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CONCEPT:

Weighted Average Atomic Mass

• The weighted average atomic mass of an element is the average mass of its isotopes, weighted by their fractional abundances in nature.
• The formula for calculating the weighted average atomic mass is:

Average Atomic Mass = (fractional abundance of isotope 1 × atomic mass of isotope 1) + (fractional abundance of isotope 2 × atomic mass of isotope 2) + ...

EXPLANATION:

• In the given problem, the isotopic composition of element 'X' is:
• 200X: 90% or 0.90 (atomic mass = 200 u)
• 199X: 8% or 0.08 (atomic mass = 199 u)
• 202X: 2% or 0.02 (atomic mass = 202 u)
• The weighted average atomic mass is calculated as:
• Average Atomic Mass = (0.90 × 200) + (0.08 × 199) + (0.02 × 202)
• Average Atomic Mass = 180 + 15.92 + 4.04
• Average Atomic Mass ≈ 199.96 u

Therefore, the weighted average atomic mass of element 'X' is approximately 200 u.

CONCEPT:

Oxidizing Agents in Groups of Representative Elements

• An oxidizing agent is a substance that gains electrons in a chemical reaction, thereby causing the oxidation of another substance.
• Generally, the most potent oxidizing agents are found in groups where elements have high electronegativity and the tendency to accept electrons.
• The halogens (Group 17) are known to be the most potent oxidizing agents due to their high electronegativity and their ability to gain an electron easily to form negative ions.

EXPLANATION:

• Group 17 (Halogens) includes elements like fluorine, chlorine, and iodine. Among these, fluorine is the most potent oxidizing agent, as it has the highest electronegativity of any element, making it very effective at accepting electrons.
• Fluorine, with its ability to easily gain an electron and form fluoride ions (F^-), is the most potent oxidizing agent in this group.
• Group 1 (Alkali metals) contains elements that are highly reactive, but they tend to lose electrons rather than gain them, making them strong reducing agents rather than oxidizing agents.
• Group 16 (Chalcogens) and Group 3 (Boron Group) contain elements that are not as potent as oxidizing agents as the halogens.

Therefore, the most potent oxidizing agent is found in Group 17.

CONCEPT:

Electron Configuration and Ion Formation

• Elements with an outer-shell electron configuration of ns²np⁴ belong to Group 16 (also known as the chalcogens) in the periodic table.
• These elements have 6 valence electrons and need 2 more electrons to complete their octet (achieve a stable electron configuration similar to a noble gas).
• To achieve a full outer shell, these elements tend to gain 2 electrons, resulting in the formation of ions with a charge of -2.

EXPLANATION:

• The given outer-shell electron configuration is ns²np⁴, which corresponds to elements like oxygen, sulfur, and other Group 16 elements.
• When these elements react chemically:
• They gain 2 electrons to fill their valence shell, achieving a stable octet configuration.
• This results in the formation of an anion with a charge of -2.
• For example:
• Oxygen (O) with configuration 1s² 2s² 2p⁴ gains 2 electrons to form an O^2- ion.
• Sulfur (S) with configuration [Ne] 3s² 3p⁴ gains 2 electrons to form an S^2- ion.

The atoms of element 'X' are most likely to produce ions with a charge of -2.

CONCEPT:

Iron Refining Process

• The process of obtaining pure iron from impure forms typically involves removing impurities from cast iron (which is obtained from the blast furnace).
• The purest form of commercial iron is called "wrought iron," which is produced by refining pig iron.
• A reverberatory furnace is used in the process to oxidize the impurities (mainly carbon, sulfur, and silicon) from the pig iron to produce wrought iron.
• Limestone is often used in the furnace to remove impurities such as silica and sulfur, which forms slag that can be removed easily.

EXPLANATION:

• In the given statement, the purest form of commercial iron is "wrought iron," which is produced by refining pig iron.
• Pig iron is the product of the blast furnace and contains impurities like carbon, sulfur, and silicon.
• These impurities are removed by heating the pig iron in a reverberatory furnace, and limestone is used to help in the purification process by forming slag with the impurities.
• The reverberatory furnace helps oxidize the impurities, and the final product is "wrought iron."

