Question

Calculate the equivalent mass of $\mathrm{HCl}$ in the given reaction : ${\mathrm{K}}_2{\mathrm{Kr}}_2{\mathrm{O}}_7+14\mathrm{HCl}\to 2\mathrm{KCl}+2{\mathrm{CrCl}}_3+3{\mathrm{Cl}}_2+{\mathrm{H}}_2\mathrm{O}$

Answer 1

Equivalent Mass

• An equivalent weight of a solution is defined as the molecular weight of the solute divided by the valency of the solute.
• Equivalent weight is used for predicting the mass of a substance that reacts with one atom of hydrogen in an acid-base analysis like in titration.

Calculation of Equivalent mass

Step 1: Calculating change in oxidation state of HCl

$\underset{\mathrm{Potassium}\mathrm{dichromate}}{{\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\left(\mathrm{s}\right)}+\underset{\mathrm{Hydrochloric}\mathrm{acid}}{14\mathrm{HCl}\left(\mathrm{l}\right)}\to \underset{\mathrm{Potassium}\mathrm{chloride}}{2\mathrm{KCl}\left(\mathrm{s}\right)}+\underset{\mathrm{Chromium}\mathrm{chloride}}{2{\mathrm{CrCl}}_3\left(\mathrm{s}\right)}+\underset{\mathrm{Chlorine}}{3{\mathrm{Cl}}_2\left(\mathrm{g}\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$

In the given equation, $\mathrm{HCl}$ is oxidised to ${\mathrm{Cl}}_2$
Change in oxidation number = 0 - (-1) = 1
Total change in oxidation number = 6(1) = 6

Step 2: Calculating the equivalent mass of HCl

Molecular mass of $\mathrm{HCl}$ = 36.5
Equivalent mass =$\frac{\mathrm{Number}\mathrm{of}\mathrm{moles}\mathrm{of}\mathrm{HCl}\mathrm{present}\times \mathrm{Molecular}\mathrm{mass}\mathrm{of}\mathrm{HCl}}{\mathrm{Total}\mathrm{change}\mathrm{in}\mathrm{oxidation}\mathrm{state}}$
Equivalent mass = $\frac{14\times 36.5}{{\displaystyle 6}}$
Equivalent mass = 85.1

Hence, 85.1 is the equivalent mass of $\mathrm{HCl}$.