Aqueous Solutions Of Salts - Chemistry LibreTexts

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Salts of Polyprotic Acids

Do not be intimidated by the salts of polyprotic acids. Yes, they're bigger and "badder" then most other salts. But they can be handled the exact same way as other salts, just with a bit more math. First of all, we know a few things:

  • It's still just a salt. All of the rules from above still apply. Luckily, since we're dealing with acids, the pH of a salt of polyprotic acid will always be greater than 7.
  • The same way that polyprotic acids lose H+ stepwise, salts of polyprotic acids gain H+ in the same manner, but in reverse order of the polyprotic acid.

Take for example dissociation of \(\ce{H2CO3}\), carbonic acid.

\[\ce{H2CO3(aq) + H2O(l) <=> H3O^{+}(aq) + HCO^{-}3(aq)} \nonumber\]

with \(K_{a1} = 2.5 \times 10^{-4} \)

\[\ce{HCO^{-}3(aq) + H2O(l) <=> H3O^{+}(aq) + CO^{2-}3(aq)} \nonumber \]

with \(K_{a2} = 5.61 \times 10^{-11}\).

This means that when calculating the values for Kb of CO32-, the Kb of the first hydrolysis reaction will be \(K_{b1} = \dfrac{K_w}{K_{a2}}\) since it will go in the reverse order.

Summary of Acid Base Properties of Salts
Type of Solution Cations Anions pH
Acidic

From weak bases NH4+, Al3+, Fe3+

From strong acids: Cl-, Br-, I-, NO3-, ClO4-

 < 7
Basic From strong bases: Group 1 and Group 2, but not Be2+

From weak acids: F-, NO2-, CN-, CH3COO-

 > 7
Neutral

From strong bases: Group 1 and Group 2, but not Be2+.

From strong acids: Cl-, Br-, I-, NO3-, ClO4-

 = 7

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