Equilibrium NEET 2005: pH Order and Salt Hydrolysis

Q: What is the correct pH order in the Equilibrium NEET 2005 question?

Option D is correct: pH₁ > pH₂ > pH₃ > pH₄. For the equal-molar preparations in the question, the order is Na₂O > Na₂S > Na₂Se > Na₂Te.

What is the correct pH order in the Equilibrium NEET 2005 question?

Option D is correct in the Equilibrium NEET 2005 question: sodium oxide gives the highest pH, followed by sodium sulphide, sodium selenide and sodium telluride. The task compares aqueous preparations made from equal molar amounts per unit volume of these four compounds, labelled respectively:

N a_{2} O : {p H}_{1}; N a_{2} S : {p H}_{2}; N a_{2} S e : {p H}_{3}; N a_{2} T e : {p H}_{4}

Isomolar means equal molar concentration, not equal mass concentration. Equal masses contain different numbers of moles because the compounds have different molar masses.

The supplied options are:

{p H}_{1} > {p H}_{2} = {p H}_{3} > {p H}_{4}

{p H}_{1} < {p H}_{2} < {p H}_{3} < {p H}_{4}

{p H}_{1} < {p H}_{2} < {p H}_{3} = {p H}_{4}

D, correct

{p H}_{1} > {p H}_{2} > {p H}_{3} > {p H}_{4}

This question bank tags the problem hard, with an expected solving time of 90 seconds. These are question-bank labels, not student-performance statistics.

Why does sodium oxide give the highest pH?

Sodium oxide reacts with water to form sodium hydroxide, which dissociates to supply hydroxide ions. The official solution places it highest in pH among these equal-molar preparations. Unlike the other anions here, oxide does not persist as an ordinary dissolved dianion: it reacts with the solvent.

The balanced reaction is: $N a_{2} O + H_{2} O \to 2 N a O H$

Sodium hydroxide dissociates in water: $N a O H \to N a^{+} + O H^{-}$

The hydroxide ions make the solution basic. The balanced equation gives the amount of sodium hydroxide formed from one mole of sodium oxide:

1 m o l N a_{2} O \times \frac{2 m o l N a O H}{1 m o l N a_{2} O} = 2 m o l N a O H

Complete dissociation supplies two moles of hydroxide ions. This is a stoichiometric check, not a numerical pH calculation: no solution concentration has been assigned.

How does hydride acidity determine sulphide, selenide and telluride basicity?

The official solution ranks sulphide as the strongest base of these three dianions, followed by selenide and telluride. The governing rule is the reversal: a stronger acid has a weaker conjugate base. Carrying an acid-strength trend directly into a base-strength trend gives the wrong direction.

The group-16 parent-hydride acidity order used in the solution is: $H_{2} S < H_{2} S e < H_{2} T e$

Acid strength increases down this sequence. The dianion basicity order used in the official solution runs in the opposite direction:

S^{2 -} > S e^{2 -} > T e^{2 -}

The three hydrolysis reactions connect this comparison to hydroxide formation:

S^{2 -} + H_{2} O ⇌ H S^{-} + O H^{-}

S e^{2 -} + H_{2} O ⇌ H S e^{-} + O H^{-}

T e^{2 -} + H_{2} O ⇌ H T e^{-} + O H^{-}

Each dianion accepts a proton from water, leaving a hydroxide ion behind. Under comparable conditions, the stronger base accepts that proton more readily and produces more hydroxide.

Precision note: the immediate conjugate acids of the dianions are hydrogensulphide, hydrogenselenide and hydrogentelluride:

H S^{-}, H S e^{-}, H T e^{-}

They are not the neutral parent hydrides, which differ from the dianions by two protons. The official solution uses the parent-hydride acidity trend as its qualitative comparison. Keep that shorthand separate from the exact conjugate pairs shown in the hydrolysis equations.

How do we turn the basicity order into the final pH order?

At the same temperature, more hydroxide means lower pOH and therefore higher pH. Combine sodium oxide’s formation of sodium hydroxide with the decreasing hydrolysis basicity of sulphide, selenide and telluride.

The preparations decrease in basicity and pH in this order:

N a_{2} O > N a_{2} S > N a_{2} S e > N a_{2} T e

Mapping back to the question’s labels gives:

{p H}_{1} > {p H}_{2} > {p H}_{3} > {p H}_{4}

Option D is correct. An ordering question does not require numerical pH values. Compare hydroxide formation rather than searching for missing concentrations or acidity constants.

Use this method chain: reaction with water → acid-strength trend → reversed base-strength trend → pH order.

