Arrange the following acids in increasing order of acidic strength.

Correct option is D

The correct answer is (d) I < II < III.

Solutions:

The three acids shown in the image correspond to:

1. H₃PO₂ (Hypophosphorous acid)
2. H₃PO₃ (Phosphorous acid)
3. H₃PO₄ (Phosphoric acid)

• The acidity of oxyacids of phosphorus depends on the number of ionizable -OH groups (hydroxyl groups).
• More electronegative oxygen atoms increase the stability of the conjugate base, making it easier for the acid to lose a proton (H⁺), thereby increasing acidity.

• H₃PO₂ (Hypophosphorous acid) → Weakest Acidic

  • Contains one -OH group (only one ionizable proton).
  • Least tendency to donate H⁺.

• H₃PO₃ (Phosphorous acid) → Moderately Acidic

  • Contains two -OH groups (two ionizable protons).
  • More acidic than H₃PO₂.

• H₃PO₄ (Phosphoric acid) → Strongest Acidic

  • Contains three -OH groups (three ionizable protons).
  • Most acidic among the three.

Since acidic strength increases with the number of ionizable -OH groups, the correct increasing order is:

H3PO2<H3PO3<H3PO4\mathbf{H₃PO₂ < H₃PO₃ < H₃PO₄}H3​PO2​<H3​PO3​<H3​PO4​

Which matches Option (D): I < II < III.