The strength of an aqueous NaOH solution is most accurately determined by titrating: (Note: consider that an…

Step 1 — Principles of accurate titration:

For accurate titration:

• The standard solution (known concentration) should be in the burette or measured by pipette.
• The analyte (unknown) should be in the conical flask.
• The primary standard should be used to standardise the solution.

Step 2 — Evaluate each option:

(1) NaOH in pipette, oxalic acid in burette: NaOH is the analyte (unknown), so it should be in the flask, not the pipette. Oxalic acid (primary standard) should be in the flask. Incorrect setup ✗

(2) NaOH in burette, oxalic acid in conical flask:

• Oxalic acid ($\mathrm{H_2C_2O_4 \cdot 2H_2O}$) is a primary standard — stable, pure, known molecular weight.
• Known amount of oxalic acid in flask; NaOH added from burette.
• This is the correct, most accurate method ✓

(3) NaOH in burette, conc. $\mathrm{H_2SO_4}$ in flask: Concentrated $\mathrm{H_2SO_4}$ is hygroscopic and cannot be weighed accurately → not a primary standard ✗

(4) NaOH in volumetric flask: Volumetric flask is for preparing solutions, not for titration ✗

Answer: Option (2)