QCM : Basic Concepts of Chemistry — 48 questions

Explication

Chemistry is the branch of science concerned with the preparation, properties, structure, and reactions of material substances.

Explication

Modern chemistry took shape in eighteenth-century Europe after earlier alchemical traditions had been introduced by the Arabs. Alchemy and iatrochemistry were more prominent during 1300–1600 CE.

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Ancient Indian chemistry was known by names such as Rasayan Shastra, Rastantra, Ras Kriya, and Rasvidya, and covered areas including metallurgy, medicine, cosmetics, glass, and dyes.

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Harappans increased copper hardness by using tin and arsenic. Their technological achievements also included baked and glazed pottery, faience, and forged metal objects.

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Acharya Kanda proposed that substances consist of eternal, indestructible, spherical, indivisible particles called Paramãnu, which could combine into pairs or triplets through unseen forces.

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Matter is defined as anything that has mass and occupies space. The other properties may apply to some matter but do not define it.

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Chemistry contributes to healthcare by developing drugs, including cisplatin and taxol for cancer therapy. AZT is another chemical drug used to help AIDS patients.

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Safer alternatives have been developed because chlorofluorocarbons contribute to depletion of stratospheric ozone. Methane and carbon dioxide remain separate greenhouse-gas concerns.

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Pure substances are classified further into elements and compounds. Mixtures, in contrast, contain two or more pure substances in variable proportions.

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A homogeneous mixture has a uniform composition because its components are distributed evenly throughout. A non-uniform composition characterizes a heterogeneous mixture.

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Distillation separates mixture components by using differences in boiling points. Filtration, for example, separates suitable solids from fluids based on particle size or insolubility.

Explication

A compound contains different elements in a definite ratio and can be separated into simpler substances only by chemical methods. Mixture components remain physically combined and may be separated by physical methods.

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Boiling point is a physical property that can be measured without producing a new substance. Flammability, rusting, and reaction with acid involve chemical change.

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A quantitative observation includes a numerical value and its unit, such as 250 millilitres. The other observations describe qualities without necessarily giving numerical measurements.

Explication

The kelvin is the SI base unit for thermodynamic temperature. The mole measures amount of substance, while the joule is not an SI base unit.

Explication

Density is mass divided by volume: ρ = 2.0 kg ÷ 0.5 m³ = 4.0 kg m⁻³. Mass is the numerator in this relationship, not the density itself.

Explication

Using F = (9/5)C + 32 gives F = (9/5)(20) + 32 = 68°F. The Celsius-to-Kelvin relationship is a different conversion.

Explication

Mass describes the amount of matter and remains constant, while weight is the gravitational force acting on an object and can change with location. Therefore, they are distinct physical quantities.

Explication

Scientific notation expresses a number as N × 10ⁿ, with N ranging from 1.000… to 9.999… and n an integer.

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Zeros before the first non-zero digit are not significant, while the zero between non-zero digits and terminal zeros to the right of the decimal point are significant.

Explication

The measurements are close to one another, so they are precise, but they disagree substantially with the accepted value, so they are not accurate.

Explication

For addition, the result cannot have more digits to the right of the decimal point than the least precise measurement. Since 3.2 has one decimal place, the result is 15.3.

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Dimensional analysis uses unit factors equal to one, chosen so unwanted units cancel and the desired units remain.

Explication

Multiplying 2 × 24 × 60 × 60 gives 172,800 seconds.

Explication

The law states that matter cannot be created or destroyed, so total mass remains constant even though chemical reactions can produce different substances.

Explication

A compound follows definite proportions when it contains the same elements in the same mass proportions regardless of its source. Both natural and synthetic cupric carbonate contain 51.35% copper, 9.74% carbon, and 38.91% oxygen by mass.

Explication

The masses of Y that combine with a fixed mass of X are in the small whole-number ratio 2:6, or 1:3, demonstrating the law of multiple proportions.

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Avogadro’s law states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.

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Dalton proposed that matter is composed of atoms, compounds contain atoms in fixed ratios, and reactions rearrange conserved atoms. Later discoveries showed that atoms are divisible, but this was not part of Dalton’s original theory.

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Dalton’s theory accounted for the laws of chemical combination but did not explain the laws of gaseous volumes or the reasons atoms combine.

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One atomic mass unit, or u, is exactly one-twelfth of the mass of a carbon-12 atom. Hydrogen was an earlier arbitrary reference, not the modern definition.

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Average atomic mass accounts for both isotope masses and their natural relative abundances, so it is a weighted mean rather than simply one isotope’s mass.

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The molecular mass is found by multiplying each element’s atomic mass by the number of its atoms in the molecule and summing the results: M = Σᵢ nᵢAᵢ.

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Ionic substances such as sodium chloride are described by formula units rather than discrete molecules. Their formula mass is the sum of the atomic masses represented in the formula unit.

Explication

The mole is the SI unit of amount of substance and contains exactly 6.02214076 × 10²³ specified entities. Molarity, in contrast, describes concentration.

Explication

The Avogadro constant is N_A = 6.02214076 × 10²³ mol⁻¹. The unit mol⁻¹ distinguishes it from the dimensionless number of entities in one mole.

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Molar mass is the mass of one mole and is expressed in g mol⁻¹. Its numerical value matches the corresponding mass expressed in atomic mass units.

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Mass percentage compares an element’s mass contribution with the compound’s molar mass and multiplies by 100. A mole fraction instead compares numbers of moles.

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The empirical formula represents the simplest whole-number ratio of atoms. The molecular formula represents the actual number of each atom in one molecule.

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Assuming a 100 g sample makes each percentage numerically equal to the corresponding mass in grams. These masses are then converted to moles and simplified into a whole-number ratio.

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Stoichiometry calculates quantitative relationships, such as the masses or volumes of reactants and products. Balancing an equation establishes ratios but does not itself calculate these amounts.

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Coefficients may be adjusted to obtain equal numbers of each type of atom on both sides. Subscripts must remain unchanged because changing them changes the substances involved.

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The balanced equation is CH₄ + 2O₂ → CO₂ + 2H₂O. It has one carbon, four hydrogen, and four oxygen atoms on each side.

Explication

The limiting reagent is consumed first and therefore determines the maximum amount of product. Once it is exhausted, further reaction cannot occur.

Explication

Mole fraction uses the component’s moles divided by the total moles of every component in the solution. Moles per litre is molarity, not mole fraction.

Explication

Molarity equals moles of solute divided by litres of solution: 0.50 mol ÷ 2.0 L = 0.25 M.

Explication

For dilution, the amount of solute remains constant, giving the relationship M₁V₁ = M₂V₂. The initial and final concentration-volume products are equal.

Explication

Molality is defined as the number of moles of solute present in one kilogram of solvent. It differs from molarity, which uses the volume of the entire solution.