3) Citric acid is a weak organic acid found in citrus fruits and some soft drink

Question

3) Citric acid is a weak organic acid found in citrus fruits and some soft drinks. It is an important intermediate in the citric acid cycle and therefore occurs in the metabolism of almost all living things. It is a triprotic acid, three moles to remove all the acidic protons. Titration of citric acids, occurs in a stepwise fashion in which one acid, as with all triprotic proton is a time. The stepwise dissociation can therefore be represented by three removed at equations, each with an acid-dissociation equilibrium constant. (Kal) (Kaz) H H HC6HSO The following graph shows how the concentration of each species changes when NaoH is added to a solution containing 100 mmoles citric acid.

Explanation / Answer

Molecular weight of Citric acid:
H3C6H5O7 = 3(1.0004)+6(12)+5(1.0004)+7(16) = 192.0032 gm/mol = 0.1920032 gm/mmol

Mass of the citric acid used in this titration = 100 mmoles * 0.1920032 gm/mmol = 19.20032 gms

[H3C6H5O7 ] = H3A ; [H2C6H5O7-] = H2A- ; [HC6H5O7-2] = HA2- ; [C6H5O73-] = A3-
Look for the Intersection of curves  H2A- and A3- i.e) exactly at 200 mmoles , so after addition
of 200 mmoles of base [H2C6H5O7-] =  [C6H5O73-]
Similarly look for the intersection of H2A- and HA2- which is at 150 mmoles of base, so after addition of
150 mmoles of base [H2C6H5O7-] = [HC6H5O7-2]
After addition of 100 mmoles of base [H3C6H5O7 ] =  [HC6H5O7-2]

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