Question

For ortho phosphoric acid,

H3PO4 (aq) + H2O (aq) $\rightleftharpoons$ H3O+ (aq) + H2$\mathrm { PO } _ { 4 } ^ { - }$ (aq) ; Ka 

H2PO4 (aq) + H2O (aq) $\rightleftharpoons$ H3O+ (aq) +$\mathrm { HPO } _ { 4 } ^ { 2 - }$(aq) ; Ka2

$\mathrm { HPO } _ { 4 } ^ { 2 - }$(aq) + H2O (aq) $\rightleftharpoons$ H3O+ (aq) +$\mathrm { PO } _ { 4 } ^ { 3 - }$ (aq) ; Ka3

The correct order of Ka values is :

• A. > $\mathrm { K } _ { \mathrm { a } _ { 2 } }$ < $K _ { a _ { 3 } }$
• B. < $\mathrm { K } _ { \mathrm { a } _ { 2 } }$ <
• C. > $\mathrm { K } _ { \mathrm { a } _ { 2 } }$ >
• D. < $\mathrm { K } _ { \mathrm { a } _ { 2 } }$ >

Answer 1

Answer: (C)

> $\mathrm { K } _ { \mathrm { a } _ { 2 } }$ >

Answer 2

For each weak polyprotic acid Ka1 > Ka2 > Ka3 ................