Question

In a redox reaction, dichromate ion is reduced to $C{r^{ + 3}}$ ion, the equivalent mass of ${K_2}C{r_2}{O_7}$ in this reaction is:
A.$\dfrac{\text{molecular mass}}{3}$
B.$\dfrac{\text{molecular mass}}{6}$
C.$\dfrac{\text{molecular mass}}{1}$
D.$\dfrac{\text{molecular mass}}{2}$

Answer 1

At first think about the reduction reaction of potassium dichromate to chromate ion and the definition of equivalent weight of the compound. Balance the redox reaction so that you can get some idea to calculate equivalent mass.

Complete step by step answer:
The overall redox reaction is given as,
$C{r_2}{O_7}^{2 - } + 14{H^ + } + 6{e^ - } \to 2C{r^{3 + }} + 7{H_2}O$
From the above reaction the change in oxidation state of chromium is $+ 6$.
Equivalent weight is defined as the mass of one equivalent, that is the mass of a given substance which will combine with or displace a fixed quantity of another substance.
For redox reactions, the equivalent weight of each reactant supplies or reacts with one mole of electrons in a redox reaction.
The equivalent weight of potassium dichromate is given by,
Equivalent mass $= \dfrac{\text{molecular mass}}{6}$

So the answer is B.

Additional Information:- Equivalent weights were originally determined by the experiment, but are now derived from molar masses. The equivalent weight of a compound can be calculated by dividing the molecular mass by the number of positive or negative electrical charges that result from the dissolution of the compound. The equivalent weights of the elements were often determined by studying their reactions with oxygen.

Note:
In determining the normality of a solution equivalent mass of a compound is used. For acid-base reactions, the equivalent weight of an acid or base is the mass which supplies or reacts with one mole of hydrogen cations.