Laws of chemical combination

Correct answer: C. The total mass is equal

The governing concept is the law of conservation of mass. In an ordinary chemical reaction, atoms are rearranged, but they are neither created nor destroyed. Therefore, the sum of the masses of all reactants equals the sum of the masses of all products, provided the reaction is considered in a closed system. This law does not say that the mass of each individual product must equal the mass of each individual reactant. For example, if 12 g of carbon reacts with 32 g of oxygen, the product carbon dioxide has a total mass of 44 g. The correct statement is therefore option C. Option A wrongly compares separate substances, option B contradicts the law, and option D concerns an irrelevant physical property.

Correct answer: D. Tea is a mixture

Tea can have variable composition so it is not a pure compound. Exam tip: treat beverages as mixtures.

Correct answer: A. Different compounds made from the same two elements are needed

The governing concept is the law of multiple proportions. This law applies when the same two elements form two or more different compounds. If a fixed mass of one element combines with different masses of the other element, those masses show a simple whole-number ratio. For example, carbon and oxygen form CO and CO₂; for the same mass of carbon, the oxygen masses combine in a 1:2 ratio. Two samples of H₂O are still samples of the same compound, so they cannot demonstrate multiple proportions. A mixture is also unsuitable because its composition is variable rather than fixed. Hence option A is correct: compounds formed from the same two elements, but having different compositions, must be compared. Options B, C and D do not state the required condition.

Correct answer: B. Litre

Gay-Lussac’s law of combining volumes states that, at the same temperature and pressure, reacting gases combine in simple whole-number ratios of their volumes. Therefore, a volume unit such as litre is the most useful clue. Gram and kilogram measure mass, so they are not the key indicator for this volume-based law. Exam tip: words such as gas, same temperature and pressure, and volume ratio usually point to Gay-Lussac’s law.

Correct answer: C. Mass percentage

The law of definite proportions states that a pure compound always contains its constituent elements in a fixed ratio by mass. Mass percentage gives the percentage by mass of each element in a compound, so it is useful for understanding and verifying this law. Colour, odour, and container shape do not indicate the mass ratio of the elements. Exam tip: link a fixed mass ratio with percentage composition or mass percentage.

Correct answer: D. 25 grams

According to the law of conservation of mass, in a closed system the total mass of reactants before a reaction equals the total mass of products after the reaction. Therefore, total product mass = 11 g + 14 g = 25 g. The value 3 g is only the difference between the given masses, not the product mass. Exam tip: For a closed-system reaction, add the masses of all reactants to obtain the total mass of products.

Correct answer: A. 41 grams

In a closed system, the law of conservation of mass applies. Hence, the total mass of the products equals the total mass of the reactants: 23 g + 18 g = 41 g. The value 5 g is only the difference between the given masses, not the product mass. Exam tip: for a closed-system question, add the masses of all reactants.

Correct answer: B. 40 grams

According to the law of conservation of mass, in a closed system the total mass of reactants before a reaction equals the total mass of products after the reaction. Therefore, total product mass = 31 g + 9 g = 40 g. Values such as 31 g or 9 g represent the mass of only one reactant, not the total products. Exam tip: for a closed-system question, add the masses of all reactants.

Correct answer: C. 43 grams

According to the law of conservation of mass, the total mass remains the same before and after a reaction in a closed system. Therefore, total product mass = 17 g + 26 g = 43 g. The value 9 g is only the difference between the reactant masses, not the total product mass. Exam tip: in closed-system questions, add the masses of all reactants.

Correct answer: D. 41 grams

According to the law of conservation of mass, mass is neither created nor destroyed during a reaction in a closed system. Therefore, total mass of products = 28 g + 13 g = 41 g. The value 15 g is the difference between the reactant masses, not the total product mass. Exam tip: In closed-system questions, add the masses of all reactants to obtain the total mass of products.

Correct answer: A. 47 grams

According to the law of conservation of mass, in a closed system the total mass before a reaction equals the total mass of the products after the reaction. Therefore, total product mass = 35 g + 12 g = 47 g. Values such as 35 g or 12 g represent the mass of only one reactant, not the total products. Exam tip: In closed-system questions, add the masses of all reactants.

