Why is the n-Factor of H3PO2 only 1? (Structure & Basicity)
One of the most common trick questions in JEE and NEET Chemistry involves the oxoacids of phosphorus. At first glance, looking at the molecular formula of Hypophosphorous acid (H3PO2), one might easily assume its n-factor (basicity) is 3 because there are three hydrogen atoms.
The Reality: H3PO2 has an n-factor of 1! Let's find out why.
Video Explanation: Decoding the Structure
Watch Abhishek Sengar sir from CHEMCA explain the structural reason behind this exception. Understanding this will prevent you from making silly mistakes in your exams!
The Secret lies in the Chemical Structure
For oxoacids (acids containing oxygen), the n-factor is determined by the number of replaceable H⁺ ions. Not all hydrogens in a molecule are replaceable. Only those attached to a highly electronegative atom (like Oxygen) can ionize and act as acidic protons.
Let's look at the structure of H3PO2 (Hypophosphorous acid). In this molecule, the central Phosphorus atom is sp³ hybridized and is attached to:
- One P=O double bond.
- One P-OH bond (Hydroxyl group).
- Two P-H bonds (Hydrogen directly attached to Phosphorus).
Acidic vs. Reducing Hydrogens:
- The single hydrogen attached to the oxygen (in the P-OH group) is the Acidic Hydrogen. This is the only one that can detach as an H⁺ ion.
- The two hydrogens directly attached to the phosphorus atom are non-ionizable. These are known as Reducing Hydrogens, and they give H3PO2 its strong reducing character.
Since H3PO2 has only one P-O-H bond, its basicity is 1, making it a monobasic acid with an n-factor of 1.
Practice Questions for JEE & NEET
At the end of the video, Sir gave you a task! Test your knowledge on the other two important oxoacids of phosphorus. Can you predict their n-factors?
Question 1: What is the n-factor (basicity) of Phosphorous acid (H3PO3)?
Answer: 2 (Dibasic Acid)
Reasoning: In the sp³ hybridized structure of H3PO3, the central Phosphorus is attached to:
- One P=O double bond.
- Two P-OH bonds (These contain the 2 acidic hydrogens).
- One P-H bond (This is 1 reducing hydrogen).
Because there are 2 ionizable hydrogens (attached to oxygen), the n-factor is 2.
Question 2: What is the n-factor (basicity) of Phosphoric acid (H3PO4)?
Answer: 3 (Tribasic Acid)
Reasoning: In the structure of H3PO4, the central Phosphorus is attached to:
- One P=O double bond.
- Three P-OH bonds (These contain the 3 acidic hydrogens).
- Zero P-H bonds (No reducing hydrogens).
Because all three hydrogens are attached to oxygen atoms, they are all ionizable, making the n-factor 3.
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