Structure and Bonding Effects on Acid Strength

• Polyprotic acids: successive Ka values decrease by about 105.

  H3PO4 7.5 x 10-3
  
  H2PO4- 6.2 x 10-8
  HPO42- 2.2 x 10-13

• It is easier to pull H+ away from a neutral molecule than from an anion and the difficulty increases with increasing negative charge.

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