Using the Pauling methods of evaluating Ka1, and where applicable, K a2 and K a3

Question

Using the Pauling methods of evaluating Ka1, and where applicable, Ka2 and Ka3 values, identify the correct numerical value for the indicated constant and write a chemical equation depicting the ionization process via the Bronsted-Lowery Theory of acids and bases.

The Pauling formula for oxy acids is HpXOm(OH)n . The "p" values are all 0 herein.

a) H3AsO4 , Ka1=          M; Acid Dissoc. Rxn.

                    Ka2=        M; Acid Dissoc. Rxn.

                    Ka3=          M; Acid Dissoc. Rxn.

b) Consider the following series of bromine oxy acids: HBrO4, HBrO3, HBrO2, and HBrO

                    1) Verify the descending order of Ka1 w/exp of 10

                    2) Rationalize the trend in terms of Br-O bonding

Explanation / Answer

H3AsO4 <==> H+ + H2AsO4^- Ka1= 5.8 *10^-3

H2AsO4^- <==> H+   + HAsO4^2- Ka2 =1.1*10^-7

HAsO4^2- <===> H+ + AsO4^3-     Ka3 = 3.2*10^-12

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The order is : HBrO4>HBRO3>HBrO2>HBrO

Every acid contains one Br-O-H bond and except HOBr , all contain atleast one Br=O. Greater the number of Br=O bonds, greater will be the +ve charge on Br (as O will pull the electrons towards itself). If Br has +ve charge is will in turn pull the electrons from -O-H bond. Thus H+ can be easily liberated.

Thus greater the number of O atom in the acid greater will be the acid dissociation constant.

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