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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3g2041

Use pKa values of the conjugate acids in water: methylammonium 10.6, imidazolium 7.0, pyridinium 5.2, guanidinium 13.6. At pH 7.4 which species exist predominantly (more than 50%) in the protonated form?

Amethylammonium ion
Bimidazolium ion
Cpyridinium ion
Dguanidinium ion

Answer: A, D

Protonated fraction

= 1/ (1 + 1 0^{pH - p K_{a}})

. Methylammonium:

0.9994

. Imidazolium:

0.28

. Pyridinium:

0.0063

. Guanidinium:

0.999999

. A species is mostly protonated only if pKa exceeds the pH, so methylammonium and guanidinium.

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