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How to Use the Redox Equation to Find the Number of Waters of Crystallization in Oxalic Acid

Oxalic acid is a strong and toxic organic acid with the molecular formula C2H2O4. When you add potassium permanganate, which is a strong oxidizing agent, you break the oxalic acid down into CO2 and water. If you've already titrated a sample of oxalic acid with potassium permanganate, you can use the redox equation to determine how much water was originally present in the crystal, assuming there were no other impurities. The water molecules incorporated into the crystal structure are also called waters of crystallization.

Instructions

- 1
  Write down the balanced redox equation for oxalic acid and permanganate ion, which is as follows: 2 MnO4- + 5 C2H2O4 + 16 H+ -----> 2 (Mn2+) + 10 CO2 + 8 H2O.
- 2
  Notice that two permanganate ions are required to oxidize five molecules of oxalic acid. Using your data from your titration, multiply the number of moles of permanganate ion you added by (5/2). If you add 0.01 moles of permanganate ion, for example, this would mean there are (5/2) * (0.01) = 0.025 moles of oxalic acid originally present.
- 3
  Multiply the number of moles of oxalic acid by the molar mass of oxalic acid, which is 90 grams per mole. If you had 0.025 moles of oxalic acid, for instance, you would have 0.025 x 90 = 2.25 grams of oxalic acid.
- 4
  Subtract the calculated weight of the oxalic acid from the mass of your starting material. You should have weighed your starting material before you began your titration. If the mass of the starting material was 2.7 grams, for example, 2.7 - 2.25 = 0.45 grams.
- 5
  Divide the mass of the water you calculated in the previous step by 18 grams per mole, which is the molar mass of water. For example, 0.45 / 18 is 0.025 moles of water.
- 6
  Divide the number of moles of water molecules present by the number of moles of oxalic acid originally present to determine the ratio of water molecules to oxalic acid molecules in the original crystal. In the example, 0.025 / 0.025 = 1, so for every oxalic acid molecule there was one molecule of water in the original sample.

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