The leaves of the rhubarb plant are considered poisonous because they contain high levels of oxalic acid - Scottish Highers Chemistry - Question 3 - 2017

To determine the mass of oxalic acid in a rhubarb leaf, one could use a method involving titration. The following steps outline this procedure:

1. Preparation of Sample:

   • Homogenize a known weight of the rhubarb leaf to ensure it represents the entire leaf's oxalic acid content.

2. Extraction of Oxalic Acid:

   • Use a solvent, such as distilled water, to extract the soluble oxalic acid from the homogenized sample. This can be done by mixing the leaf sample with the solvent and allowing it to sit to dissolve all solubles.

3. Filtering and Dilution:

   • Filter the mixture to remove solid plant materials, collecting the liquid which contains the oxalic acid.

4. Titration:

   • Perform a titration using a standard solution of a strong base, such as sodium hydroxide (NaOH). The oxalic acid will react with the base, neutralizing it and resulting in a measurable change in pH, typically monitored with a pH indicator or pH meter.

5. Determination of Concentration:

   • Calculate the concentration of oxalic acid in the solution based on the volume of base used in the titration. The balanced chemical equation for the reaction indicates that 1 mole of oxalic acid reacts with 2 moles of base:
   $$$$

ightarrow Na_2C_2O_4 + 2H_2O$$

6. Calculating Mass:
   • Finally, use the concentration and the volume of the extracted oxalic acid solution to find the total moles of oxalic acid, then convert this to mass using the molar mass of oxalic acid, which is approximately 90.03 g/mol. The formula to find the mass is: $ext{Mass (g)} = ext{Moles} imes ext{Molar Mass (g/mol)}.$

This systematic approach allows for an accurate determination of the mass of oxalic acid present in the rhubarb leaf sample.