Question

4. A sample of orange juice was titrated with 0.0286 M I2 solution. A 100 mL sample of juice took 9.89 mL of I2 to reach the starch end point.

a. Write the reaction of vitamin C with iodine.

b. What is the stoichiometric ratio of vitamin C to iodine in the reaction?

c. What is the concentration of the vitamin C in the sample titrated? ( In units of mg VitC/ 100 mL juice).

d. How much of the juice does one need to drink to meet the RDA standard amount of vitamin C for one day?

Answer 1

a) C6H8O6 +I2 ----> C6H6O6 +2H+ +2I-

ascorbic acid(vit C)+ iodine ---> dehydroascorbic acid +hydrogen ions +iodide ions

b) stoichiometric ratio of vitamin C to iodine in the reaction= 1:1 (1 mol vit C reacts with 1 mol I2)

c) M(I2)=molarity of iodine=0.0286M

M(vit )=molarity of vit C=?

Volume of I2 used up in titration=9.89 ml=V(I2)

Volume of vit C(juice ) titrated =100 ml=V(vit C)

Using equation,

M(I2) *V(I2)= M(vit C)*V(vit C)

M(vit C)=M(I2) *V(I2)/V(vit C)=0.0286M*9.89ml/100ml=0.00282M
M(vit C)=0.00282 mol/L=0.00282 mol/L *molar mass=0.00282 mol/L* 176.12g/mol=0.498162g/L=0.498162g/L* (1000mg/g)=498.162 mg/L=498.162 mg/L*(1L/1000ml)=0.498162mg/ml

So , concentration of vit per 100 ml=0.498162mg/ml *100ml=49.816 mg or 49.816mg Vit C/100 ml

d)RDA (recommended dietary allowance) standard amount of vitamin C for one day is 90 mg/day for Adults men and 75 mg/day for adult women=75

volume to be consumed per day by adult men=(90 mg/day )/(0.498162mg/ml)=180.7 ml

volume to be consumed per day by adult women=(75 mg/day )/(0.498162mg/ml)=150.5 ml