Laws of chemical combination

Correct answer: B. The experiment should have been done in a closed system

According to the law of conservation of mass, total mass remains constant in a chemical reaction. In the open beaker, the gas produced escaped, so the measured mass of the beaker and remaining contents appeared lower. Gases do have mass; if the escaped gas were included, the total mass would remain the same. Therefore, the experiment should have been performed in a closed system. Exam tip: mention a closed container when testing conservation of mass in a gas-producing reaction.

Correct answer: A. Law of definite proportions

The law of definite proportions states that a pure compound has the same composition by mass, regardless of the sample or experiment. The repeated carbon dioxide samples therefore illustrate this law.

Correct answer: A. Law of definite proportions

The law of definite proportions says that a pure compound has a fixed composition by mass. A salt-and-sand mixture is not a compound, and its proportions can vary, so this law does not require a fixed ratio for it.

Correct answer: B. Multiple proportions

The direct answer is option B: the law of multiple proportions. The concept is used when the same two elements form more than one compound, and a fixed mass of one element combines with different masses of the other element in a simple whole-number ratio. CO and CO₂ contain the same two elements, carbon and oxygen, but in different compounds. To reason step by step, fix the mass of carbon, compare the oxygen masses combined with it in CO and CO₂, and reduce their ratio; the ratio is a simple whole-number ratio. Option A, conservation of mass, concerns equality of total mass before and after a reaction, not comparison between two compounds. Option B is correct because it compares different compounds made from the same two elements. Option C, definite proportions, applies to one pure compound and says its elements have a fixed mass ratio; it does not explain the comparison between CO and CO₂. Option D, gaseous volumes, concerns volume ratios of gases at the same temperature and pressure, not these mass combinations. Memory cue: same two elements, different compounds, different masses in a simple ratio means multiple proportions.

Correct answer: A. Law of multiple proportions

The law of multiple proportions applies when the same two elements form more than one compound. For a fixed mass of one element, the masses of the other that combine with it are in a simple whole-number ratio.

Correct answer: B. Law of definite proportions

The law of definite proportions states that a pure compound always contains the same elements in the same mass ratio. Thus, pure water has the same composition whether it comes from one source or another.

Correct answer: B. Fixed

The governing law is the law of definite proportions, also called the law of constant composition. It states that a pure chemical compound always contains the same elements combined in the same fixed mass ratio, regardless of where the sample comes from or how much of it is examined. Therefore, option B, fixed, is correct. The ratio need not be 1:1; for example, water has a fixed mass ratio of hydrogen to oxygen of 1:8, not equal masses. Option A describes a mixture, whose composition can vary, and option D is chemically meaningless for a compound containing elements. The word “pure” is important because impurities or mixtures can change the measured overall ratio.

Correct answer: A. Because a gas's volume changes with temperature and pressure

A gas's volume depends on its temperature and pressure. Comparing volumes under the same conditions makes the ratios in the law meaningful and comparable.

Correct answer: C. Gay-Lussac’s law of gaseous volumes

The governing concept is Gay-Lussac’s law of combining volumes. It states that when reacting gases and gaseous products are measured at the same temperature and pressure, their volumes stand in simple whole-number ratios. The ratio 1:1:2 is exactly such a simple whole-number volume ratio, so option C is correct. The law of conservation of mass concerns total mass rather than gas volume. The law of definite proportions concerns the fixed mass composition of a pure compound, not the volumes of reacting gases. Reciprocal proportions is a different mass-ratio relationship among elements. Thus the decisive clue is the explicit gaseous volume ratio, especially its simple integer form and the implied common temperature and pressure conditions.

Correct answer: B. Law of multiple proportions

These formulas illustrate the law of multiple proportions. Hydrogen and oxygen form two different compounds, H₂O and H₂O₂. For a fixed mass of hydrogen, the masses of oxygen that combine in the two compounds are in a simple whole-number ratio: the oxygen amount in H₂O₂ is twice that in H₂O. The same two elements therefore combine in more than one ratio, giving option B. The law of definite proportions would describe one pure compound having one fixed composition, not two compounds with different formulas. Conservation of mass concerns the total mass before and after a reaction, and solubility is unrelated to these formulas. The essential clues are the same two elements and different, simple whole-number compositions.

Correct answer: B. Carry out the reaction in a closed vessel and measure the total mass before and after

In a closed vessel, matter cannot enter or escape. Measuring the total mass of the whole system before and after the reaction lets you check that the mass is conserved.

Correct answer: A. Law of definite proportions

The law of definite proportions states that a given pure compound always contains its elements in a fixed ratio by mass. That is exactly the condition described in the question.

Correct answer: B. Law of multiple proportions

The law of multiple proportions applies when the same two elements form different compounds. For a fixed mass of one element, the masses of the other element in those compounds are in a simple whole-number ratio.

Correct answer: D. 12 g

The compound has a fixed mass ratio: B/A = 6 g ÷ 2 g = 3. For 4 g of A, the required mass of B is 3 × 4 g = 12 g.

