You want to measure the oxalic acid contents in 20 ml of a solution. To this end, you add 100 mmol sodium dichromate to the solution. Then you titrate the remainder of the dichromate with a Fe2+ solution of 1 mM, of which you need 11 ml. What was the concentration of oxalic acid in the original solution?
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100 mmol Na2Cr2O7 initially. Some reacted with the oxalic acid, the excess is titrated with Fe^+2. 11 mL x 1 mM Fe^+2 = mmoles Fe^+2 titrant. Convert the Fe^+2 to mmoles Cr2O7^-2 using the coefficients in the balanced equation,then subtract mmoles Cr2O7^-2 from the 100 there initially. The difference is the amount of Cr2O7^-2 that reacted with the oxalic acid. Then convert mmoles Cr2O7^-2 that reacted to mmoles oxalic acid. The concn oxalic acid will be mmoles/20 mL.
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