Laws of chemical combination

Correct answer: B. Law of conservation of mass

In this law the total mass before and after reaction remains equal. Exam tip: focus on the word total.

Correct answer: C. Pure compound

In a pure compound elements are in a fixed mass ratio. Exam tip: remember the pure compound clue.

Correct answer: D. Different compounds of the same two elements

Direct answer: Option D, different compounds formed from the same two elements. The law of multiple proportions applies when two elements combine in more than one compound. We compare the masses of one element that combine with a fixed mass of the other element. These masses show a simple whole-number ratio, such as 1:2. This is a law about composition by mass, not about appearance or one isolated measurement. Option A is wrong because colour differences in one mixture do not show how elements combine. Option B is wrong because the volume of one gas alone does not compare compounds or mass ratios. Option C is wrong because hardness is a physical property of a metal, not a comparison of chemical compositions. Option D is correct because it states the essential condition: the same two elements form different compounds, and their combining masses are compared. Remember: same two elements, different compounds, simple whole-number mass ratio.

Correct answer: A. At the same temperature and pressure

Gay-Lussac's law of combining volumes states that, when gases react, their volumes and the volume of gaseous products are in simple whole-number ratios, provided all volumes are measured at the same temperature and pressure. Therefore, option A is correct. Colour, taste, shape, lustre, or the metallic nature of substances has no role in defining this gas-volume relationship.

Correct answer: B. Three

The direct answer is B: three elements are involved. The law of reciprocal proportions compares the masses of three elements. Suppose elements A and B separately combine with a fixed mass of element C. The masses of A and B that combine with the same mass of C are then compared with the ratio in which A and B combine with each other. This is why the law is called reciprocal: one relationship is checked against another related combination. Step by step, the key clue is not simply “a compound has two elements”; it is the comparison of three elements across different compounds or combinations. Option A, one element, is impossible because a combination law must compare substances. Option B, three elements, is correct because the law studies their mass relationships. Option C, two elements, describes the basic participants in some individual compounds but not the complete reciprocal-proportion relationship. Option D, four elements, is not the standard definition of this law. Do not confuse it with conservation of mass, which says total mass remains constant, or definite proportions, which concerns a fixed composition of one pure compound. Memory cue: reciprocal proportions = three elements and linked mass ratios.

Correct answer: C. Law of constant composition

The law of definite proportions states that a pure chemical compound always contains the same elements in the same fixed ratio by mass. For example, water contains hydrogen and oxygen in a constant composition, no matter where the sample comes from or how it is prepared. The word definite means fixed, while composition refers to the elements and their proportions in the compound.

Because the law describes a fixed or constant composition, it is also called the law of constant composition. The pressure-volume law concerns gases, evaporation is a physical change, and thermal conductivity concerns the transfer of heat. None of these names describes fixed elemental ratios in a compound. Therefore choice C is the correct alternative name.

Correct answer: D. Lavoisier

The direct answer is D: Lavoisier. The law of conservation of mass says that in a chemical reaction matter is not created or destroyed. The total mass of the reactants before a reaction equals the total mass of the products after it, when the system is properly closed. Lavoisier established this law through careful experiments and accurate weighing. Option A, Proust, is associated with the law of definite proportions, which says a compound has elements in a fixed mass ratio. Option B, Dalton, is associated with atomic theory and the law of multiple proportions. Option C, Gay-Lussac, is associated with gas-volume relationships and combining volumes. Option D is correct because Lavoisier is the scientist linked with conservation of mass. Memory cue: Lavoisier means “mass is preserved.”

Correct answer: A. Proust

The law of definite proportions states that a pure chemical compound always contains the same elements combined in the same fixed proportion by mass. For example, a pure compound does not change its composition merely because it is obtained from a different source. This law helped establish that compounds have definite and measurable composition.

Option A, Proust, is correct. Joseph Louis Proust proposed the law of definite or constant proportions. Lavoisier is strongly associated with conservation of mass, Dalton with atomic theory and multiple proportions, and Richter with reciprocal proportions. Remembering the specific law linked with each scientist prevents confusion between the major laws of chemical combination.

Correct answer: B. Dalton

The law of multiple proportions is associated with John Dalton. It states that when two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in simple whole-number ratios. Proust proposed the law of definite proportions, so it is a close but incorrect option. Exam tip: remember the law of multiple proportions with Dalton’s atomic theory.

