When $MnO_2$ is fused with KOH in air, it gives : (A) $KMnO_4$ (B) $K_2MnO_4$ (C) $Mn_2O_7$ (D) $Mn_2O_3$

When MnO₂ is fused with KOH in air, it gives : (A) KMnO₄ (B) K₂MnO₄ (C) Mn₂O₇ (D) Mn₂O₃

Question

MnO_2

CBSECBSE Class XII Board 2024MCQ· 1mImportance★★★★★

✓ Free question

Concept understanding — Inorganic Synthesis

Inorganic Synthesis – What It Really Means

Imagine you want to build a house. You need bricks, cement, steel, and a plan to put them together. Inorganic synthesis is exactly that — but for making chemical compounds that do not contain carbon-hydrogen bonds (the domain of organic chemistry). You take simple starting materials (elements or simple compounds) and, through a controlled chemical reaction, build a more complex inorganic product.

The intuition is simple: you are a chemist-craftsman. You decide what to make, choose the right ingredients, set the right conditions (temperature, pressure, solvent, time), and then isolate the pure product. The "synthesis" part is the entire journey from idea to pure substance.

The Precise Statement

Inorganic synthesis is the branch of chemistry concerned with the design, planning, and execution of chemical reactions to prepare inorganic compounds — including metals, alloys, coordination complexes, main-group compounds, solid-state materials, and nanomaterials — with controlled purity, structure, and properties.

It is not just "mixing chemicals." It involves:

Choosing the correct starting materials (precursors) — often simple salts, oxides, or elements.
Selecting a reaction method — solid-state heating, solution precipitation, electrochemical deposition, sol-gel, hydrothermal, etc.
Controlling reaction conditions — temperature, pressure, pH, concentration, atmosphere (inert gas, air, vacuum).
Purifying the product — recrystallization, distillation, sublimation, chromatography.
Characterising the product — proving you actually made what you intended (X-ray diffraction, spectroscopy, elemental analysis).

A Concrete Example: Making Copper(II) Sulfate Pentahydrate

You want to make the familiar blue crystal, $\ce{CuSO4·5H2O}$ .

Intuition: You have copper metal (a wire) and dilute sulfuric acid. Copper does not react with dilute acid directly — you need an oxidising agent. So you add nitric acid or simply heat copper with concentrated sulfuric acid.

Reaction:

\ce{Cu + 2H2SO4(conc.) -> CuSO4 + SO2 + 2H2O}

Then you evaporate the solution carefully. Blue crystals of $\ce{CuSO4·5H2O}$ appear.

What you did: You synthesised an inorganic compound from elemental copper and an acid. You controlled the concentration, temperature, and evaporation rate. You then filtered and dried the crystals.

Why It Matters

Inorganic synthesis is the foundation of:

Catalysts (e.g., $\ce{Pt}$ on alumina for car exhausts)
Electronic materials (silicon wafers, gallium arsenide for LEDs)
Medicinal compounds (cisplatin for cancer therapy)
Pigments (titanium dioxide white, Prussian blue)
Batteries (lithium cobalt oxide electrodes)

Without inorganic synthesis, modern technology would not exist.

A Common Misconception

Watch out

Many students think inorganic synthesis is just "mixing two solutions and collecting a precipitate." That is only one method — and often the least controlled. Real synthesis requires understanding why a reaction happens (thermodynamics, kinetics, solubility) and how to drive it to completion (Le Chatelier's principle, removing a product, using excess reagent).

The Big Picture

Inorganic synthesis is purposeful creation. You are not a passive observer — you are an active designer. You decide the target molecule, then work backwards to find the best route. It is the most creative part of inorganic chemistry, and it demands both theoretical knowledge and practical skill.

Tip

When you first meet a synthesis problem, ask yourself: What is the simplest, cheapest, safest way to get from my starting materials to my target? That question is the heart of inorganic synthesis.

