Chemistry Archives

231. Which one of the following compounds does not exhibit a different oxid

Which one of the following compounds does not exhibit a different oxidation number of the same element?

[amp_mcq option1=”Pb₃O₄” option2=”Fe₃O₄” option3=”Fe₂O₃” option4=”Mn₃O₄” correct=”option3″]

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The correct option is C) Fe₂O₃. In Fe₂O₃, iron exists in only one oxidation state.

We need to determine the oxidation state of the metal element in each compound:
A) Pb₃O₄ is a mixed oxide, often written as 2PbO·PbO₂. In PbO, O is -2, so Pb is +2. In PbO₂, O is -2 (total -4 for 2 O atoms), so Pb is +4. Lead has oxidation states +2 and +4 in Pb₃O₄.
B) Fe₃O₄ is a mixed oxide, FeO·Fe₂O₃. In FeO, O is -2, so Fe is +2. In Fe₂O₃, O is -2 (total -6 for 3 O atoms), so 2 Fe atoms are +6, meaning each Fe is +3. Iron has oxidation states +2 and +3 in Fe₃O₄.
C) Fe₂O₃: Assuming oxygen is -2, the total charge from 3 oxygen atoms is 3 * (-2) = -6. For the compound to be neutral, the total charge from 2 iron atoms must be +6. Thus, the oxidation state of each Fe atom is +6 / 2 = +3. All iron atoms in Fe₂O₃ are in the +3 oxidation state.
D) Mn₃O₄ is a mixed oxide, MnO·Mn₂O₃. In MnO, O is -2, so Mn is +2. In Mn₂O₃, O is -2 (total -6 for 3 O atoms), so 2 Mn atoms are +6, meaning each Mn is +3. Manganese has oxidation states +2 and +3 in Mn₃O₄.

Mixed oxides like Pb₃O₄, Fe₃O₄, and Mn₃O₄ contain the metal element in more than one oxidation state combined within the same compound structure. Simple binary oxides like Fe₂O₃ (or FeO, FeO₂, etc.) typically have the metal in a single oxidation state throughout the compound.

232. Which one of the following statements about the law of conservation of

Which one of the following statements about the law of conservation of mass is correct?

[amp_mcq option1=”A given compound always contains exactly same proportion of elements.” option2=”When gases combine in a reaction, they do so in a simple ratio by volume provided all gases are at room temperature.” option3=”Matter can neither be created nor destroyed.” option4=”Equal volumes of all gases at same temperature and pressure contain equal number of molecules.” correct=”option3″]

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The Law of Conservation of Mass, first stated by Antoine Lavoisier, posits that mass is conserved in a chemical reaction or physical change. This means that the total mass of the reactants before a chemical reaction is equal to the total mass of the products after the reaction. The statement “Matter can neither be created nor destroyed” is a fundamental principle expressing this law.

The Law of Conservation of Mass is a cornerstone of classical chemistry, stating that the total quantity of matter remains constant in an isolated system. It is often expressed as matter cannot be created or destroyed, though it can change form.

Option A describes the Law of Definite Proportions (Proust). Option B describes Gay-Lussac’s Law of Combining Volumes. Option D describes Avogadro’s Law. These are different fundamental laws of chemistry. While related to how matter behaves in chemical reactions, they are distinct from the Law of Conservation of Mass.

233. Which one of the following is the chemical formula of gypsum?

Which one of the following is the chemical formula of gypsum?

[amp_mcq option1=”CaSO₄·2H₂O” option2=”Ca₂SiO₄” option3=”2CaSO₄·H₂O” option4=”CaSO₄” correct=”option1″]

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Gypsum is a mineral commonly found in evaporite deposits. Its chemical composition is calcium sulfate dihydrate, meaning it contains calcium sulfate (CaSO₄) and two molecules of water of crystallization per formula unit. The chemical formula for gypsum is CaSO₄·2H₂O.

The degree of hydration of calcium sulfate determines whether it is gypsum (dihydrate), plaster of Paris (hemihydrate), or anhydrite (anhydrous).

When gypsum (CaSO₄·2H₂O) is heated, it loses water to form calcium sulfate hemihydrate (CaSO₄·½H₂O), which is commonly known as Plaster of Paris. Further heating removes all water, resulting in anhydrous calcium sulfate (CaSO₄), or anhydrite. Gypsum is widely used in plaster, drywall (gypsum board), cement production, and as a soil conditioner.

234. Which one of the following is not used as fertilizer?

Which one of the following is not used as fertilizer?

