Laws of chemical combination

Correct answer: C. 25 grams

This is a direct application of conservation of mass, one of the basic laws of chemical combination. The total mass before the reaction must equal the total mass after the reaction when the reaction is considered in a closed system. Let the second reactant weigh x grams. Then x + 16 = 41. Solving gives x = 41 − 16 = 25 grams. Hence, option C is correct. Option A confuses the known reactant with the unknown one, option B states the total product rather than the missing quantity, and option D is the result of an incorrect subtraction. A quick check confirms the answer because 25 + 16 = 41 grams.

Correct answer: D. 20 grams

The ratio A:B is 2:5, so the total number of parts is 2 + 5 = 7. One part has mass 28/7 = 4 g. Therefore, the mass of B, representing 5 parts, is 5 × 4 = 20 g. Choosing 16 g would represent only 4 parts for B, which does not match the given ratio. Exam tip: In ratio questions, first add the ratio parts and find the value of one part.

Correct answer: A. 20 grams

The mass ratio A:B is 3:4, so the total number of parts is 3 + 4 = 7. Each part has mass 35 ÷ 7 = 5 g. Since B has 4 parts, its mass is 4 × 5 = 20 g. The value 15 g is the mass of A, not B. Exam tip: first add the ratio parts, then divide the total mass by their sum.

Correct answer: B. 28 grams

The relevant concept is the law of definite proportions: a pure compound always contains its elements in a fixed mass ratio. Here A:B = 5:7, so the total number of ratio parts is 5 + 7 = 12. The mass represented by one part is 48 g ÷ 12 = 4 g. Element B occupies 7 parts, so its mass is 7 × 4 g = 28 g. Therefore, option B is correct. Option A corresponds to the mass of A, option C incorrectly assigns the entire compound mass to B, and option D does not follow the stated ratio. The two masses also check: A = 20 g and B = 28 g, giving 48 g in total and a ratio of 5:7.

Correct answer: C. 36 grams

The governing principle is the law of definite proportions, which says that elements in a pure compound occur in a fixed mass ratio. The ratio A:B is 4:9, giving 4 + 9 = 13 total parts. Since the compound weighs 52 g, one ratio part weighs 52 ÷ 13 = 4 g. B represents 9 parts, so its mass is 9 × 4 = 36 g. Thus, option C is correct. Option A is the mass of A, not B. Option B is the total mass of the compound, and option D would correspond to treating the ratio incorrectly. The complementary mass of A is 52 − 36 = 16 g, which confirms both the total and the 4:9 ratio.

Correct answer: D. 15 grams

This problem uses the law of definite proportions, according to which a pure compound has a constant composition by mass. The stated ratio A:B = 7:3 contains 7 + 3 = 10 equal ratio parts. The total mass is 50 g, so each part represents 50 ÷ 10 = 5 g. B occupies 3 parts; therefore, its mass is 3 × 5 = 15 g. Option D is correct. Option A is the mass of A because 7 × 5 = 35 g. Option B is the total mass and cannot be the mass of only B, while option C does not result from the given ratio. Adding 35 g and 15 g gives the stated total of 50 g.

Correct answer: A. 44 grams

The governing concept is the law of definite proportions: the elements of a pure compound are present in a fixed ratio by mass. Here the ratio A:B is 6:11, so the compound contains 6 + 11 = 17 ratio parts. The mass of one part is 68 g ÷ 17 = 4 g. Since B accounts for 11 parts, its mass is 11 × 4 = 44 g. Therefore, option A is correct. Option B corresponds to A because 6 × 4 = 24 g. Option C is the complete compound mass, not B alone, and option D is inconsistent with the ratio. The check 24 g + 44 g = 68 g confirms the result.

Correct answer: B. 40 grams

For A:B = 1:8, the total number of parts is 1 + 8 = 9. One part has a mass of 45/9 = 5 g. Therefore, the mass of B, which has 8 parts, is 8 × 5 = 40 g. The value 45 g is the total mass, not the mass of B alone. Exam tip: first add the ratio parts, then find the value of one part.

Correct answer: C. 24 grams

The relevant governing concept is the law of definite proportions. It states that a pure compound contains its constituent elements in a fixed mass ratio. For A:B = 5:6, the total number of ratio parts is 5 + 6 = 11. The total mass is 44 g, so one part has mass 44 ÷ 11 = 4 g. B contributes 6 parts, giving 6 × 4 = 24 g. Hence, option C is correct. Option A is the mass of A, option B is the total compound mass, and option D is not obtained from the ratio. As a check, A weighs 5 × 4 = 20 g; 20 + 24 = 44 g and 20:24 simplifies to 5:6.

