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Ascorbic Acid Amount Calculation (stoichiometry/)

  1. Oct 4, 2014 #1
    1. Ascorbic acid in vitamin tablet
    Vitamin tablets with advertised contents including 750 mg ascorbic acid were analysed.

    A student took a whole tablet, crushed and dissolved all of it in water. He then diluted the tablet solution to a final volume of 250.0 mL. The student analysed 20.0 mL aliquots of the solution for ascorbic acid by titration using a standardised iodine solution. Data

    Final volume containing whole tablet: 250.0 mL
    Volume of tablet solution analysed: 20.0 mL
    Standard Iodine solution concentration: 0.0198 mol L-1
    Average titration volume 17.71 mL

    Calculate the mass of ascorbic acid in the tablet. Enter your answer as a whole number of mg.

    Attempt: So for this question I calculated the amount of mols of ascorbic acid per 20ml (it is in a 1:1 ratio with iodine) - 0.0198mol/L x 0.01771L = 0.00350658mol. Then I multiplied it by 250ml/20ml (12.5) to find the amount of mols in the entire 250ml solution - 12.5x 0.00350658mol = 0.004383225mol. I then multiplied this by the molar mass of ascorbic acid (176.14g/mol) to get grams of ascorbic acid - 176.14g/mol x 0.004383225mol = 0.7718859225g. I then multiplied this by 1000mg/1g to convert grams to milligrams - 0.7718859225g x 1000mg/1g = 771.8859225mg = 772mg (answer as whole number).

    Is this right or did I forget something? I can do all of the math it's just that my problem solving skills aren't that great.


    2. Ascorbic Acid in Juice
    A selection of commercial fruit juices and fruit extracts were analysed for ascorbic acid content.

    A student took kiwifruit juice to analyse. This juice is reputed to have ascorbic acid concentrations about 3 times greater than that of orange juice. The student diluted the juice by taking a 25.00 mL sample to a final volume of 100.0 mL. The student analysed 25.00 mL aliquots of the diluted juice solution for ascorbic acid by titration using a standardised iodine solution. Data

    Volume juice for dilution: 25.00 mL
    Final diluted volume: 100.0 mL
    Volume of diluted juice analysed: 25.00 mL
    Standard Iodine solution concentration: 0.00496 mol L-1
    Average titration volume 18.44 mL

    Calculate the concentration of ascorbic acid in the original juice in units of mg / 100 mL. Enter your answer as a whole number of mg

    Attempt: I think I know how to do this one but the dilutions at the beginning are throwing me off, I'm not sure if I need to use C1V1=C2V2 to change anything before I start. Regardless this is how I did it:
    I calculated the amount of mols of ascorbic acid per 20ml (it is in a 1:1 ratio with iodine) - 0.00496mol/L x 0.01844L = 0.0000914624mol. I then multiplied this by 4 to find the amount of mols in the 100mL solution = 0.0003658496mol/100ml. I then multiplied this by the molar mass of ascorbic acid 176.14g/mol = 0.06444074854g/100ml. I then multiplied this by 1000mg/1g to convert it to milligrams = 64.44074854mg/100ml.
    64mg/100ml. I don't know if this is right or not as we did a lab with orange juice and got 72.3mg/100ml. It says the juice is reputed to have though so it could be false.

    Sorry for the wall of text, thanks for the help!
     
  2. jcsd
  3. Oct 4, 2014 #2

    Borek

    User Avatar

    Staff: Mentor

    Both look OK to me.
     
  4. Oct 4, 2014 #3
    So I didn't need to do any additional dilution calculations for 2? Sweet! Thank you so much, I'v been stuck on this question for ages. I'm way to indecisive.
     
  5. Oct 4, 2014 #4

    Borek

    User Avatar

    Staff: Mentor

    You don't have information for that. You did all that could be done.
     
  6. Oct 4, 2014 #5
    I got the same thing as you as well Devon
     
    Last edited: Oct 4, 2014
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