Correct answer: B. 40 grams

According to the law of conservation of mass, the total mass remains the same before and after a reaction in a closed system. Therefore, total mass of products = 19 g + 21 g = 40 g. The value 21 g is the mass of only one reactant, not of all the products. Exam tip: In closed-system questions, add the masses of all reactants.

Correct answer: C. 50 grams

According to the law of conservation of mass, in a closed system the total mass of substances before a reaction equals the total mass of products after the reaction. Therefore, total product mass = 42 g + 8 g = 50 g. The value 34 g is only the difference between the two masses, not the total mass of products. Exam tip: In closed-system questions, add the masses of all reactants to obtain the total mass of products.

Correct answer: D. 45 grams

According to the law of conservation of mass, in a closed system the total mass of reactants remains equal to the total mass of products. Therefore, the total product mass is 16 + 29 = 45 grams. The value 13 grams is only the difference between the two given masses, not the total product mass. Exam tip: in closed-system questions, add the masses of all reactants to find the total mass of products.

Correct answer: A. 55 grams

According to the law of conservation of mass, in a closed system the total mass of reactants before a reaction equals the total mass of products after the reaction. Therefore, total product mass = 24 g + 31 g = 55 g. The value 7 g is only the difference between the given masses, not the product mass. Exam tip: For a closed-system reaction, add the masses of all reactants to obtain the total mass of products.

Correct answer: B. 16 grams

The governing idea is the fixed composition of a pure compound: its constituent masses remain in a definite ratio. The ratio A:B = 4:7 contains 4 + 7 = 11 total parts. Since these 11 parts together weigh 44 g, one part weighs 44 ÷ 11 = 4 g. Element A occupies 4 parts, so its mass is 4 × 4 = 16 g. Equivalently, the mass fraction of A is 4/11 and 44 × 4/11 = 16 g. Therefore option B is correct. Option A, 28 g, is the mass corresponding to seven parts and would represent B; option C incorrectly assigns the entire sample to A; option D does not follow from the given ratio. The calculation also confirms that A + B = 16 + 28 = 44 g.

Correct answer: C. 45 grams

This problem uses the definite-proportions principle and the parts method. The ratio A:B = 2:9 has 2 + 9 = 11 total parts. The complete compound weighs 55 g, so the mass represented by one part is 55 ÷ 11 = 5 g. B corresponds to 9 parts; therefore its mass is 9 × 5 = 45 g. Thus option C is correct. Another check is to calculate the mass fraction of B: 9/11, and 55 × 9/11 also equals 45 g. Option A is only 2 parts and would correspond to A, not B. Option B is the mass of the whole compound, not one constituent, while option D is greater than the total mass and is physically impossible. The two constituent masses are 10 g and 45 g, whose sum is the stated 55 g.

Correct answer: D. 25 grams

The relevant governing concept is definite proportion: a pure compound contains its elements in a constant mass ratio. Here A:B = 5:8, so the total number of ratio parts is 5 + 8 = 13. The total mass is 65 g, hence one part has mass 65 ÷ 13 = 5 g. A occupies 5 parts, giving mass 5 × 5 = 25 g. Therefore option D is correct. This can also be written as A = 65 × 5/(5+8) = 25 g. Option A, 40 g, is the mass corresponding to B’s eight parts; option B is the total mass of the compound rather than A’s mass. Option C does not satisfy the stated ratio. As a consistency check, A = 25 g and B = 40 g add to 65 g, and 25:40 simplifies to 5:8.

Correct answer: A. 30 grams

The question is based on the law of definite proportions, expressed here through a simple mass-ratio calculation. The ratio A:B = 6:5 contains 6 + 5 = 11 parts altogether. Since the complete compound has a mass of 66 g, one ratio part equals 66 ÷ 11 = 6 g. B represents 5 parts, so its mass is 5 × 6 = 30 g. Thus option A is correct. Option B, 36 g, corresponds to A’s six parts; option C is the mass of the whole sample, not B alone; and option D does not result from the ratio. A useful check is that A has mass 36 g and B has mass 30 g, giving a total of 66 g. Their ratio is 36:30, which reduces by 6 to 6:5, exactly as required.

