The strength of an aqueous NaOH solution is most accurately determined by titrating :
(Note : consider that an appropriate indicator is used)

Solution

Chemistry ma’am · 4 steps

Titrate NaOH against a primary standard: oxalic acid, not concentrated sulphuric acid.

1. Need a standard. To find the strength of NaOH, you must titrate it against an acid whose concentration you know exactly. That acid has to be a primary standard: pure, stable, and easy to weigh accurately.
2. Pick the acid. Oxalic acid, H₂C₂O₄.2H₂O, is a stable crystalline solid, so its solution can be made to an exact strength by weighing. Concentrated H₂SO₄ absorbs moisture, so its exact strength is unknown, and it releases a lot of heat when it mixes with water. NaOH is not a primary standard either, because it absorbs moisture and CO₂ from air.

   Option	Acid used	Setup	Verdict
   (a)	oxalic acid, a primary standard	NaOH in pipette, acid in burette	less accurate: the fading pink end point with phenolphthalein is hard to judge
   (b)	oxalic acid, a primary standard	NaOH in burette, known volume of acid in flask	most accurate
   (c)	conc. H₂SO₄, not a standard	volumetric flask cannot deliver titrant	not a valid titration
   (d)	conc. H₂SO₄, not a standard	NaOH in burette	acid strength unknown, so the result is unreliable

3. Best arrangement. Pipette a known volume of standard oxalic acid into the conical flask and add NaOH from the burette. With phenolphthalein, the end point is the first permanent pale pink, which is easy to spot.
4. So. The answer is (b): NaOH in the burette and aqueous oxalic acid in the conical flask.

The idea to keep. To standardise NaOH, always titrate it against a primary standard such as oxalic acid. Take the standard in the flask and add the unknown from the burette.

Answer from the NTA final answer key. Solution by PrepWiser.