Inorganic synthesis routes are discussed in the NCERT/CBSE Class 12 Chemistry chapters on d- and f-Block Elements and Coordination Compounds, and ‘inorganic synthesis important questions’ is a common search among students preparing for board exams and JEE Main. Understanding the logic behind choosing a synthesis route is also useful preparation for reasoning-based NEET and CET chemistry questions.

Why this formula?

Inorganic Synthesis: Why the Key Formulae Hold

Inorganic synthesis is the branch of chemistry concerned with the preparation of inorganic compounds — from simple salts to complex coordination compounds, organometallics, and solid-state materials. The key formulae in this field are not arbitrary; they arise from fundamental principles of stoichiometry, thermodynamics, kinetics, and coordination chemistry.

Let’s break down the reasoning behind the most important formulae.

1. The Yield Formula: Why It’s Not Just “Product/Reactant”

The most basic formula in any synthesis is:

\text{Percentage Yield} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100\%

Why this holds:

Theoretical yield is calculated from the limiting reagent — the reactant that runs out first. This is based on the law of conservation of mass and the stoichiometric coefficients from the balanced chemical equation.
Actual yield is always less than theoretical because of:
- Side reactions (competing pathways)
- Incomplete reactions (equilibrium limitations)
- Loss during purification (filtration, crystallization, etc.)
The formula is a ratio because yield is a fractional measure of efficiency — it tells you how much of the maximum possible product you actually obtained.

Key insight: The formula works only if you correctly identify the limiting reagent. For example, in the synthesis of $\ce{FeCl3}$ from $\ce{Fe}$ and $\ce{Cl2}$ , if you have 1 mol Fe and 2 mol $\ce{Cl2}$ , Fe is limiting (1:1.5 stoichiometry), so theoretical yield is based on Fe.

2. The Atom Economy Formula: Why It Measures “Greenness”

\text{Atom Economy} = \frac{\text{Molecular Mass of Desired Product}}{\text{Sum of Molecular Masses of All Reactants}} \times 100\%

Why this holds:

This formula was introduced by Barry Trost (1991) to quantify how much of the starting materials ends up in the product.
It is not a yield — it’s a theoretical maximum based on the balanced equation. It assumes 100% yield.
The denominator includes all reactants (including solvents if they are consumed, but usually only stoichiometric reagents).
A high atom economy (e.g., 100% for addition reactions like $\ce{A + B -> C}$ ) means less waste. A low atom economy (e.g., substitution reactions with leaving groups) means more byproducts.

Example: In the synthesis of $\ce{NaCl}$ from $\ce{Na}$ and $\ce{Cl2}$ :

$2\ce{Na} + \ce{Cl2} \rightarrow 2\ce{NaCl}$

Atom economy = $\frac{2 \times 58.44}{2 \times 22.99 + 70.90} \times 100\% = 100\%$ — because all atoms end up in the product.

3. The Solubility Product and Precipitation: Why $K_{sp}$ Controls Synthesis

For a sparingly soluble salt like $\ce{AgCl}$ :

\ce{AgCl(s) <=> Ag+(aq) + Cl-(aq)}

K_{sp} = [\ce{Ag+}][\ce{Cl-}]

Why this holds:

$K_{sp}$ is an equilibrium constant derived from the law of mass action. It applies only to saturated solutions.
In synthesis, you use $K_{sp}$ to predict whether a precipitate will form when mixing solutions. If the ion product $Q = [\ce{Ag+}][\ce{Cl-}]$ exceeds $K_{sp}$ , precipitation occurs.
The formula is temperature-dependent (because $\Delta G^\circ = -RT \ln K_{sp}$ ). So you must control temperature to control precipitation.

Reasoning: The equilibrium constant arises from the balance between the lattice energy (holding the solid together) and the hydration energy (stabilizing ions in solution). A very small $K_{sp}$ means the solid is very stable — useful for gravimetric synthesis.