Correct answer: B. 12 grams

The governing principle is that a pure compound has a definite composition by mass. The stated ratio A:B = 3:10 gives 3 + 10 = 13 total parts. The total mass of those 13 parts is 52 g, so one part weighs 52 ÷ 13 = 4 g. Since A occupies 3 parts, its mass is 3 × 4 = 12 g. Therefore option B is correct. Option A, 40 g, corresponds to the ten parts of B, not the three parts of A. Option C is the total mass of the entire compound, and option D is inconsistent with the ratio. A final check confirms the result: A = 12 g and B = 40 g, their sum is 52 g, and 12:40 simplifies to 3:10. This confirms both the arithmetic and the interpretation of the ratio.

Correct answer: C. 30 grams

This is an application of definite proportions using ratio parts. The mass ratio A:B = 7:6 has 7 + 6 = 13 total parts. The total mass is 65 g, therefore each part represents 65 ÷ 13 = 5 g. B occupies 6 parts, so its mass is 6 × 5 = 30 g. Hence option C is correct. Option A, 35 g, is the mass of A because A has seven parts. Option B gives the mass of the complete compound and cannot be the mass of only B. Option D is not obtained from the given ratio and would also make the corresponding value of A inconsistent with the total. Checking the result, A = 35 g and B = 30 g add to 65 g, while 35:30 reduces by 5 to 7:6, confirming the answer.

Correct answer: D. 32 grams

The governing concept is the law of definite proportions: the elements in a pure compound occur in a constant mass ratio. For A:B = 8:11, the total number of parts is 8 + 11 = 19. The total mass of 19 parts is 76 g, so one part has a mass of 76 ÷ 19 = 4 g. A represents 8 parts, giving 8 × 4 = 32 g. Therefore option D is correct. Option A, 44 g, corresponds to B’s eleven parts; option B is the mass of the whole compound, not A alone. Option C does not satisfy the ratio. The check is straightforward: A = 32 g and B = 44 g total 76 g, and 32:44 simplifies by 4 to 8:11. Thus the result is consistent with both the stated total and the fixed composition.

Correct answer: A. 25 grams

The question uses the fixed-composition principle for a pure compound. The ratio A:B = 9:5 contains 9 + 5 = 14 total parts. Since the total mass is 70 g, the mass of one part is 70 ÷ 14 = 5 g. B corresponds to 5 parts, so its mass is 5 × 5 = 25 g. Thus option A is correct. Option B, 45 g, belongs to A because A has nine parts. Option C is the total mass of the entire compound, not the mass of B alone. Option D is not produced by the given ratio. A consistency check gives A = 45 g and B = 25 g; their sum is 70 g, and their ratio 45:25 simplifies by 5 to 9:5. Therefore the selected answer satisfies the governing law and the numerical data.

Correct answer: B. 35 grams

The governing idea is the law of definite proportions: a pure compound contains its constituents in a fixed mass ratio. The ratio 1:7 means the whole sample has 1 + 7 = 8 equal mass parts. Since 8 parts weigh 40 g, one part weighs 40 ÷ 8 = 5 g. Element or constituent B occupies 7 parts, so its mass is 7 × 5 = 35 g. Therefore, option B is correct. Option A gives only one part, option C incorrectly treats the entire sample as B, and option D is greater than the stated total mass, so it cannot be possible. No molecular mass or mole conversion is required here; only the given mass ratio must be applied.

Correct answer: C. 40 grams

This problem uses the fixed-composition principle for a pure compound. The mass ratio A:B = 10:3 contains 10 + 3 = 13 total parts. The complete sample has a mass of 52 g, so the mass represented by one ratio part is 52 ÷ 13 = 4 g. Constituent A occupies 10 parts; therefore, its mass is 10 × 4 = 40 g. Hence option C is correct. Option A corresponds to 3 parts, which would represent B, while option B assigns the entire sample to A and ignores B. Option D is greater than the total mass and is physically impossible. The calculation demonstrates how a stated composition can be converted directly into the mass of one constituent without needing any additional chemical data.