Question

h3po4 is weaker acid than h3po3 why​

Answer 1

For this question, first draw the Lewis structures of each acid. You will see that for H3PO3 one of the hydrogens is bonded directly to phosphorus. This is not an acidic hydrogen and therefore H3PO3 is diprotic. On the other hand, H3PO4 is triprotic, all hydrogens bonded to oxygens. Looking at the pKa values we see that pKa1 for H3PO3 is 1.3 where as pKa1 for H3PO4 is 2.1. Also the pKa2 for H3PO3 is greater than pKa2 for H3PO4. From this experimental data we can see that H3PO3 is more acidic. Another way of thinking about this is to consider the polarization of each acid. H3PO3 is more polar, so the O-H bonds will be weaker, making the H+ easier to come off, and causing H3PO3 to be more acidic than H3PO4.