The question
Name: ______________________ Date: ____________ How much oxalic acid is in a rust remover? (10 points)
Data table
Titrating a rust remover
A powdered rust remover contains oxalic acid, H₂C₂O₄ (molar mass 90.04 g/mol), mixed with substances that do not react with bases. Oxalic acid is a weak acid, present in water mostly as H₂C₂O₄ molecules. In the titration each molecule gives up both of its acidic protons to hydroxide ions, forming the oxalate ion, C₂O₄²⁻.
A student weighs 1.2500 g of the powder, dissolves it in distilled water and makes the solution up to exactly 250.0 mL. She titrates 25.00 mL portions of this solution with 0.1045 M NaOH, using phenolphthalein as the indicator.
| Trial | Initial reading (mL) | Final reading (mL) |
|---|---|---|
| 1 | 0.42 | 24.18 |
| 2 | 1.10 | 24.81 |
| 3 | 0.75 | 24.47 |
(a) Describe how the student should prepare the 250.0 mL of solution from the weighed powder. Name the glassware. [2 points]
(b) Calculate the average volume of NaOH used. [1 point]
(c) Calculate the mass percent of oxalic acid in the powder. Show your setup. [3 points]
(d) Write the balanced net ionic equation for the reaction between oxalic acid and sodium hydroxide in the titration, and identify the Brønsted-Lowry base in it. [1 point]
(e) The student rinsed the buret with distilled water but did not rinse it with the NaOH solution before filling it. Predict whether the calculated mass percent of oxalic acid is too high, too low or unchanged, and justify your answer. [2 points]
(f) Another student says the 25.00 mL portions should be titrated in dry flasks, because water in the flask would change the result. Explain why this is not necessary. [1 point]