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 follow a titration method. This involves a few key steps:

1. Sample Preparation: Start by preparing a sample of the rhubarb leaves. This can be done by drying and grinding the leaves to a fine powder to ensure a uniform sample.

2. Extraction of Oxalic Acid: The ground leaf material can be treated with distilled water to extract the soluble oxalic acid. This mixture should be filtered to separate the solid plant material from the liquid containing the dissolved oxalic acid.

3. Titration Setup: The resulting solution can then be titrated using a standardized base, such as sodium hydroxide (NaOH). A suitable indicator, such as phenolphthalein, could be used to identify the endpoint of the titration.

4. Calculation of Concentration: During the titration, the volume of base used to neutralize the oxalic acid can be noted. From the known concentration of the base and the volume used, the number of moles of the base can be calculated.

5. Determining Moles of Oxalic Acid: The stoichiometry of the neutralization reaction needs to be applied here. Given the reaction of oxalic acid with sodium hydroxide is: $H_2C_2O_4 + 2NaOH \rightarrow Na_2C_2O_4 + 2H_2O$, it shows that one mole of oxalic acid reacts with two moles of NaOH. Thus, the moles of oxalic acid can be calculated based on the moles of NaOH used.

6. Calculating Mass: Finally, with the number of moles of oxalic acid determined, the mass can be calculated using the molar mass of oxalic acid, which is approximately 90.03 g/mol. The formula to find the mass is given by: $\text{Mass (g)} = \text{Moles} \times \text{Molar Mass (g/mol)}$

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