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?

Question

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?

Answer 1

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.

Answer 2

To determine the concentration of oxalic acid in the original solution, we need to make use of the concept of stoichiometry and the balanced chemical equation for the reaction between sodium dichromate (Na2Cr2O7) and oxalic acid (H2C2O4).

The balanced chemical equation is:
3 Na2Cr2O7 + 16 H2C2O4 → 8 H2O + 3 Cr2(SO4)3 + 16 CO2

From the balanced equation, we can see that the mole ratio between Na2Cr2O7 and H2C2O4 is 3:16. This means that for every 3 moles of Na2Cr2O7 used, 16 moles of H2C2O4 will react.

Given that we added 100 mmol (millimoles) of Na2Cr2O7 and titrated the remainder of Na2Cr2O7 with 1 mM (millimolar) Fe2+ solution, we can calculate the moles of Na2Cr2O7 (and consequently the moles of H2C2O4) used as follows:

Moles of Na2Cr2O7 = 100 mmol / 1000 = 0.1 moles

Since the mole ratio between Na2Cr2O7 and H2C2O4 is 3:16, we can calculate the moles of H2C2O4 as follows:

Moles of H2C2O4 = (0.1 moles Na2Cr2O7) * (16 moles H2C2O4 / 3 moles Na2Cr2O7) = 0.533 moles

Now we know that we have 0.533 moles of H2C2O4 in 20 ml of solution. To determine the concentration, we need to convert the volume to liters:

Volume of solution = 20 ml / 1000 = 0.02 liters

Concentration of oxalic acid = Moles of H2C2O4 / Volume of solution
Concentration = 0.533 moles / 0.02 liters = 26.65 M

Therefore, the concentration of oxalic acid in the original solution is 26.65 M (Molar).