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Can you help me check my calculations for molarity and help me…

Can you help me check my calculations for molarity and help me understand how to calculate the percentage of vitamin c in the juice?  I included the instructions for comparison: 

 

Goal

To determine the concentration of vitamin c in juices and freshly squeezed lemons juice.  A redox titration involving an iodometric method will be used to do the analysis.  The samples will be ranked by their vitamin c content.

 

  • Vitamin C can be determined in food by use of an oxidation-reduction reaction. 
  • The redox reaction is preferable to an acid-base titration because a number of other species in juice can act as acids, but relatively few interfere with the oxidation of ascorbic acid by iodine.
  • The solubility of iodine is increased by complexation with iodide to form triiodide. The triiodide will then oxide vitamin C to yield dehydroascorbic acid.
  • The endpoint is indicated by the reaction of iodine with starch suspension, which produces a blue-black product.
  • As long as vitamin C is present, the triiodide is quickly converted to iodide ion, and no-blue-black iodine-starch product is observed.
  • However, when all the vitamin C has been oxidized, the excess triiodide (in equilibrium with iodine) reacts with starch to form the expected blue-black color.

 

Preparation Instructions

 

Part 1 – Preparation of reagents

 

  1. Dissolve 5.00 gram potassium iodide (KI), and 0.268 gram potassium iodate (KIO3) in 200 mL of distilled water in a 400 mL beaker. 

 

  1. Add 30 mL of 3M sulfuric acid.

 

  1. Pour the solution into a 500 mL volumetric flask, and dilute to a final volume of 500 mL with distilled water, and mix thoroughly.

 

  1. Dissolve 0.250 gram vitamin C in 100 mL water.  Dilute in a 250 mL volumetric flask.

 

Part 2Standardization of the iodine solution with the vitamin C standard solution

 

  1. Add 25.00 mL of vitamin C solution into a 125 mL Erlenmeyer flask.

 

  1. Add 10 drops of 1% starch solution.

 

  1. Rinse the buret twice with 5-10 mL of iodine solution, and then fill it.

 

  1. Record your initial buret volume.

 

  1. Titrate the solution until the endpoint is reached. The first sign of blue color that remains after 20 seconds of swirling.

 

  1. Record the final volume.

 

  1. Repeat this titration three (3) times.

 

Part 3 – Determination of Vitamin C in your samples.

 

  1. Add 25.0 mL of your beverage sample into a 125 mL Erlenmeyer flask.  Repeat titration from part 2.

 

  1. Repeat the iodometric titration until you have enough measurements to feel confident that your data is accurate.  What does this mean?

 

  1. Add 10.0 mL freshly squeezed lemon juice into a 125 mL Erlenmeyer flask. 

 

  1. Repeat titration from part 2.

 

  1. Repeat the iodometric titration until you have enough measurements to feel confident that your data is accurate. What does this mean?

 

Compound Measured Weight (g)
Potassium iodide 5.0025 grams
Potassium iodate 0.2685 grams
Vitamin C / Ascorbic acid 0.2513 grams

 

 

Sample # Drops of Starch Added

Initial Buret Volume

mL

Volume of titrated solution used mL Color Observed
Beaker #1 13 drops 50 mL 19.3 mL Dark Red
Beaker #2 13 drops 50 mL 19.1 mL Light Blue
Beaker #3 13 drops 50 mL 18.5 mL Dark Purple

 

Sample with Orange Juice # Drops of Starch Added

Initial Buret Volume

mL

Volume of titrated solution used mL Color Observed
Beaker #1 13 drops 50 mL 6.4 mL Dirty yellow
Beaker #2 13 drops 50 mL 6.4 mL Dirty yellow
Beaker #3 13 drops 50 mL 6.4 mL Dirty yellow

 

 

Known Values

Potassium iodide m.w. = 166.0028 grams/ mole


Potassium iodide weight measured = 5.0025 grams 

Solution Volume: 500 mL / 0.500 Liters 

 

Moles = (5.0025) g / (166.0028) g/mole = 0.03013 moles 

Potassium iodide Molarity = moles / Liter


M = 0.03013 moles / 0.500 Liters = 0.06027 moles / Liter Potassium iodide 

 

 

 

Known Values

Potassium iodate m.w. = 214.007 grams/ mole


Potassium iodate weight measured = 0.2685 grams 

Solution Volume: 500 mL / 0.500 Liters 

 

Moles = (0.2685) g / (214.007) g/mole = 0.0012546 moles 

Potassium iodate Molarity = moles / Liter


M = 0.0012546 moles / 0.500 Liters = 0.002509 moles / Liter Potassium iodate 

 

Known Values

Ascorbic acid m.w. = 176.12 grams/ mole


Ascorbic acid weight measured = 0.2513 grams 

Solution Volume: 500 mL / 0.500 Liters 

 

Moles = (0.2513) g / (176.12) g/mole = 0.001426 moles 

Ascorbic acid Molarity = moles / Liter


M = 0.0014626 moles / 0.500 Liters = 0.00285   moles / Liter Ascorbic acid

 

Known Values

Sulfuric acid m.w. = 98.08 grams/ mole


Sulfuric acid weight measured =  30 mL 

Solution Volume: 500 mL / 0.500 Liters 

 

Moles = (3) g / (98.08) g/mole = 0.03058 moles 

Sulfuric acid Molarity =  3 moles / Liter


M = 0.03058 moles / 0.500 Liters = 0.06117 moles / Liter Ascorbic acid

 

 

 

Calculate 

 

  1. The percentage % of Vitamin C in your juices and freshly squeezes lemon juice samples with the standard deviation or confident limit.

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