4. The Coordination Number and Ligand Field Stabilization Energy (LFSE)

For an octahedral complex, the LFSE is:

\text{LFSE} = \left( -0.4 \times n_{t_{2g}} + 0.6 \times n_{e_g} \right) \Delta_o

Why this holds:

This formula comes from crystal field theory (CFT). In an octahedral field, the five $d$ orbitals split into two sets: the lower-energy $t_{2g}$ (three orbitals) and the higher-energy $e_g$ (two orbitals).
The splitting energy $\Delta_o$ is the energy difference between these sets.
Electrons fill the $t_{2g}$ orbitals first (Hund’s rule), and each electron in $t_{2g}$ stabilizes the complex by $-0.4\Delta_o$ relative to the barycenter (average energy). Each electron in $e_g$ destabilizes by $+0.6\Delta_o$ .
The formula explains why certain coordination numbers are preferred: for example, $\ce{[Co(H2O)6]^{2+}}$ (high-spin $d^7$ ) has LFSE = $-0.8\Delta_o$ , while $\ce{[CoCl4]^{2-}}$ (tetrahedral) has a smaller LFSE — so the octahedral form is more stable.

Key insight: The formula is not arbitrary — it’s derived from the geometry of the ligand field and the electron configuration of the metal ion. It predicts magnetic properties, color, and stability.

5. The Nernst Equation in Redox Synthesis

For a half-cell reaction $\ce{M^{n+} + ne- -> M(s)}$ :

E = E^\circ - \frac{RT}{nF} \ln Q

Why this holds:

This is derived from thermodynamics: $\Delta G = \Delta G^\circ + RT \ln Q$ , and $\Delta G = -nFE$ .
In synthesis, you use it to predict whether a redox reaction will occur spontaneously. For example, to reduce $\ce{Fe^{3+}}$ to $\ce{Fe^{2+}}$ using $\ce{Zn}$ , you compare $E^\circ$ values.
The formula shows that concentration matters: even a non-spontaneous reaction (positive $E^\circ$ cell) can be driven by changing concentrations (Le Chatelier’s principle).

Example: In the synthesis of $\ce{CuCl}$ from $\ce{Cu^{2+}}$ and $\ce{Cu}$ (disproportionation), the Nernst equation tells you the equilibrium constant — and thus the yield.

Summary Table: Why Each Formula Holds

Formula	Core Principle	Why It’s Not Just a Definition
Percentage Yield	Stoichiometry + Conservation of Mass	Depends on limiting reagent identification
Atom Economy	Mass balance in balanced equation	Measures waste, not yield
$K_{sp}$	Equilibrium constant from $\Delta G^\circ$	Predicts precipitation — key for purification
LFSE	Crystal field splitting + electron filling	Explains stability, color, and coordination preference
Nernst Equation	$\Delta G = -nFE$	Links concentration to spontaneity — essential for redox synthesis

Final Takeaway

Every formula in inorganic synthesis is a mathematical expression of a physical law — conservation of mass, equilibrium thermodynamics, or quantum mechanical splitting. Understanding why they hold (not just memorizing them) lets you predict outcomes, optimize conditions, and design new syntheses. For exams, always ask: “What principle does this formula come from?” — that’s the key to solving problems correctly.

Fusing $MnO_2$ with KOH in air oxidises Mn(IV) to Mn(VI), forming the green manganate ion $MnO_4^{2-}$ . The product is potassium manganate, $K_2MnO_4$ , option (B).

This is a classic example of an oxidation reaction in a fused alkaline medium. The key is to track the oxidation state of manganese and the role of the environment.

Why this approach works: In solid-state or fused-salt reactions, the strong alkaline medium (KOH) and the oxidising power of atmospheric oxygen work together. $MnO_2$ is already a common starting material for manganese chemistry. When you fuse it with KOH, you create a melt rich in $OH^-$ ions. Air ( $O_2$ ) acts as the oxidising agent, pulling electrons away from manganese. The Mn(IV) in $MnO_2$ cannot stay at +4 in such a strongly oxidising, basic melt — it gets pushed to a higher stable state. The +6 state (manganate) is particularly stable in alkaline conditions, while the +7 state (permanganate) requires even stronger oxidising conditions or a different workup.

Let’s walk through the reasoning step by step.

