Consider the following information about the diprotic acid, ascorbic acid. (H 2

Question

Consider the following information about the diprotic acid, ascorbic acid. (H2As for short, molar mass 176.1)

H2As HAs

Consider the following information about the diprotic acid, ascorbic acid. (H2As for short, molar mass 176.1) The titration curve for disodium ascorbate, Na2As, with standard HCl is shown below: 1. What major species is (are) present at point III? 2. What is the pH at point I (V1/2 HCl added)? 3. What is the pH at point III? 4. Which of the following is a major species present at point IV?

Explanation / Answer

Na2 As (Sodium Ascorbate ) being a salt solution reacts with standard HCl solution being gradually added to it yielding a diprotic acid H2 As. The reaction therefore would take place in two diferent well categorized steps-

( ! ) Formation of NaAsH representing half reaction

( 2 ) Na As H would again react till complete neutralization /reaction.

Note that HCl solution has been used to estimate the alkaline / basic nature of the reaction medium caused by the hydrolysis of the salt.

With these considerations the answers are-

1. E ( ie. H2As , H+ ) - since the solution would remain acidic at thisstage ,

2. A ( pH would remain acidic even after adding !/2 HCl ie. 4.2 ) at point I

3. B ( because at this stage at point III the end point is closer to neutral nature of the reaction medium.It should not be acidic or highly alkaline as noted with other options provided for pH values )

4 C As2- (The major species present when the reaction is almost complete at point IV )

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