HSC M/C qn (1 Viewer)

[amberbambi]

Could someone please explain the reasoning behind this question? It was from the 2010 HSC, qn 7

Equal volumes of four 0.1mol/L acids were titrated with the same sodium hydroxide solution. Which one requires the greatest volume of base to change the colour of the indicator?
A. Citric acid
B. Acetic acid
C. Sulfuric acid
D. Hydrochloric acid
Could someone explain why the answer is A and not C? Does the strength of the acids affect the result? Thank you

 

[zhertec]

Citric acid is a triprotic acid, hence would require the greatest amount of base to neutralise as the number of OH- ions in order to neutralise the three protons released would be 3, compared to the diprotic sulfuric acid (2 OH- ions) and the other two (B and D) are monoprotic.
Although strength of acid is dependent on the ionisation of molecules, remember that the ionisation of weak acids (citric acid in this case) are in a constant equilibrium:
Stage 1: C6H8O7 <---> (equilibrium sign) C6H7O7 + H+
Stage 2: C6H7O7 <---> (equilibrium sign) C6H6O7 + H +
Stage 3: C6H6O7 <---> (equilibrium sign) C6H5O7 + H+

As the ionisation of the acid is an equilibrium, that means the products/reacts amount can change dependent on the conditions. Hence if you add OH- ions in the acid, they will neutralise with the H+ ions, causing a deficiency of H+, resulting in the molecular acid to constantly ionise and produce H+ ions as more base is added.

Hence as a result, the ionisation percentage/strength of the acid does not matter, as the equilibrium is constantly shifting to replace the amount of H+ ions being reacted.

Therefore the answer will be dependent on which molecular acid is able to donate the most protons, hence A.

P.S. correct me if any of the information is incorrect :3