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What is the equivalent mass of IO4- when it is converted into acidic I2 in Acidic medium?
1. M/6
2. M/7
3. M/5
4. M/4

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Correct Answer - Option 2 : M/7


Equivalent mass:

  • It is the amount of substance that combines with 1g of hydrogen or 8 g of oxygen or 35.5g of chlorine.
  • The formula for calculating the equivalent mass of a substance undergoing redox reaction is given by:

\(Equivalent\;weight = \frac{{formula\;mass}}{{no\;of\;electrons\;exchanged}}\)

Equivalent weight = Molecular weight / Valency factor


Equivalent weight =  Molecular weight / n-factor

Eq Wt = M / x

  • Where n-factor varies for different species as follows:
    • For elements: x = valency of element
    • For acid: x = no. of replaceable H+ ions per acid molecule
    • For the base: x = no. of replaceable OHions per basic molecule
    • For reducing or oxidizing agent, x = total change in oxidation state per molecule.



The reaction is IO4→ I2.

  • Balancing the Iodine and oxygen atoms on both sides we get,

2IO4→ I2 + 8H2O

  • Balancing the Hydrogen atoms we get,

2IO4 + 16H→ I2 + 8H2O

  • Balancing the charge, we get:

2IO4 + 16H+ 14e- → I2 + 8H2O

  • The oxidation state of Iodine in  IO4is +7.
  • The oxidation state of I in I2 is 0.
  • The change in oxidation state = + 7.
  • Hence, seven electrons are exchanged in total per IO4ion in the reaction.
  • Let the mass of  IO4be 'M' . As it undergoes redox reaction, the equivalent mass will be given by:

\(Equivalent\;weight = \frac{{formula\;mass}}{{no\;of\;electrons\;exchanged}}\)

Equivalent mass = M / 7.

Hence,  the equivalent mass of IO4- when it is converted into acidic I2 in an acidic medium is M/7.

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