Laws of chemical combination

The law of multiple proportions compares different compounds made from the same two elements. Two identical samples of one pure compound do not provide different compounds to compare; they instead have the same fixed composition.

The volume ratio of gas A to gas B is 20:30. Dividing both terms by their greatest common factor, 10, gives 2:3. Option 3:2 is a close distractor, but it reverses the order of the gases. Exam tip: write ratios in the order in which substances are given and reduce them to the simplest whole-number form.

The volume ratio of gas A to gas B is 15:5. Dividing both terms by 5 gives the simplest ratio, 3:1. Thus, 3 volumes of gas A react with 1 volume of gas B. The ratio 1:3 represents the reverse order, gas B : gas A. Exam tip: always retain the order of substances stated in the question while writing a ratio.

A ratio is simplified by dividing every term by the greatest common divisor shared by all terms. For 12, 12, and 24, the greatest common divisor is 12. Dividing term by term gives 12 ÷ 12 = 1, 12 ÷ 12 = 1, and 24 ÷ 12 = 2. Thus the simplest ratio is 1:1:2, making option A correct. Options B and C rearrange the positions of the numbers, which changes the original order, while option D divides only some terms and is therefore not a valid equivalent ratio. The fact that the quantities are gaseous does not change this basic ratio-reduction procedure.

At the same temperature and pressure, gas volumes are directly proportional to their amounts in moles. The given ratio is 6:18:12. Dividing every term by 6 gives the simplest ratio, 1:3:2. The ratio 1:2:3 does not match the given volumes in the stated order. Exam tip: reduce all gas volumes by their highest common factor without changing the order of the gases.

The correct answer is D, same colour. Gay-Lussac’s law states that the volumes of reacting gases and gaseous products are in simple whole-number ratios when measured at the same temperature and pressure. Colour is irrelevant to this relationship.

The correct answer is A. Gay-Lussac’s law states that, at the same temperature and pressure, the volumes of reacting gases and gaseous products have a simple whole-number ratio. It does not say that all gases have equal volumes.

In a binary compound, the percentages of A and B together must add up to 100%. Therefore, percentage of B = 100 - 62 = 38%. The value 62% is the given percentage of A, not B. Exam tip: in percentage-composition questions, the percentages of all constituents must total 100%.

The ratio of the percentages of A and B is 48:52. Dividing both terms by their greatest common divisor, 4, gives 12:13. The ratio 13:12 reverses the order, while 4:5 is not the simplest form of 48:52. Exam tip: reduce percentage ratios by dividing both values by their greatest common divisor.

The correct answer is B, 30%. Mass percentage = (mass of A ÷ total mass of compound) × 100 = (45 g ÷ 150 g) × 100 = 30%.

The correct answer is B, 25%. Mass percentage = (mass of X ÷ total mass of compound) × 100 = (50 g ÷ 200 g) × 100 = 25%.

The correct answer is A. In a closed system, no matter enters or leaves, so the total mass of the reactants equals the total mass of the products in a chemical reaction. In an open system, escaping gas can make a measurement appear lower if it is not collected.

The law of definite proportions, also called the law of constant composition, states that a particular pure compound always contains the same constituent elements in the same fixed proportion by mass, regardless of the source or amount of the sample. Therefore, option A correctly describes its scope. A mixture or solution can have variable composition because its components may be present in different amounts, so options B and the mixture-related interpretation are wrong. The law is not restricted to gases or to open vessels; consequently, options C and D introduce conditions that are not part of the law.

The law of multiple proportions applies when the same two elements combine in more than one compound. If the mass of one element is fixed, the different masses of the other element that combine with it are in a simple whole-number ratio. Thus, option A states both essential conditions: the compounds contain the same two elements, and there is more than one compound. Option B is incomplete because one compound cannot provide a comparison. A beverage is generally a mixture, not necessarily a set of compounds formed by the same elements, so C is unsuitable. Option D confuses this mass-ratio law with Gay-Lussac’s law of gaseous volumes.

The correct answer is A. These laws describe quantitative relationships, such as how mass and gas volumes relate in chemical reactions. Such regularities helped provide a foundation for atomic theory and the study of chemical formulae.

The correct answer is D. Gay-Lussac is associated with the law of combining volumes, not the law of reciprocal proportions. Lavoisier, Proust and Dalton are correctly matched with the laws listed in the other options.

The correct answer is A. Gay-Lussac's law of combining volumes states that the volumes of reacting gases and gaseous products are in simple whole-number ratios when measured at the same temperature and pressure.

The correct answer is C. In an ordinary chemical reaction, matter is not created or destroyed; in a closed system, the total mass before and after the reaction is the same. So the claim that matter disappears contradicts mass conservation.

The law of definite proportions states that the constituent elements of a pure compound are always present in a fixed mass ratio. Therefore, the statement that this ratio may be arbitrary in a pure compound contradicts the law. The ratio in a mixture can vary because mixtures are not governed by this law. Exam tip: distinguish carefully between a pure compound and a mixture.

The law of multiple proportions applies when the same two elements form two or more compounds. For a fixed mass of one element, the masses of the other element combine in simple whole-number ratios; for example, oxygen in CO and CO₂ is in the ratio 1:2 for the same mass of carbon. Option D describes the law of definite proportions, not multiple proportions. Exam tip: the phrase “one compound” usually points to the law of definite proportions.

The correct answer is D. Gay-Lussac's law relates the volumes of reacting gases and gaseous products, measured at the same temperature and pressure; taste has no role in determining those volume ratios.

In questions based on laws of chemical combination, check whether the calculated masses or ratios are in the simplest whole-number form and whether the unit is appropriate for the question. Therefore, option A is the correct final-checking habit. Ignoring the mass of a gas is incorrect because it may be part of the reaction calculation. Exam tip: before finalising an answer, recheck the ratio, unit, and calculation.

The correct answer is A. The law of definite proportions states that a pure compound always contains the same elements in the same fixed ratio by mass. A mixture can have a variable composition, so the law applies to compounds, not mixtures.

The governing concept is the law of multiple proportions. It applies when the same two elements combine to form two or more different compounds. Their masses are compared while the mass of one element is kept fixed; the resulting ratios are simple whole numbers. For example, carbon and oxygen form CO and CO2, where the oxygen masses combining with the same carbon mass are in the ratio 1:2. Therefore, option A is correct. Definite proportions concerns one pure compound, conservation of mass compares total reactant and product masses, and Gay-Lussac’s law concerns gaseous volume ratios at the same temperature and pressure.