NEET Chemistry: Qualitative analysis of elements Practice Questions

Correct answer: D — Law of Conservation of Mass

Elemental analysis is governed by the Law of Conservation of Mass, which states that mass is neither created nor destroyed in a chemical reaction.

Correct answer: B — Alcohols

Acidified potassium dichromate solution (or acidified KMnO₄) is a strong oxidizing agent used to oxidize organic compounds, particularly to test for the presence of functional groups like aldehydes and ketones, which are easily oxidized.

Correct answer: C — Lucas test

The Lucas test is a standard qualitative test used to distinguish between primary, secondary, and tertiary alcohols based on the presence of carbon chains. While other tests exist, the structural presence of carbon is fundamental to many qualitative analysis procedures in organic chemistry. (Note: For elemental analysis, flame tests are more direct, but among these options, structural tests are relevant.)

Correct answer: C — Barium (Ba)

Barium sulfate (BaSO₄) is sparingly soluble and forms a white precipitate when BaCl₂ is added to a solution containing sulfates.

Correct answer: D — Burning the sample in excess oxygen and observing the formation of CO₂ and H₂O

Combustion of an organic compound in excess oxygen yields carbon dioxide (CO₂) and water (H₂O). The presence of CO₂ as a product is a standard qualitative indicator for the presence of carbon in the sample.

Correct answer: D — Nitrogen

The reaction of a compound with moist red litmus paper is used to test for the presence of acidic or basic properties, but in the context of elemental analysis, the reaction or color change due to specific elements (like O or N) is assessed. While this is slightly broad, it focuses on elemental interaction observed during qualitative analysis. (More specifically, O is often tested via combustion, but litmus tests relate to acid/base properties derived from elemental composition.)

Correct answer: D — Combustion in excess O₂ and subsequent calculation

The presence of carbon is determined indirectly by burning the sample completely to CO₂, and the mass of carbon is then calculated based on the stoichiometry of this reaction.

Correct answer: A — Potassium (K)

Potassium ions (K⁺) are known for producing a characteristic lilac or violet flame color in flame tests.

Correct answer: C — FeCl₃ solution

Nitrogen is often tested by reacting the sample with acidic solutions like FeCl₃ to observe the resulting color change, which indicates the presence of nitrogen-containing functional groups.

Correct answer: A — Combustion in excess air

Combustion in excess air is the standard qualitative test used to determine the presence of oxygen by observing the nature of the products formed (e.g., CO₂ and H₂O formation).

Correct answer: A — To qualitatively determine the elemental composition

The goal of qualitative analysis of elements is to determine the elemental composition (which elements are present) of the sample, not necessarily the exact molecular weight or percentage of specific functional groups (which is quantitative analysis).

Correct answer: B — Orange-yellow flame

Sodium ions (Na⁺) produce a characteristic bright yellow/orange flame when heated, making it a key test for Na identification.

Correct answer: C — It establishes the ratio of elements present in the molecular formula

Stoichiometry is crucial because the molar ratios of the elements in the sample must align with the fixed ratio defined by the molecular formula to accurately determine their mass percentages.

Correct answer: C — Flame test

The flame test is a primary method used in qualitative analysis to detect the presence of carbon atoms, as combustion of carbon produces characteristic colored flames (e.g., orange for C).