Government and industry analysts routinely measure the quantity of ascorbic acid

Question

Government and industry analysts routinely measure the quantity of ascorbic acid (vitamin C, C_6H_8O_6) in commercial products such as fruit juices and vitamin tablets. The ascorbic acid reacts with excess iodine, and the amoun of I_2 remaining is determined with sodium thiosulfate (Na_2S_2O_3). It is suspected by the Montreal Fraud Division that a prominent natural foods importer is defrauding his customers by claiming that his vitamin C tablets contain 45% of the active ingredient ascorbic acid. In one analysis, a 2.55- gram vitamin tablet is mixed with water and treated with 53.20 mL of 0.1030 MI_2 according to the equation: C_6H_8O_6(s) +I_2 (aq) rightarrow C_6H_6O_6(aq) + 2 HI (aq) After all the ascorbic acid reacts, the excess l_2 reacts with 27.54 mL of 0.115 M Na_2S_2O_3 according to the equation: l_2(aq) + 2 Na2_S_2O_3(aq) rightarrow 2 Nal (aq) Na_2S_4O_6(aq) How many grams of ascorbic acid are in the vitamin tablet? Should the Montreal police arrest the importer? Why?

Explanation / Answer

Moles of Thosulphate used = vol of thiosulphate in L * molarity

                                        = 0.02754 L * 0.115 M =3.17*10^-3 moles

2 moles Na2s2O3 reacts with 1 mol I2.

Moles of I2 = 1.584*10^-3 moles

Moles of ascorbic acid present = 1.584*10^-3 moles

Mass of ascorbic acid = 1.584*10^-3 moles * molar mass of ascorbic acid

                                 = 1.584*10^-3 moles *176.12g/mol =0.28 g

% of ascorbic acid = 0.28/2.55gm *100% =10.9 %

% od ascorbic acid is only 10% when it claims to be 45%. Police should arrest the imposter.

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