Identify the starting oxidation state. In $MnO_2$ , oxygen is $-2$ (usual for oxides). Let the Mn oxidation state be $x$ . Then $x + 2(-2) = 0$ , so $x = +4$ . Manganese is in the +4 oxidation state.
Recognise the reaction conditions. “Fused with KOH in air” means:
- High temperature (fusion) — the mixture is molten.
- Strongly basic medium — excess KOH provides $OH^-$ ions.
- Presence of atmospheric oxygen ( $O_2$ ) — a good oxidising agent.
Predict the likely product. In alkaline conditions, manganese can exist in several oxidation states. The +6 state, as the manganate ion $MnO_4^{2-}$ , is well-known and stable in basic solution. The +7 state, as permanganate $MnO_4^-$ , is more stable in acidic or neutral conditions. Here, the basic melt favours the manganate. Also, $O_2$ is a moderately strong oxidiser — it can take Mn from +4 to +6, but not easily to +7 (that usually requires a stronger oxidant like $KNO_3$ or $KClO_3$ ).
Write the balanced chemical equation. The reaction is:

2 MnO_2 + 4 KOH + O_2 \rightarrow 2 K_2MnO_4 + 2 H_2O

Check: Mn goes from +4 to +6 (loss of 2 electrons per Mn). $O_2$ goes from 0 to $-2$ (gain of 4 electrons per $O_2$ ). Two Mn atoms lose 4 electrons total, exactly balancing the gain by one $O_2$ molecule. The KOH provides the potassium ions and the oxygen for the water.

Confirm the product identity. $K_2MnO_4$ is potassium manganate. It is a green solid, distinctly different from the purple $KMnO_4$ . This green colour is often observed during the fusion process.

Watch out

A common mistake is to jump straight to $KMnO_4$ because it is more familiar. But $KMnO_4$ forms only if you then dissolve the fused mass in water and acidify it, or if you use a stronger oxidant in the fusion. In the direct fusion with air, the product is always the manganate, $K_2MnO_4$ .

Tip

Remember the mnemonic: Alkaline melt + air → Manganate (green). Acidify or add stronger oxidant → Permanganate (purple). The colour change from green to purple is a classic test in qualitative analysis.

Eliminate the other options.
- (A) $KMnO_4$ — requires further oxidation (e.g., electrolysis or $Cl_2$ bubbling) of the manganate solution. Not formed directly in the fusion.
- (C) $Mn_2O_7$ — a highly unstable, explosive oily liquid formed by reacting $KMnO_4$ with cold concentrated $H_2SO_4$ . Not formed in alkaline fusion.
- (D) $Mn_2O_3$ — a reduction product (Mn in +3 state). The reaction is oxidative, not reductive.

✓Final answer

K_2MnO_4

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 23 on this concept.

CBSE 2025Set ANNUAL1 mark
$Q. How will you prepare K2MnO4 from pyrolusite? (Give chemical equation only)$
›Reveal solutionSolution

Fusion of pyrolusite (MnO2) with KOH in the presence of an oxidising agent (air/O2 or KNO3) gives potassium manganate.

Pyrolusite ( $MnO_2$ ) is fused with $KOH$ in presence of air (or an oxidising agent like $KNO_3$ ):

$2MnO_2 + 4KOH + O_2 \xrightarrow{\text{fuse}} 2K_2MnO_4 + 2H_2O$

This gives dark green potassium manganate ( $K_2MnO_4$ ), which is later disproportionated (by electrolytic oxidation or with $Cl_2$ / $CO_2$ ) to $KMnO_4$ .

✓Final answer
$2MnO2 + 4KOH + O2 \to 2K2MnO4 + 2H2O$
CBSE 2025Set ANNUAL1 mark
$Q. How will you prepare Potassium dichromate from Sodium dichromate? (Give chemical equation only)$
›Reveal solutionSolution

KCl is added to a solution of sodium dichromate; the less soluble potassium dichromate crystallises out.

Sodium dichromate solution is treated with potassium chloride:

$Na_2Cr_2O_7 + 2KCl \to K_2Cr_2O_7 + 2NaCl$

Since $K_2Cr_2O_7$ is less soluble than $NaCl$ , it crystallises out on cooling and can be separated by filtration.

