents of mass of Bromine.
This in order to phenol, its molecular option (D) is the correct answer.

Note: The phenol contains one $O{H^ - }$ group but we didn’t divide molecular mass of phenol with $1$ because $B{r_2}$ did not replace $OH$ in this reaction. Only $6$ hydrogen atoms are replaceable. Hence we divide its molecular mass with $6$.