Question

Oxalic acid (C2H2O4) is a diprotic acid. A student was testing a lab sample that contained some diprotic acid. The student measured out 2.50 g of the sample and added it to a 250 mL volumetric flask. The student then added water to make it up to 250 mL. The student took out 25 mL aliquot into a conical flask. This was then titrated with potassium hydroxide solution with a concentration of 1.20 M. The following titres were obtained: 15.25 mL, 15.10 mL, 15.00 mL and 15.02 mL.

A. Calculate the concentration of oxalic acid in the 25 mL aliquot.
B. Calculate the concentration of oxalic acid in the 250 mL volumetric flask.

Answer 1

To find the oxalic acid concentration in the aliquot, you calculate the average volume of KOH used, determine the moles of KOH and hence the moles of oxalic acid, and calculate the molarity. For the volumetric flask, you use the total moles of oxalic acid in the aliquot and the volume of the flask to find the concentration.

Step-by-step explanation:

To calculate the concentration of oxalic acid (C2H2O4) in the 25 mL aliquot and the 250 mL volumetric flask, we must first compute the molar concentration of oxalic acid that reacted with potassium hydroxide (KOH). Since oxalic acid is diprotic, we must account for its ability to donate two protons.

Step 1: Calculating the average volume of KOH used in the titration

Average volume of KOH = (15.25 + 15.10 + 15.00 + 15.02 mL) / 4 = 15.0925 mL

Step 2: Calculating moles of KOH used

Moles of KOH = Volume (L) × Molarity = 0.0150925 L × 1.20 M = 0.018111 moles

Step 3: Calculating moles of oxalic acid reacted

Since the reaction between oxalic acid and KOH is 1:2 (due to diprotic nature of oxalic acid), the moles of oxalic acid = moles of KOH / 2 = 0.018111 / 2 = 0.0090555 moles

Step 4: Calculate the molarity of oxalic acid in the 25 mL aliquot

Molarity of oxalic acid = Moles of oxalic acid / Volume (L) = 0.0090555 moles / 0.025 L = 0.36222 M

Step 5: Calculate the concentration of oxalic acid in the 250 mL volumetric flask

To find the concentration in the 250 mL flask, we use the fact that moles of oxalic acid in the flask = moles of oxalic acid in aliquot. Moles in the flask = 0.0090555 moles (as calculated before), and volume is 0.250 L, therefore concentration = 0.0090555 moles / 0.250 L = 0.036222 M.