✓Final answer
$Na2Cr2O7 + 2KCl \to K2Cr2O7 + 2NaCl$
CBSE 2024Set 56/2/11 markMCQ
$MnO_2$
›Reveal solutionSolution
Fusing $MnO_2$ with KOH in air oxidises Mn(IV) to Mn(VI), forming the green manganate ion $MnO_4^{2-}$ . The product is potassium manganate, $K_2MnO_4$ , option (B).

This is a classic example of an oxidation reaction in a fused alkaline medium. The key is to track the oxidation state of manganese and the role of the environment.

Why this approach works: In solid-state or fused-salt reactions, the strong alkaline medium (KOH) and the oxidising power of atmospheric oxygen work together. $MnO_2$ is already a common starting material for manganese chemistry. When you fuse it with KOH, you create a melt rich in $OH^-$ ions. Air ( $O_2$ ) acts as the oxidising agent, pulling electrons away from manganese. The Mn(IV) in $MnO_2$ cannot stay at +4 in such a strongly oxidising, basic melt — it gets pushed to a higher stable state. The +6 state (manganate) is particularly stable in alkaline conditions, while the +7 state (permanganate) requires even stronger oxidising conditions or a different workup.

Let’s walk through the reasoning step by step.

Identify the starting oxidation state. In $MnO_2$ , oxygen is $-2$ (usual for oxides). Let the Mn oxidation state be $x$ . Then $x + 2(-2) = 0$ , so $x = +4$ . Manganese is in the +4 oxidation state.

Recognise the reaction conditions. “Fused with KOH in air” means:

High temperature (fusion) — the mixture is molten.

Strongly basic medium — excess KOH provides $OH^-$ ions.

Presence of atmospheric oxygen ( $O_2$ ) — a good oxidising agent.

Predict the likely product. In alkaline conditions, manganese can exist in several oxidation states. The +6 state, as the manganate ion $MnO_4^{2-}$ , is well-known and stable in basic solution. The +7 state, as permanganate $MnO_4^-$ , is more stable in acidic or neutral conditions. Here, the basic melt favours the manganate. Also, $O_2$ is a moderately strong oxidiser — it can take Mn from +4 to +6, but not easily to +7 (that usually requires a stronger oxidant like $KNO_3$ or $KClO_3$ ).

Write the balanced chemical equation. The reaction is:

$2 MnO_2 + 4 KOH + O_2 \rightarrow 2 K_2MnO_4 + 2 H_2O$
Check: Mn goes from +4 to +6 (loss of 2 electrons per Mn). $O_2$ goes from 0 to $-2$ (gain of 4 electrons per $O_2$ ). Two Mn atoms lose 4 electrons total, exactly balancing the gain by one $O_2$ molecule. The KOH provides the potassium ions and the oxygen for the water.

Confirm the product identity. $K_2MnO_4$ is potassium manganate. It is a green solid, distinctly different from the purple $KMnO_4$ . This green colour is often observed during the fusion process.

Watch out

A common mistake is to jump straight to $KMnO_4$ because it is more familiar. But $KMnO_4$ forms only if you then dissolve the fused mass in water and acidify it, or if you use a stronger oxidant in the fusion. In the direct fusion with air, the product is always the manganate, $K_2MnO_4$ .

Tip

Remember the mnemonic: Alkaline melt + air → Manganate (green). Acidify or add stronger oxidant → Permanganate (purple). The colour change from green to purple is a classic test in qualitative analysis.

Eliminate the other options.

(A) $KMnO_4$ — requires further oxidation (e.g., electrolysis or $Cl_2$ bubbling) of the manganate solution. Not formed directly in the fusion.

(C) $Mn_2O_7$ — a highly unstable, explosive oily liquid formed by reacting $KMnO_4$ with cold concentrated $H_2SO_4$ . Not formed in alkaline fusion.

(D) $Mn_2O_3$ — a reduction product (Mn in +3 state). The reaction is oxidative, not reductive.