Therefore, the correct answer is  Wrought iron, pig iron, limestone

CONCEPT:

Anomalous Behavior of First Members of s and p Block Elements

• The first member of each group in the s and p block elements (like Lithium, Beryllium, Boron, etc.) shows anomalous behavior compared to the rest of the group due to their unique physical and chemical properties.
• These anomalies arise because of their small size, high charge-to-radius ratio, and relatively high electronegativity compared to the subsequent elements in the same group.
• These factors make the first members of the s and p block elements exhibit characteristics that are different from the rest of their group, like stronger covalent bonding, less tendency to form ions, and more acidic oxides.

EXPLANATION:

• The first member of each group in the s and p block elements, such as Lithium (Li), Beryllium (Be), Boron (B), Carbon (C), etc., is much smaller in size compared to the rest of the elements in the same group.
• Due to their smaller size, these elements have a larger charge-to-radius ratio and higher electronegativity, which gives rise to their anomalous behavior. For example, they tend to form more covalent bonds and do not easily form cations, unlike the larger members of the group.
• The small size, large charge-to-radius ratio, and high electronegativity lead to distinct chemical and physical properties that make them behave differently from the rest of the group elements.

Therefore, the correct answer is Small size, large charge / radius ratio, high electronegativity

CONCEPT:

High Octane Fuels and Their Properties

• Octane rating is a measure of a fuel's ability to resist knocking (pre-detonation) in internal combustion engines. Higher octane fuels resist knocking better.
• Iso-octane (C₈H₁₈) is used as the standard for 100 octane, while n-heptane (C₇H₁₆) is used as the standard for a 0 octane rating.
• Knocking is caused by premature ignition of the fuel-air mixture, leading to the formation of radicals that react with oxygen and oxidize further.
• The structure of the hydrocarbon (branched vs straight-chain) influences the formation of radicals during combustion, which in turn affects the knocking tendency of the fuel.

EXPLANATION:

• Statement A: "n-heptane has a 'zero' octane rating, and iso-octane has a 'hundred'."
• This is true. n-Heptane is used as the reference fuel for the lowest octane rating (0), and iso-octane is used as the reference for the highest octane rating (100).
• Statement B: "During combustion, branched-chain hydrocarbons produce 39 radicals, and straight-chain hydrocarbons produce 1° radicals."
• This statement is not correct in terms of the number of radicals. While branched-chain hydrocarbons do tend to form fewer radicals (because their structure is more stable and resistant to knocking), the number of radicals produced is not precisely 39 or always compared to the 1° radicals from straight-chain hydrocarbons.
• Statement C: "3° radicals react immediately with oxygen and undergo further oxidation."
• This is true. Tertiary (3°) radicals are more stable and react readily with oxygen, leading to further oxidation, which helps in reducing knocking.
• Statement D: "Branched-chain hydrocarbons produce more knocking as compared to straight-chain hydrocarbons."
• This statement is incorrect. Branched-chain hydrocarbons tend to resist knocking better than straight-chain hydrocarbons, as their structure makes it more difficult for the fuel-air mixture to auto-ignite.

Therefore, the correct answer is:

3) (A), (C) and (D)

CONCEPT:

Chemical Reactions Involving Aromatic Alcohol, Oxidation, and Grignard Reagents

• The first reaction involves the oxidation of an aromatic alcohol using potassium dichromate (K₂Cr₂O₇) in an acidic medium (H⁺). This reaction typically oxidizes the -OH group of the alcohol to a carbonyl group, forming an aldehyde or carboxylic acid.
• The second reaction involves the use of a Grignard reagent (PhMgBr) that reacts with a carbonyl compound (such as an aldehyde or ketone) to form an alcohol after hydrolysis.
• The third reaction involves the heating of a compound with CrO₃ (chromic acid), which further oxidizes aldehydes to carboxylic acids, completing the reaction sequence.

