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

To decide which experimental arrangement gives the most accurate value for the strength (normality or molarity) of an aqueous $$\text{NaOH}$$ solution, let us recall how an acid-base titration is normally carried out.

In any titration we finally apply the relation

$$N_1V_1 = N_2V_2$$

where

$$N_1 \;$$ and $$\;V_1$$ are respectively the normality and volume of one reactant, and $$N_2 \;$$ and $$\;V_2$$ are respectively the normality and volume of the other reactant.

Our purpose is to determine $$N_{\text{NaOH}}$$ as accurately as possible, so we must minimise the uncertainty in every other quantity that appears in the above equation. Let us analyse all four options with this criterion in mind.

1. Which solution should be the primary standard? Oxalic acid ($$\text{H}_2\text{C}_2\text{O}_4$$) is a typical primary standard acid. It is pure, stable, non-hygroscopic and can be weighed accurately, so its concentration can be prepared with very small uncertainty. An aqueous $$\text{NaOH}$$ solution, on the other hand, absorbs $$\text{CO}_2$$ from air and its concentration changes slowly with time; therefore it must always be standardised. Hence the known (standard) solution has to be oxalic acid, not NaOH.

2. Which apparatus gives the smaller volume uncertainty? A pipette delivers one fixed volume (for example $$10.00\ \text{mL}$$) with a very small relative error (typically $$\pm 0.04\ \text{mL}$$). A burette, though also precise, involves two separate readings (initial and final) and its total uncertainty is roughly twice the reading uncertainty (for a class A burette $$\pm 0.10\ \text{mL}$$ per reading). Consequently, the pipette is the more accurate device.

3. Where should the standard oxalic acid go? Since the standard solution should be measured with the most accurate device, the oxalic acid must be transferred with a pipette into the conical (titration) flask. The unknown NaOH should be taken in the burette so that we can vary its delivered volume until the end-point is reached.

Arrangements that follow the above reasoning:

• Oxalic acid → pipette → conical flask • NaOH (unknown) → burette.

Let us now examine each option:

Option A: NaOH in a pipette, oxalic acid in a burette - puts the unknown in the more accurate device and the standard in the less accurate one, which is the opposite of what we want.

Option B: NaOH in a burette, oxalic acid in a conical flask - exactly fits the preferred arrangement (the wording implicitly means the acid has first been transferred to the flask with a pipette).

Option C: Uses concentrated $$\text{H}_2\text{SO}_4$$, which is not a primary standard, and its heat of dilution would make a direct titration unreliable.

Option D: Mentions a volumetric flask for NaOH (a storage/standard-making vessel, not a titration device) and again concentrated $$\text{H}_2\text{SO}_4$$ in the flask - unsuitable for high-precision titration.

Hence, on the basis of minimising experimental uncertainty, Option B is the arrangement that gives the most accurate determination of the strength of the NaOH solution.

Hence, the correct answer is Option B.