Correct answer: C. Gay-Lussac

Gay-Lussac proposed the law of gaseous volumes. It states that, at the same temperature and pressure, the volumes of reacting gases and gaseous products are in simple whole-number ratios. Avogadro’s law instead relates equal volumes of gases at the same temperature and pressure to equal numbers of molecules. Exam tip: associate simple ratios of gaseous volumes with Gay-Lussac.

Correct answer: D. 20 grams

The total reactant mass is 8 plus 12 which equals 20 grams. Exam tip: keep total mass equal in a closed system.

Correct answer: A. 23 grams

The total reactant mass is 14 plus 9 which equals 23 grams. Exam tip: keep total mass equal in a closed system.

Correct answer: B. 25 grams

The total reactant mass is 6 plus 19 which equals 25 grams. Exam tip: keep total mass equal in a closed system.

Correct answer: C. 25 grams

The total reactant mass is 21 plus 4 which equals 25 grams. Exam tip: keep total mass equal in a closed system.

Correct answer: D. 27 grams

The total reactant mass is 10 plus 17 which equals 27 grams. Exam tip: keep total mass equal in a closed system.

Correct answer: A. 24 grams

The total reactant mass is 13 plus 11 which equals 24 grams. Exam tip: keep total mass equal in a closed system.

Correct answer: B. 30 grams

The total reactant mass is 7 plus 23 which equals 30 grams. Exam tip: keep total mass equal in a closed system.

Correct answer: C. 34 grams

The total reactant mass is 16 plus 18 which equals 34 grams. Exam tip: keep total mass equal in a closed system.

Correct answer: D. 33 grams

The total reactant mass is 5 plus 28 which equals 33 grams. Exam tip: keep total mass equal in a closed system.

Correct answer: A. 37 grams

The total reactant mass is 22 plus 15 which equals 37 grams. Exam tip: keep total mass equal in a closed system.

Correct answer: B. 25 grams

The governing concept is the law of conservation of mass: in a reaction carried out without loss of material, the total mass of reactants equals the total mass of products. Let the unknown reactant mass be x. Then 11 g + x = 36 g. Subtracting 11 g from both sides gives x = 36 − 11 = 25 g. Therefore, option B is correct. Option A simply repeats the known mass, option C gives the entire product mass rather than the missing reactant, and option D results from an incorrect subtraction. This calculation assumes the stated masses represent the complete reacting substances.

Correct answer: C. 27 grams

The governing concept is conservation of mass in a closed chemical reaction. The masses of all reactants together must equal the mass of the product, so if the unknown reactant is x, the equation is 18 g + x = 45 g. Isolating x requires subtracting 18 g from the total: x = 45 − 18 = 27 g. Thus option C is correct. Option A repeats the known reactant, option B mistakes the complete product for the missing part, and option D does not satisfy the equation because 18 + 9 equals only 27 g. The answer follows directly from the stated complete-mass assumption.

Correct answer: D. 25 grams

This question applies the law of conservation of mass: in a chemical reaction, the total mass of the reactants equals the total mass of the products, assuming no material escapes or enters the system. Therefore, if the product mass is 52 g and one reactant contributes 27 g, let the unknown reactant be x. Then x + 27 = 52, so x = 52 − 27 = 25 g. Thus, option D is correct. Option A merely repeats the known reactant mass, option B gives the entire product mass rather than the missing part, and option C results from an incorrect subtraction.

Correct answer: A. 23 grams

This question uses the law of conservation of mass. In a chemical reaction carried out in a closed system, the total mass of the reactants equals the total mass of the products. If the product has a mass of 30 grams and one reactant contributes 7 grams, the unknown reactant must provide the remaining mass. We calculate it as \\(30-7=23\\) grams. Therefore option A is correct. The calculation assumes that these are the complete reactants and that no material has escaped or entered the system.

The method is to write total reactant mass as the sum of the known and unknown masses: \\(7+x=30\\). Subtracting 7 grams from both sides gives \\(x=23\\) grams. Option B repeats the known mass, option C gives the total rather than the missing part, and option D does not satisfy the mass balance. Thus 23 grams is the only value consistent with conservation of mass.

Correct answer: B. 35 grams

The governing idea is the law of conservation of mass. For a closed chemical system, the combined mass of all reactants equals the mass of the product formed. If the unknown reactant has mass x, the data give x + 29 g = 64 g. Rearranging the equation, x = 64 g − 29 g = 35 g. Therefore, option B is the correct answer. Option A is only the mass of the known reactant, while option C is the total product mass and not the missing reactant. Option D comes from subtracting in the wrong way or making an arithmetic error. The units remain grams throughout the calculation.