S Block — NEET UG practice

1. Recall the purpose of Lassaigne's test.

Lassaigne's test is a qualitative technique used to detect the presence of nitrogen, sulphur, halogens, and phosphorus in an organic compound by fusing it with sodium metal to convert these elements into water-soluble sodium salts.

2. Recall the typical reactions involved in this test.

When an organic compound containing these elements is fused with sodium, characteristic reactions occur, such as:

$$\mathrm{Na} + \mathrm{C} + \mathrm{N} \xrightarrow{\Delta} \mathrm{NaCN}$$ $$2\mathrm{Na} + \mathrm{S} \xrightarrow{\Delta} \mathrm{Na_2S}$$ $$\mathrm{Na} + \mathrm{X} \xrightarrow{\Delta} \mathrm{NaX}\ (\mathrm{X} = \mathrm{Cl, Br, I})$$

These all involve sodium reacting directly with the elements present in the organic compound to form detectable sodium salts.

3. Examine option B and see if it fits this pattern.

The reaction $2\mathrm{CuO} + \mathrm{C} \xrightarrow{\Delta} 2\mathrm{Cu} + \mathrm{CO_2}$ involves copper oxide reacting with carbon, which is unrelated to sodium fusion and does not detect nitrogen, sulphur, phosphorus, or halogens. This reaction is not part of Lassaigne's test at all.

4. Conclude which reaction does not belong.

Hence, the reaction in option B does not belong to Lassaigne's test.

1. Identify the reaction being described.

Heating sodium ethanoate (sodium acetate, $\mathrm{CH_3COONa}$) with sodium hydroxide in the presence of calcium oxide is a classic reaction that follows decarboxylation, producing an organic gas:

$$\mathrm{CH_3COONa} + \mathrm{NaOH} \xrightarrow[\Delta]{\mathrm{CaO}} \mathrm{CH_4} + \mathrm{Na_2CO_3}$$

2. Identify the organic compound formed.

The organic product of this reaction is methane ($\mathrm{CH_4}$), which has a molar mass of $16\ \mathrm{g/mol}$.

3. Calculate the weight of $2$ moles of methane.

$$\text{weight} = 2 \times 16 = 32\ \mathrm{g}$$

Hence, the weight of $2$ moles of the organic compound (methane) is $32\ \mathrm{g}$, matching Option A.

1. Evaluate the Assertion (A) about lithium and beryllium's ionic character.

Actually, due to their small size and high polarizing power, lithium and beryllium tend to form compounds with pronounced covalent character, not ionic character, unlike the other heavier members of their respective groups. So Assertion (A) is false.

2. Evaluate the Reason (R) about the diagonal relationship.

The diagonal relationship is a well-known phenomenon in the periodic table where certain elements in Period 2 show similarities to elements diagonally placed in Period 3. Lithium (Group 1) and Magnesium (Group 2) do indeed show such similarities due to comparable ionic size and charge density. So Reason (R) is true.

3. Combine the two conclusions.

Since Assertion (A) is false but Reason (R) is true, the correct choice reflects this combination.

Hence, the answer is B.