Correct answer: A. 1:2

For the same fixed mass of carbon, the masses of oxygen are 16 g and 32 g. Therefore, the ratio is 16:32 = 1:2. The ratio 2:1 is the reverse order, so it is not correct. Exam tip: simplify a ratio by dividing both terms by their greatest common factor.

Correct answer: A. Law of definite proportions — fixed composition of a pure compound

The law of definite proportions is identified by the fixed composition of a pure compound: its elements occur in a constant mass ratio. The other clues do not describe the stated laws.

Correct answer: A. At the same temperature and pressure

The correct answer is A. Gay-Lussac’s law compares the volumes of reacting gases and gaseous products at the same temperature and pressure; under these conditions, the volume ratios are simple whole numbers.

Correct answer: A. Gas escaped

Limestone is mainly calcium carbonate, and an acid reacting with a carbonate produces a salt, water, and carbon dioxide gas. In an open flask, the carbon dioxide can leave the apparatus. Consequently, the balance records a lower mass for the material remaining in the flask, even though mass has not been destroyed. Therefore, option A is the correct explanation. To demonstrate conservation of mass experimentally, the reaction should be carried out in a closed system that retains the gas, and the complete apparatus should be weighed before and after reaction. Option B contradicts conservation of mass, while option C is false because solids have mass and option D is false because acid is matter. The observed decrease is a measurement-system issue caused by escaped gas.

Correct answer: B. Law of conservation of mass

The total mass of the reactants is 6 g + 4 g = 10 g, which equals the mass of the substance formed. Therefore, this illustrates the law of conservation of mass: mass is neither created nor destroyed in a chemical reaction. The law of definite proportions concerns a fixed mass ratio of elements in a compound, not simply equality of total reactant and product masses. Exam tip: if total reactant mass equals total product mass, choose the law of conservation of mass.

Correct answer: A. Yes, because the total mass of reactants and products is 10 g

The total mass of reactants is 3 g + 7 g = 10 g. The total mass of products is 9 g + 1 g of gas = 10 g. Therefore, the law of conservation of mass is satisfied. Option D may be generally true, but it does not establish equality of the given masses. Exam tip: always include the mass of every product, including gases, before comparing both sides.

Correct answer: A. Both ratios are equivalent

Dividing both terms of 6:10 by 2 gives 3:5. Therefore, the ratio of A to B is the same in both samples, as expected for a pure compound under the law of definite proportions. Option B is incorrect because 6:10 is only a multiple form of 3:5, not a different ratio. Exam tip: reduce ratios to their simplest form before comparing them.

Correct answer: A. Simplifying the ratio

The governing mathematical concept is simplifying a ratio by dividing both terms by their greatest common factor. In 2:4, the greatest common factor is 2. Dividing each term by 2 gives 2 ÷ 2 : 4 ÷ 2 = 1:2. The relationship represented by the ratio has not changed; only its numbers have been written in the smallest whole-number form. Therefore, option A is correct. The process does not destroy mass, create a gas, or change a compound into a mixture. It is simply a numerical representation of the same relative quantities. In chemistry, reducing ratios to their simplest whole-number form is useful when expressing elemental composition or deriving an empirical formula, provided all terms are divided by the same nonzero factor.

Correct answer: A. Gay-Lussac's law of gaseous volumes

The direct answer is option A: Gay-Lussac's law of gaseous volumes. The statement gives volumes of gases: 1 volume of chlorine, 1 volume of hydrogen, and 2 volumes of hydrogen chloride. These are in the simple ratio 1:1:2, under the usual condition that the gas volumes are measured at the same temperature and pressure. Step 1: identify that the data are volumes, not masses. Step 2: note that all substances involved are gases in this gaseous-volume example. Step 3: write the ratio as 1:1:2. Step 4: match a simple gas-volume ratio with Gay-Lussac's law. Option A is therefore correct. Option B concerns fixed mass composition in one compound, not gas-volume ratios. Option C compares mass ratios in different compounds made by the same elements, so it is not relevant. Option D, reciprocal proportions, deals with mass relationships among elements and compounds, not the stated gas volumes. The important exam cue is simple volume ratios in a gas reaction; that points directly to Gay-Lussac's law.

Correct answer: A. Sulfur and oxygen form SO₂ and SO₃

The law of multiple proportions applies when the same two elements form more than one compound. In SO₂ and SO₃, sulfur and oxygen are the same pair of elements, but they combine in different whole-number ratios. If the mass of sulfur is kept fixed, the masses of oxygen that combine with it are in a simple ratio, because SO₃ contains one more oxygen atom per sulfur atom than SO₂. Therefore, option A gives a genuine example of this law. Option B concerns physical mixing, while C and D do not compare compounds formed by the same two elements.

Correct answer: A. In pure NaCl, sodium and chlorine are present in a fixed mass ratio

According to the law of definite proportions, the constituent elements of a pure compound are always present in a fixed ratio by mass. Therefore, pure NaCl contains sodium and chlorine in a fixed mass ratio. Tea, soil and lemonade are mixtures, so the amounts of their components can vary. Exam tip: a compound has fixed composition, whereas a mixture has variable composition.