EXPLANATION:

• In the first step, the aromatic alcohol undergoes oxidation by potassium dichromate in acidic conditions. This oxidation converts the -OH group to a carbonyl group, forming a benzaldehyde (C₆H₅CHO).

• In the second step, the Grignard reagent (PhMgBr) is added to the aldehyde (benzaldehyde). The Grignard reagent attacks the carbonyl carbon, forming a tetrahedral intermediate, which upon hydrolysis, gives a secondary alcohol (a benzyl alcohol).

• The final step involves heating the intermediate compound with CrO₃, which oxidizes the alcohol to a carboxylic acid (benzoic acid, C₆H₅COOH).

Therefore, the compound is B

CONCEPT:

Chloride-Bridged Structure and Its Formation

• A chloride-bridged structure is a type of molecular arrangement where chloride ions bridge between two metal centers. This structure is typically observed in vapour or gaseous states of certain compounds.
• In certain metal halides, such as BeCl₂, the molecules in the vapour phase can form a bridge using chloride ions, leading to a polymeric structure.
• This bridging behavior is largely due to the low coordination number and the polarizing power of the metal cations involved, which allows them to form such bridge-like structures with chloride ions.

EXPLANATION:

• In the vapour state, some metal halides such as BeCl₂ form a chloride-bridged structure. This occurs because the beryllium ion (Be²⁺) has a high charge-to-radius ratio, which allows it to polarize the chloride ions and form a polymeric chain with bridging chloride ions between the metal centers.
• When BeCl₂ is vaporized, the molecules exist as a dimeric or polymeric structure with chloride ions bridging between the Be²⁺ ions. This is different from other halides that do not show this type of bridging behavior in the vapour phase.
• In contrast, compounds like LiCl and MgCl₂ do not exhibit such chloride-bridged structures in the vapour phase because their ions do not form similar interactions with chloride ions.

Therefore, the correct answer is BeCl₂ and AlCl₃

CONCEPT:

Polymers Used in Clothing

• Polymers are large molecules composed of repeated subunits, and their properties make them suitable for various applications, including textiles.
• Different types of polymers are used in clothing based on their characteristics, such as strength, flexibility, breathability, and moisture-wicking ability.
• Polymers like Nylon and Polyester are synthetic fibers commonly used in making sportswear and activewear due to their durability and moisture-resistant properties.

EXPLANATION:

• Nylon: A synthetic polymer known for its elasticity, strength, and quick-drying nature. It is widely used in sportswear and activewear.
• Polyester: Another synthetic polymer that is lightweight, durable, and moisture-wicking, commonly used in sportswear.
• Jute: A natural fiber used mainly in making ropes, mats, and sacks, but not suitable for sportswear.
• Cotton: A natural fiber that is breathable and comfortable but lacks the moisture-wicking and quick-drying properties required for activewear.

Therefore, the polymer used in making sportswear and activewear clothing is Nylon.

CONCEPT:

Alternatives to Tetrachloroethylene in Dry Cleaning

• Tetrachloroethylene (also known as perchloroethylene or PERC) is a commonly used solvent in the dry cleaning industry, but it has been identified as a potential human carcinogen and poses environmental hazards.
• Due to these health and environmental concerns, "green" alternatives to PERC have been developed, which are safer for both the environment and human health.

EXPLANATION:

• Ethyl acetate, supercritical liquid CO₂, and other non-toxic solvents are considered "green" alternatives to PERC in dry cleaning.
• Among these, supercritical liquid CO₂ is one of the most effective and eco-friendly alternatives. It uses carbon dioxide in a supercritical state (where it behaves like both a liquid and a gas) to clean clothes. It is non-toxic, non-flammable, and does not produce hazardous waste.
• Ethyl acetate is another alternative, as it is less harmful than PERC but is not as commonly used in dry cleaning as supercritical CO₂.
• Petrol (gasoline) and CFCs (chlorofluorocarbons) are not suitable as green alternatives. Petrol is highly flammable and toxic, while CFCs are environmentally harmful due to their contribution to ozone depletion.

Therefore, the correct answer is Supercritical liquid CO₂