Rubex back Titration calculation

shenno

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

studiot

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:

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.