Why does failing to reverse the acidity trend produce option B?

Option B incorrectly makes pH increase down the group by carrying the hydride acidity trend directly into anion basicity. Its proposed order is:

{p H}_{1} < {p H}_{2} < {p H}_{3} < {p H}_{4}

This method treats “stronger acid” as if it meant “stronger associated base”, then assigns a higher salt-solution pH. That inference is invalid: conjugate acid and base strengths move in opposite directions.

Extending the faulty down-group rule to all four substances also misses sodium oxide’s direct formation of sodium hydroxide. The error has two parts: forgetting the acid–base reversal and treating oxide like the other dissolved anions.

Repair check: write one hydrolysis equation, then identify which ion accepts a proton from water more strongly. Rank proton acceptance first and pH second. Keep oxide separate because its reaction with water must be considered before the hydrolysis comparison.

Can you apply the same reasoning to three related Equilibrium questions?

Track one proton to identify a conjugate acid; compare hydrolysis and base strength to rank salt-solution pH. The following are three original related practice questions, not verified NEET past-paper questions. Attempt each before reading its worked answer.

What is the immediate conjugate acid of sulphide in its first hydrolysis step?

Answer: hydrogensulphide. Sulphide gains one proton from water:

S^{2 -} + H_{2} O ⇌ H S^{-} + O H^{-}

Adding one positively charged proton changes the ion’s charge from two minus to one minus. Hydrogen sulphide requires a second proton, so it is not the immediate conjugate acid in this step.

At equal molar concentration and the same temperature, does sodium fluoride or sodium chloride have higher pH?

Answer: sodium fluoride. Fluoride is the conjugate base of the weak acid hydrogen fluoride and produces hydroxide by hydrolysis:

F^{-} + H_{2} O ⇌ H F + O H^{-}

Chloride undergoes negligible hydrolysis in the standard NEET treatment because it is the conjugate base of the strong acid hydrochloric acid. Sodium fluoride therefore has the higher pH under the stated conditions.

If HA is stronger than HB, which equal-molar sodium salt solution has higher pH, NaA or NaB?

Answer: NaB, when HA and HB are weak monoprotic acids. The weaker acid has the stronger conjugate base:

Acid strength: H A > H B Base strength: B^{-} > A^{-}

The stronger base accepts a proton from water more readily:

B^{-} + H_{2} O ⇌ H B + O H^{-}

Its solution produces more hydroxide and has higher pH at the same temperature. Before selecting an option, write the acid order and the reversed base order on separate lines.

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Frequently asked questions

What is the correct pH order in the Equilibrium NEET 2005 question?

Option D is correct: pH₁ > pH₂ > pH₃ > pH₄. For the equal-molar preparations in the question, the order is Na₂O > Na₂S > Na₂Se > Na₂Te.

Why does sodium oxide give the highest pH?

Sodium oxide reacts with water to form sodium hydroxide: Na₂O + H₂O → 2NaOH. One mole of sodium oxide forms two moles of sodium hydroxide, which dissociates to supply hydroxide ions. The official solution places this preparation highest in pH among the four.

What does isomolar mean in this pH question?

Isomolar means equal molar concentration: equal numbers of moles per unit volume. It does not mean equal mass concentration, because the four compounds have different molar masses.

Why must we reverse the acidity trend when comparing basicity?

A stronger acid has a weaker conjugate base. The official solution uses the parent-hydride acidity order H₂S < H₂Se < H₂Te as a qualitative guide to the reversed dianion basicity order S²⁻ > Se²⁻ > Te²⁻. Strictly, the immediate conjugate acids of these dianions are HS⁻, HSe⁻ and HTe⁻, not the neutral parent hydrides.

Equilibrium NEET 2005: pH Order and Salt Hydrolysis

What is the correct pH order in the Equilibrium NEET 2005 question?

Why does sodium oxide give the highest pH?

How does hydride acidity determine sulphide, selenide and telluride basicity?

How do we turn the basicity order into the final pH order?

Why does failing to reverse the acidity trend produce option B?

Can you apply the same reasoning to three related Equilibrium questions?

What is the immediate conjugate acid of sulphide in its first hydrolysis step?

At equal molar concentration and the same temperature, does sodium fluoride or sodium chloride have higher pH?

If HA is stronger than HB, which equal-molar sodium salt solution has higher pH, NaA or NaB?

Frequently asked questions

What is the correct pH order in the Equilibrium NEET 2005 question?

Why does sodium oxide give the highest pH?

What does isomolar mean in this pH question?

Why must we reverse the acidity trend when comparing basicity?

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