[amp_mcq option1=”Ammonium nitrate” option2=”Ammonium sulphide” option3=”Ammonium phosphate” option4=”Ammonium sulphate” correct=”option2″]

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Fertilizers are substances used to provide essential nutrients to plants, primarily nitrogen (N), phosphorus (P), and potassium (K).
A) Ammonium nitrate (NH₄NO₃) is a common nitrogen fertilizer.
B) Ammonium sulphide ((NH₄)₂S) contains nitrogen and sulfur, but it is highly unstable and decomposes easily to ammonia and hydrogen sulfide gas. Due to its instability and potential toxicity (hydrogen sulfide), it is not used as a common agricultural fertilizer.
C) Ammonium phosphate (e.g., (NH₄)₂HPO₄ or NH₄H₂PO₄) are common fertilizers (like DAP and MAP) providing both nitrogen and phosphorus.
D) Ammonium sulphate ((NH₄)₂SO₄) is a common fertilizer providing nitrogen and sulfur.
Therefore, Ammonium sulphide is not used as a fertilizer.

Common fertilizers contain essential plant nutrients, primarily nitrogen, phosphorus, and potassium, often in the form of stable salts. Ammonium compounds are often used as sources of nitrogen, but their suitability as fertilizers depends on their stability and chemical properties in soil.

While ammonium sulphide contains nitrogen and sulfur, its instability makes it unsuitable for agricultural application. It is more relevant in laboratory chemistry or specific industrial processes rather than widespread use as a soil nutrient source.

235. Which one of the following is an oxidation-reduction reaction?

Which one of the following is an oxidation-reduction reaction?

[amp_mcq option1=”NaOH + HCl → NaCl + H₂O” option2=”CaO + H₂O → Ca(OH)₂” option3=”2Mg + O₂ → 2MgO” option4=”Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl” correct=”option3″]

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An oxidation-reduction (redox) reaction is a chemical reaction in which the oxidation states of atoms are changed. This involves the transfer of electrons.
Let’s check the oxidation states of elements in each reaction:
A) NaOH + HCl → NaCl + H₂O: Na (+1→+1), O (-2→-2), H (+1→+1), Cl (-1→-1). No change in oxidation states. This is an acid-base neutralization reaction.
B) CaO + H₂O → Ca(OH)₂: Ca (+2→+2), O (-2→-2), H (+1→+1). No change in oxidation states. This is a combination reaction.
C) 2Mg + O₂ → 2MgO: Mg (0→+2), O (0→-2). Magnesium’s oxidation state increases from 0 to +2 (oxidation). Oxygen’s oxidation state decreases from 0 to -2 (reduction). This is a redox reaction.
D) Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl: Na (+1→+1), S (+6→+6), O (-2→-2), Ba (+2→+2), Cl (-1→-1). No change in oxidation states. This is a double displacement (precipitation) reaction.

Redox reactions are characterized by changes in the oxidation states of the participating atoms. Oxidation is the loss of electrons (increase in oxidation state), and reduction is the gain of electrons (decrease in oxidation state). Elemental substances (like Mg and O₂) have an oxidation state of 0.

Many common reactions are redox reactions, including combustion, corrosion, and cellular respiration. Acid-base neutralization and simple precipitation reactions typically do not involve changes in oxidation states and are therefore not redox reactions.

236. The solution of which one of the following will have pH less than 7?

The solution of which one of the following will have pH less than 7?

[amp_mcq option1=”NaOH” option2=”KCl” option3=”FeCl₃” option4=”NaCl” correct=”option3″]

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The pH of a solution indicates its acidity or alkalinity. A pH less than 7 is acidic, pH equal to 7 is neutral, and pH greater than 7 is alkaline (basic). Let’s analyze the given options:
A) NaOH (Sodium hydroxide) is a strong base. Its solution is alkaline (pH > 7).
B) KCl (Potassium chloride) is a salt formed from a strong acid (HCl) and a strong base (KOH). Its solution is neutral (pH ≈ 7).
C) FeCl₃ (Iron(III) chloride) is a salt formed from a weak base (Fe(OH)₃) and a strong acid (HCl). When dissolved in water, metal ions with high charge density, like Fe³⁺, undergo hydrolysis, reacting with water to produce hydrated metal ions and release H⁺ ions, making the solution acidic: FeCl₃(aq) + 3H₂O(l) ⇌ Fe(OH)₃(s) + 3H⁺(aq) + 3Cl⁻(aq). Therefore, its solution will have a pH less than 7.
D) NaCl (Sodium chloride) is a salt formed from a strong acid (HCl) and a strong base (NaOH). Its solution is neutral (pH ≈ 7).

The pH of a salt solution depends on the strength of the acid and base from which the salt is formed. Salts of strong acid and strong base are neutral, salts of strong acid and weak base are acidic, and salts of weak acid and strong base are alkaline.

Hydrolysis is the reaction of a substance with water. In the case of salts formed from weak acids or bases, the ions can react with water, affecting the pH of the solution. Cations from weak bases and anions from weak acids are the ones that undergo significant hydrolysis.

237. Which one of the following is called dry ice?

Which one of the following is called dry ice?

