Some analyses are best performed via the technique of \"back titration.\" For ex

Question

Some analyses are best performed via the technique of "back titration." For example, during the analysis of vitamin C with I_3' a known excess of I_3' can be added to a sample. Then the un-reacted l_3" excess can be back titrated with thiosulfate (S_2O_3^2). The important analysis of glucose (C_6H_12O_6) in nutritional and other biochemical processes can be performed in a similar I_13 reaction. The stoichiometry is shown in the following balanced reaction of I_3 and glucose: C_6H_12O_6 + I_3- + 30H- rightarrow C_6H_11O_7 + 2H_2 O + 31 To begin the analysis, 50.0 mL of 0.0526 M solution of I_3' was added to a sample containing glucose. The excess (un-reacted) I3' required 23.7 mL of 0.0867 M S_2O_32' in a back titration. The stoichiometry of that is shown in the following balanced reaction of W reacting with S_2O_3^2. 2S_2O_3^2- + I_3 rightarrow 3I + S_4O_6^2 How many moles of glucose were in the sample?

Explanation / Answer

From the above data

total moles of I3- added = moalrity x volume

                                      = 0.0526 M x 50 ml

                                      = 2.63 mmol

1 mole of I3- reacted with 2 moles of S2O3^2-

excess I3- = 0.0867 M x 23.7 ml/2

                 = 1.0274 mmol

moles of glucose present = total moles of I3- - excess moles of I3-

                                         = 2.63 - 1.0274

                                         = 1.6026 mmol

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