Order of acidity for oxoacids of P: H3PO4 > H3PO3 > H3PO2

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The discussion centers on the order of acidity for the oxoacids of phosphorus: H3PO4, H3PO3, and H3PO2. It is established that H3PO4 is the most acidic due to its higher basicity and greater number of replaceable H+ ions. However, a participant notes that the acidity order contradicts their initial assumption when checking Ka values, which indicate a different order. The conversation highlights the importance of understanding the electron-withdrawing effects of oxygen in determining acidity. Ultimately, the need for theoretical reasoning is emphasized, as exam conditions may not provide Ka values.
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Homework Statement



What is the order of acidity of H3PO3, H3PO2 and H3PO4?

The Attempt at a Solution



H3PO4 must be most acidic because its basicity is highest. Greater the no. of repleacable H+, greater will be its acidity. On similar reasoning, H3PO2 is least acidic.

Is it right?
 
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There is a rule about acid strength and amount of oxygen in the molecule.
 
I guess you are talking about the withdrawing effect of Oxygens.
There is only one Oxygen doubly bonded to Phosphorus which withdraws electron density of the O-H group facilitating the release of H+. Since there is only one such O in every molecule given, I don't think we can compare based on that rule. Correct me if I am wrong.
 
Last edited:
Good point, my mistake.

Safest approach it to check tables for Ka values.
 
I googled their Ka values and to my surprise, found that the acidity order is opposite to that I mentioned in O.P.

Unfortunately these values are not given in my exam and I need an answer theoretically ( I am not so good at by-hearting things :biggrin:)
 

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