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If `K_(a_1)` and `K_(a_2)` are the ionisation constant of `H_3overset+NCHRCOOH` and `H_2NCHRCOO^(-)`, respectively the pH of the solution at the isoelectric point isA. `pH=(pK_(a_1)+pK_(a_2))`B. `pH=(ppK_(a_1)-pK_(a_2))^(1/2)`C. `pH=(pK_(a_1)+pK_(a_2))^(1/2)`D. `pH=(ppK_(a_1)+pK_(a_2))^(2)` |
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Answer» Correct Answer - C At a given pH, the species of amino acid exists in equilibrium `underset"Low pH"(H_3overset+N-undersetundersetR|CH-COOH) underset(H^+)overset(OH^-)hArr underset"Intermediate pH"(H_3overset+N-undersetundersetR|CH-COO^-) underset(H^+)overset(OH^-)hArr underset"High pH"(H_2NundersetundersetR|CHCOO^-)` with `K_(a_1)=([H_3Noverset+CH(R)COO^(-)][H^(+) ])/([H_3overset+NCH(R)COOH])` `K_(a_2)=([H_2NCH(R)COO^(-)][H^+])/([H_3N^(+)CHRCOO^(-)])` Thus, `K_(a_1)K_(a_2)=([H_2NCH(R)COO^(-)][H^+]^2)/([H_3N^(+)CH(R)COOH])` As isoelectric point, `[H_2NCH(R)COO^(-)]=[H_3N^(+)CH(R)COOH]` Thus, `K_(a_1)K_(a_2)=[H^(+)]^2 ` or pH=`[pK_(a_1) + pK_(a_2)]^(1//2)` |
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