[Fairy]

This is for my A-level chemistry coursework. I am investigating the acidity of wine. I have the calculation sorted for white wine, as below:

(Average titre = 16.85 + 16.80  = 16.825 cm3
                                  2

From balanced equation:

 C4H6O6  + 2NaOH ?  C4H4O6Na2  + 2H2O, 1 mole of  H2T (C4H6O6) reacts with every 2 moles NaOH

Number of moles = Concentration x Volume
                                                             1000

Number of moles of NaOH = 0.12 x 16.825 =  0.0020 moles
                                                        1000

Therefore, Number of moles of H2T = 0.0020  = 0.0010 moles
                                                              2

Concentration =  number of moles x 1000
                                Volume

25 cm3 of wine were used, so…

Concentration = 0.0010 x 1000 = 0.040 mol dm-3)
                             25

However for red wine i had to do a 1 in 10 dilution to be able to see the indicator change colour.  Could someone please tell me how to go about finding the concentration of acid in the wine sample?

The average titre i found was 4.25 cm3. there were 25cm3 of wine made up to 250 with water.  I think the results may be inaccurate as the wine was diluted too much but i can't tell without knowing how to work out the concentration.

PLease *delete me*

Thanks,
Jeni

[Pirt]

Hey,

Im not totally 100% about this but my predicion is that you were told to use a 1 in 10 dilution so that your titre volume wouldnt be so large, your titre volume was around 4cm³ if it wasnt diluted then it would have been in the region of 40cm³ so you would go about doing the calculations the same way then at the end when you find your concentration you would multiply it by 10 to take into the account of the original dilution.

Hope that helps

[Fairy]

thanks, that makes sense + it's what i was thinking of doing, but that does mean that the average titre should be similar to that found for white wine, as there should surely there would be the same number of moles of acid there. does this mean my results are wrong?

[Pirt]

Hye, I dont think your answers are wrong but im not sure about that. From your results i did a quick calcualtion of what the conc' was for red wine and it is more concentrated than the white wine. Unless your teacher has hinted that one wine should be more acidic than the other or they should be roughly the same everything seems fine, if i was in your shoes id probably double check my results with a class mate or something, but everything seems fine to me.

For your red wine conc' i got an answer of 0.102 mol dm3 but i did the calculation quite quickly so double check it.

Hope that helps

[Fairy]

hi, thanks a lot for your help,

I'm confused about whether to do the calculation assuming that there are 25cm3 of wine or 250cm3, as there were 25, made up to 250 with water.

My class all did different experiments for this so i can't compare results with someone.

[jdurg]

All that really matters is the actual number of moles of acid that you found in there.  The concentration of acid determined from the titration is meaningless as it's the number of moles that are important.  So if it took two moles of NaOH to neutralize the acid, then in the 250 mL sample you have 1 mole of acid.  That mole didn't come from the water so it could only have come from the wine.  Therefore, you take the number of moles of acid and divide it into the volume of wine it came from and there's your concentration.

By my calculations, the red wine had 0.000255 moles of acid in there.  So now you need to divide that by the volume of red wine that you initially used and you'll get your concentration.  You initially used 0.025 dm³ so the value I come up with is 0.0102 mol/dm³.