[amp_mcq option1=”Solid carbon dioxide” option2=”Liquid carbon dioxide” option3=”Liquid nitrogen” option4=”Liquid ammonia” correct=”option1″]

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Dry ice is the solid form of carbon dioxide (CO₂).

– At atmospheric pressure, solid carbon dioxide sublimes directly into a gas without melting into a liquid, hence the name “dry ice”.
– Its sublimation temperature at 1 atm is -78.5 °C.

Dry ice is used as a cooling agent in various applications, such as preserving food, creating fog effects, and cooling materials in laboratories. Liquid carbon dioxide exists under high pressure. Liquid nitrogen and liquid ammonia are different substances with different properties and uses.

238. Which one of the following statements is not correct?

Which one of the following statements is not correct?

[amp_mcq option1=”All carbons in diamond are linked by carbon-carbon single bond.” option2=”Graphite is layered structure in which layers are held together by weak van der Waals forces.” option3=”Graphite layers are formed by hexagonal rings of carbon atoms.” option4=”Graphite layers are held together by carbon-carbon single bond.” correct=”option4″]

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The statement that graphite layers are held together by carbon-carbon single bonds is incorrect. Graphite layers (graphene sheets) are themselves held together by covalent bonds within the plane, but the forces *between* these layers are weak van der Waals forces.

– Diamond has a 3D tetrahedral structure where each carbon atom is covalently bonded to four other carbon atoms by single bonds (sp³ hybridization).
– Graphite has a layered structure. Each layer consists of carbon atoms arranged in hexagonal rings, with strong covalent bonds (sp² hybridization) within the layer.
– The layers in graphite are held together by relatively weak van der Waals forces. This allows the layers to slide easily over each other, giving graphite its properties as a lubricant and its softness.

The delocalized electrons within the layers of graphite allow it to conduct electricity, unlike diamond, which is an electrical insulator. The carbon-carbon bond length within a graphite layer is about 0.142 nm, while the distance between layers is about 0.335 nm.

239. Which one of the following is a tribasic acid?

Which one of the following is a tribasic acid?

[amp_mcq option1=”Hydrochloric acid” option2=”Nitric acid” option3=”Sulphuric acid” option4=”Phosphoric acid” correct=”option4″]

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Phosphoric acid (H₃PO₄) is a tribasic acid because it can donate three protons (H⁺ ions) in a stepwise manner.

– The basicity of an acid refers to the number of acidic hydrogen atoms per molecule that can be donated as protons.
– Monobasic acids donate one proton (e.g., HCl, HNO₃).
– Dibasic acids donate two protons (e.g., H₂SO₄, H₂CO₃).
– Tribasic acids donate three protons (e.g., H₃PO₄).

The dissociation of phosphoric acid occurs in three steps:
1. H₃PO₄ ⇌ H⁺ + H₂PO₄⁻
2. H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻
3. HPO₄²⁻ ⇌ H⁺ + PO₄³⁻
Hydrochloric acid (HCl) and nitric acid (HNO₃) are monobasic. Sulphuric acid (H₂SO₄) is dibasic.

240. Which one of the following reactions will give NO (nitric oxide) gas a

Which one of the following reactions will give NO (nitric oxide) gas as one of the products?

[amp_mcq option1=”3Cu + 8HNO₃ (dilute)→” option2=”Cu + 4HNO₃ (conc.)→” option3=”4Zn + 10HNO₃ (dilute)→” option4=”Zn + 4HNO₃ (conc.)→” correct=”option1″]

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The reaction of copper (Cu) with dilute nitric acid (HNO₃) produces nitric oxide (NO) gas, copper(II) nitrate (Cu(NO₃)₂), and water. The balanced chemical equation is 3Cu + 8HNO₃ (dilute) → 3Cu(NO₃)₂ + 2NO + 4H₂O.

– Nitric acid is a strong oxidizing agent, and its reaction with metals produces nitrogen oxides rather than hydrogen gas (which is produced by the reaction of active metals with non-oxidizing acids).
– The specific nitrogen oxide produced depends on the concentration of HNO₃ and the reactivity of the metal. Dilute HNO₃ reacts with moderately reactive metals like copper to produce NO. Concentrated HNO₃ reacts with most metals (except very unreactive ones) to produce nitrogen dioxide (NO₂).

– Reaction with concentrated HNO₃: Cu + 4HNO₃ (conc.) → Cu(NO₃)₂ + 2NO₂ + 2H₂O (produces NO₂).
– Reaction with very dilute HNO₃ and highly reactive metals (like Zn): 4Zn + 10HNO₃ (very dilute) → 4Zn(NO₃)₂ + N₂O + 5H₂O (produces nitrous oxide, N₂O) or even NH₄NO₃. The reaction given in option C with dilute HNO3 producing N2O is also correct for zinc.