✓Final answer
$K_2MnO_4$
CBSE 2024Set ANNUAL1 markMCQ
$Q. The chemical formula of chromite ore is -$
(a) MnO2
(b) Na2Cr2O4
(c) FeCr2O4
(d) Na2CrO4
›Reveal solutionSolution

Chromite ore, the main source of chromium, has the formula FeCr2O4 (iron(II) chromite, a mixed oxide of iron and chromium).

Chromite crystallises in the spinel structure, in which Fe2+ ions occupy tetrahedral holes and Cr3+ ions occupy octahedral holes of a close-packed oxide lattice, giving the overall formula FeCr2O4 (equivalently FeO.Cr2O3).

This ore is fused with sodium carbonate in the presence of air to oxidise chromium to the chromate, which is the first step in the industrial preparation of potassium dichromate.

The other options are formulae of different substances: MnO2 is pyrolusite (a manganese ore), Na2Cr2O4 and Na2CrO4 are sodium chromium salts, not the natural ore.

✓Final answer
$(c) FeCr2O4.$
CBSE 2023Set ANNUAL1 markMCQ
$Q. Process of commercial production of nitric acid is$
(a) Haber process
(b) Ostwald's process
(c) Contact process
(d) Deacon's process
›Reveal solutionSolution

Ostwald's process is named specifically for industrial nitric-acid manufacture, distinguishing it from Haber's (ammonia), Contact (sulphuric acid) and Deacon's (chlorine) processes.

In Ostwald's process, ammonia is catalytically oxidised over a Pt-Rh catalyst to nitric oxide, which is further oxidised to NO2 and then absorbed in water to give nitric acid:

4NH3 + 5O2 --(Pt/Rh, 500 K, 9 bar)--> 4NO + 6H2O

2NO + O2 -> 2NO2

3NO2 + H2O -> 2HNO3 + NO

(Haber's process makes ammonia; Contact process makes sulphuric acid; Deacon's process makes chlorine from HCl.)

✓Final answer
$(b) Ostwald's process.$
CBSE 2022Set M1 mark
$Q. Name the method used for concentration of sulphide ore.$
›Reveal solutionSolution

Sulphide ores are concentrated by the froth flotation process.

The froth flotation process is used to concentrate sulphide ores. The powdered ore is mixed with water and a collector/frother (e.g. pine oil); air is blown through. The sulphide ore particles are preferentially wetted by the oil and rise with the froth, while the gangue (impurities) is wetted by water and settles down, separating the ore.

✓Final answer
$Froth flotation process.$
CBSE 2022Set ANNUAL1 markMCQ
$Q. Zone refining is used for obtaining ultra pure sample of$
(a) copper
(b) sodium
(c) germanium
(d) zinc
›Reveal solutionSolution

Zone refining purifies a metal based on the difference in solubility of impurities in the molten vs solid state of the metal.

In zone refining, a mobile induction heater melts a narrow zone of an impure metal rod at one end and moves slowly to the other end. Impurities are more soluble in the molten zone than in the solid, so they get swept along with the moving molten zone and concentrate at one end, which is then cut off. This technique gives extremely high-purity material and is specifically used for elements needed in ultra-pure form for semiconductors, most notably germanium and silicon.

✓Final answer
$(c) germanium.$
CBSE 2020Set ANNUAL1 mark
$Q. Iron scraps are advisable and advantageous than zinc scraps for reducing the low grade copper ores. Why?$
›Reveal solutionSolution

Iron and zinc both lie above copper in the reactivity series and can reduce $Cu^{2+}$ , but iron scrap is far cheaper and more abundant, so it is the economical choice.

Concept. In hydrometallurgy of copper, a low-grade ore is leached and the copper in solution is displaced by a more reactive metal:

$Cu^{2+}(aq) + M \rightarrow Cu + M^{2+}(aq)$

where $M$ must lie above copper in the activity series.