Correct answer: D. 40 grams

This question is governed by the law of definite proportions, which requires the masses of elements in a pure compound to remain in a fixed ratio. The ratio A:B = 3:8 has 3 + 8 = 11 total parts. Because the total compound mass is 55 g, one part weighs 55 ÷ 11 = 5 g. B occupies 8 parts, so its mass is 8 × 5 = 40 g. Thus, option D is correct. Option A gives the mass of A, since 3 × 5 = 15 g. Option B gives the entire compound mass, and option C does not satisfy the stated ratio. The check is 15 g + 40 g = 55 g and 15:40 simplifies to 3:8.

Correct answer: A. 33 grams

The governing principle is the law of definite proportions, stating that a pure compound has a constant mass composition. The ratio A:B is 9:11, so the total number of equal ratio parts is 9 + 11 = 20. The mass per part is 60 g ÷ 20 = 3 g. Since B represents 11 parts, its mass is 11 × 3 = 33 g. Therefore, option A is correct. Option B corresponds to A because 9 × 3 = 27 g. Option C is the total mass of the compound, not the mass of B, and option D does not agree with the given ratio. The values 27 g and 33 g add to 60 g and have the required ratio 9:11.

Correct answer: B. 1:3

The masses of B are 8 g and 24 g. Dividing both terms by 8 gives the ratio 1:3. Hence, 1:3 is correct. The ratio 3:1 is the reverse order, whereas the masses are given as 8:24 in the question. Exam tip: always reduce a ratio to the smallest whole numbers.

Correct answer: C. 2:3

The masses of B are 12 g and 18 g. Dividing both terms by their greatest common divisor, 6, gives \(12:18 = 2:3\). Therefore, option C is correct. \(3:2\) is the reverse ratio, so it is not correct. Exam tip: reduce a ratio to its simplest form by dividing both terms by their greatest common divisor.

Correct answer: D. 1:3

The masses of B are 9 g and 27 g. Dividing both terms by 9 gives the ratio 1:3, so option D is correct. The ratio 3:1 reverses the order, whereas the masses are given in the order 9:27. Exam tip: simplify a ratio by dividing both terms by their greatest common factor.

Correct answer: A. 1:2

The mass of A is fixed at 12 g in both compounds, so compare the masses of B: \(16:32=1:2\). Therefore, the correct ratio is 1:2. The ratio 2:1 is the reverse order, whereas the masses are given in the order 16:32. Exam tip: when the mass of one element is fixed, reduce the masses of the other element to the smallest whole-number ratio.

Correct answer: B. 2:3

The mass of A is fixed at 5 g in both compounds, so compare the masses of B: \(14:21\). Dividing both terms by 7 gives \(2:3\). Although \(14:21\) represents the given masses, it is not in simplest form. Exam tip: reduce a ratio by dividing both terms by their highest common factor.

Correct answer: C. 2:5

The masses of B are 10 g and 25 g. Dividing both by their greatest common divisor, 5, gives \(10:25 = 2:5\). Therefore, option C is correct. Option A, \(5:2\), is the reverse of the required ratio. Exam tip: always reduce a ratio to its simplest whole-number form.

Correct answer: D. 1:2

Since the mass of A is fixed at 3 g in both compounds, compare the masses of B: 11 g and 22 g. Their ratio is 11:22, which simplifies to 1:2 by dividing both terms by 11. Although 11:22 is the original ratio, it is not in simplest form. Exam tip: when one element has a fixed mass, form the ratio of the masses of the other element and simplify it.

Correct answer: A. 1:3

For the fixed 9 g mass of A, the masses of B are 15 g and 45 g. Therefore, their ratio is 15:45 = 1:3, obtained by dividing both terms by 15. The ratio 3:1 is the reverse order, so it is incorrect. Exam tip: reduce a mass ratio by dividing both terms by their greatest common factor.

Correct answer: B. 1:2:1

Dividing all volumes by the common greatest factor gives 1:2:1. Exam tip: simplify gas volume ratios.

Correct answer: C. 2:3:1

Dividing all volumes by the common greatest factor gives 2:3:1. Exam tip: simplify gas volume ratios.

Correct answer: D. 2:3:1

Dividing all volumes by the common greatest factor gives 2:3:1. Exam tip: simplify gas volume ratios.

Correct answer: A. 2:1:3

Dividing all volumes by the common greatest factor gives 2:1:3. Exam tip: simplify gas volume ratios.

Correct answer: B. 1:2:3

Dividing all volumes by the common greatest factor gives 1:2:3. Exam tip: simplify gas volume ratios.

Correct answer: C. 2:3:1

Dividing all volumes by the common greatest factor gives 2:3:1. Exam tip: simplify gas volume ratios.