The p-Block Elements JEE 2017: Finding Y and Z

What are Y and Z in the p-block elements JEE 2017 question?

The p-block elements JEE 2017 question has option B as its answer: Y is dinitrogen pentoxide and Z is metaphosphoric acid. Phosphorus pentoxide dehydrates nitric acid to give these products. The source is JEE Advanced 2017, Paper 2, Chemistry, Q.34, based on the paragraph shared with Q.33.

Potassium chlorate releases oxygen on heating in the presence of manganese dioxide. Phosphorus burns in that oxygen, and the resulting phosphorus pentoxide reacts with nitric acid:

The question asks for Y followed by Z, with these options:

• A:

• B: ${\mathrm{N}}_2{\mathrm{O}}_5\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{H}\mathrm{P}{\mathrm{O}}_3$
• C: ${\mathrm{N}}_2{\mathrm{O}}_4\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{H}\mathrm{P}{\mathrm{O}}_3$
• D:

The question bank rates this medium. Its 60-second target is a practice benchmark, not an official exam time limit.

How does the reaction sequence produce phosphorus pentoxide?

Heating potassium chlorate with manganese dioxide produces oxygen, which then reacts with phosphorus to form phosphorus pentoxide. These are the opening stages of the official solution:

Manganese dioxide is the catalyst in oxygen preparation. It is not a reactant in the final nitric-acid reaction.

Phosphorus then burns in the oxygen:

Phosphorus pentoxide is represented here by its molecular formula: ${\mathrm{P}}_4{\mathrm{O}}_{10}$

Its empirical formula gives only the simplest atom ratio: ${\mathrm{P}}_2{\mathrm{O}}_5$

The decisive property is that phosphorus pentoxide is a strong dehydrating agent. Use that property, not the heating condition from oxygen preparation, to choose the final products.

How does water accounting give both products?

Removing the elements of water from nitric acid gives dinitrogen pentoxide. Phosphorus pentoxide takes up those water equivalents to form metaphosphoric acid. This is the formal water accounting behind the official dehydration method.

Start with dehydration of two nitric acid units:

The question supplies four nitric acid units, so double that equation:

Account for the two water equivalents using phosphorus pentoxide:

The acid formed is metaphosphoric acid. Add these last two equations and cancel the two water equivalents appearing on opposite sides:

These component equations describe formal water transfer. They do not claim that free water first accumulates in a separate experimental stage, or that these steps form an elementary reaction mechanism.

Orthophosphoric acid cannot simply replace metaphosphoric acid. Complete hydration would require:

That needs six water equivalents, while the given reaction accounts for only two. The supplied coefficients determine which phosphorus acid fits; a memorised complete-hydration reaction does not apply unchanged.

How do you check the balance and select the answer in order?

Match the coefficients before reading off the labels: metaphosphoric acid has coefficient four, so it is Z; dinitrogen pentoxide has coefficient two, so it is Y. The question requests the reverse of their order on the written product side.

Compare directly:

Thus:

Check every element on both sides:

For oxygen, the arithmetic is:

Use oxidation states only as a supporting check. Nitrogen and phosphorus both retain their oxidation states, consistent with dehydration rather than redox:

Why does option C fail even though nitric acid can decompose?

Option C substitutes a decomposition product for the dehydration product. It assigns dinitrogen tetroxide to Y and metaphosphoric acid to Z:

The faulty route is to recall that nitric acid can decompose to nitrogen dioxide, then associate nitrogen dioxide with its dimer: $2\mathrm{N}{\mathrm{O}}_2\rightleftharpoons {\mathrm{N}}_2{\mathrm{O}}_4$

That ignores the stated dehydrating reagent. The heating used earlier to prepare oxygen does not justify replacing the final reaction with nitric acid’s thermal decomposition.

Substitute option C into the given coefficients:

The proposed products fail the oxygen count:

$O_{\text{left}}=22$ Nitrogen would also change oxidation state: $N:+5\to +4$

This is not the dehydration transformation in the official solution. Identify the reagent’s role, write the balanced transformation, then map the product labels.

Which three practice questions check the same idea?

Test the same distinctions with acid anhydrides, complete hydration and oxidation states. These are original practice questions, not additional JEE 2017 questions. Attempt each before reading its worked check.

What acid forms when dinitrogen pentoxide reacts with water?

It forms nitric acid. Adding one water unit to dinitrogen pentoxide produces two nitric acid units:

Dinitrogen pentoxide is therefore the acid anhydride of nitric acid. This reverses the formal dehydration accounting used above.

What forms when phosphorus pentoxide is completely hydrated with excess water?

Complete hydration gives orthophosphoric acid. The balanced equation requires six water equivalents:

Contrast those six equivalents with the two used to form metaphosphoric acid. The water requirement separates these products.

What is phosphorus’s oxidation state in metaphosphoric acid and phosphorous acid?

Phosphorus is plus five in metaphosphoric acid but plus three in phosphorous acid. Taking hydrogen as plus one and oxygen as minus two gives:

These acids are not interchangeable: changing between them changes phosphorus’s oxidation state.

For another same-chapter problem, use The p-Block Elements JEE 2018: Nitrogen on Heating. Before moving on, cover this solution and reproduce the two water-accounting equations, their sum and the requested product order within the 60-second practice benchmark.