Reason. Both $Fe$ and $Zn$ are more reactive than $Cu$ , so either can reduce $Cu^{2+}$ to $Cu$ :

$Cu^{2+} + Fe \rightarrow Cu + Fe^{2+}$

However, scrap iron is cheaper and more widely available than scrap zinc. Using the cheaper reducing metal lowers the cost of extraction without affecting the chemistry, so iron scraps are advisable and advantageous.

✓Final answer
$Because iron is more reactive than copper (so it can displace/reduce copper) and, unlike zinc, iron scrap is inexpensive and abundant — making the extraction economical.$
CBSE 2020Set ANNUAL1 mark
$Q. Complete the reaction XeF₆ + H₂O ⟶ ? + 2HF .$
›Reveal solutionSolution

One molecule of water partially hydrolyses $XeF_6$ to $XeOF_4$ , liberating $2HF$ .

Concept. Xenon hexafluoride is readily hydrolysed. The extent of hydrolysis depends on the amount of water. With a limited amount (1 mole of water), only partial hydrolysis occurs.

Reaction (partial hydrolysis).

$XeF_6 + H_2O \rightarrow XeOF_4 + 2HF$

Here one $O$ atom replaces two $F$ atoms, and the two displaced $F$ combine with the two $H$ of water to give $2HF$ .

Note. With excess water, complete hydrolysis gives $XeO_3$ : $XeF_6 + 3H_2O \rightarrow XeO_3 + 6HF$ . Since the given equation shows only $2HF$ , the required product is $XeOF_4$ .

✓Final answer
$XeOF_4$
CBSE 2019Set ANNUAL1 mark
$Q. What is the role of depressant (NaCN) in Froth-Flotation method?$
›Reveal solutionSolution

NaCN selectively prevents ZnS from being wetted by the collector oil (by forming a complex on its surface), so ZnS sinks while PbS floats — separating a mixed Pb–Zn sulphide ore.

Concept: Froth flotation concentrates sulphide ores: pine-oil collectors make the mineral surface hydrophobic so it rises with the froth. When two sulphides are present, a depressant is used to keep one down.

Reasoning: NaCN reacts with ZnS to form a soluble surface complex $\text{Na}_2[\text{Zn(CN)}_4]$ that is not wetted by the oil, so ZnS does not float and is depressed, while PbS floats up with the froth. This achieves selective separation.

✓Final answer
$The depressant NaCN prevents ZnS from floating (by forming a surface complex), so only PbS collects in the froth and the two sulphides are separated.$
CBSE 2019Set ANNUAL1 markMCQ
$Q. Which of the following noble gases is abundant in air?$
(i) He
(ii) Ne
(iii) Ar
(iv) Kr
›Reveal solutionSolution

Argon is the most abundant noble gas in air.

Dry air contains about 0.93% argon by volume, whereas neon, helium and krypton are present only in trace amounts (of the order of parts per million). Hence argon is by far the most abundant noble gas in the atmosphere.

✓Final answer
$(iii) Ar.$
CBSE 2019Set ANNUAL1 markMCQ
$Q. Which one is the ore of copper?$
(i) Haematite
(ii) Chalcopyrite
(iii) Dolomite
(iv) Bauxite
›Reveal solutionSolution

Chalcopyrite (CuFeS2) is the ore of copper.

An ore is a mineral from which a metal is extracted profitably. Chalcopyrite (copper pyrites), CuFeS2, is the principal ore of copper. Haematite (Fe2O3) is an iron ore, dolomite (CaCO3·MgCO3) is a calcium/magnesium mineral, and bauxite (Al2O3·2H2O) is the ore of aluminium.

✓Final answer
$(ii) Chalcopyrite.$

Q.When MnO2MnO_2MnO2​ is fused with KOH in air, it gives : (A) KMnO4KMnO_4KMnO4​ (B) K2MnO4K_2MnO_4K2​MnO4​ (C) Mn2O7Mn_2O_7Mn2​O7​ (D) Mn2O3Mn_2O_3Mn2​O3​

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Q.When $MnO_2$ is fused with KOH in air, it gives : (A) $KMnO_4$ (B) $K_2MnO_4$ (C) $Mn_2O_7$ (D) $Mn_2O_3$

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