Rubex back Titration calculation

Aug 2017
3
0
bhutan paro
A Rubex tablet, weighing 1.35g, was dissolved in 50mls of water to which
2mls of mol/L (4M) NaOH was added. This solution was made up to
the mark in a 200ml volumetric flask with water and filtered . It was found
10mls of this solution required an average titre of 6.50ml of 0.05M HCl
for neutralisation. Show your clear step-by-step calculation, in order to
determine the percentage (two decimal places) of Vitamin C (Ascorbic acid)
in this tablet based on these experimental results.

Here is my calculations :
HCl
.05 Moles =1000mls
6.5mls = .000325 moles
NaOH
4 Moles=1000mls
.008 moles=2mls
.008 moles NaOH
-.000325 moles of NaOH reacted
= .007675 moles of ascorbic acid.
X20 dilution factor of ascorbic acid in volumetric flask
=.1535 moles of ascorbic acid
X 176g molar mass of ascorbic acid
27.016 grams
This answer is clearly wrong. I cant see where I am going wrong. can someone help me please.
thanks
 
Apr 2015
120
86
Ascorbic acid is monobasic, but have you considered the effect of its structure on the strength of the acid ?

Acidity
Ascorbic acid is a vinylogous carboxylic acid and forms the ascorbate anion when deprotonated on one of the hydroxyls. This property is characteristic of reductones: enediols with a carbonyl group adjacent to the enediol group, namely with the group –C(OH)=C(OH)–C(=O)–. The ascorbate anion is stabilized by electron delocalization that results from resonance between two forms:

Ascorbate resonance.png
For this reason, ascorbic acid is much more acidic than would be expected if the compound contained only isolated hydroxyl groups.


Also, Ascorbic acid is still classed as a weak acid so you should be using Henderson Hasselbalch in you calculations.