Laws of chemical combination

Correct answer: A. 12 grams

The governing concept is the fixed mass ratio in a pure compound. The ratio A:B = 3:4 contains 3 + 4 = 7 total parts. Since the complete compound has a mass of 28 g, one part weighs 28 ÷ 7 = 4 g. Substance A occupies three parts, so its mass is 3 × 4 = 12 g. Therefore option A is correct. Option B treats the two substances as equal or divides the total incorrectly, option C corresponds to four parts and is actually the mass of B, and option D does not follow the stated ratio. A useful check is that the two calculated masses are A = 12 g and B = 16 g; their sum is 28 g and their ratio is 12:16 = 3:4.

Correct answer: C. 15 grams

7 parts of B are 21 grams so 1 part is 3 grams and 5 parts of A are 15 grams. Exam tip: use parts method.

Correct answer: C. 24 grams

The governing concept is the law of definite proportions: in pure water, hydrogen and oxygen always combine in a fixed mass ratio of 1:8. The question gives 3 g of hydrogen, which is three times the 1-part hydrogen amount. The oxygen required must therefore also be scaled by three: 8 × 3 = 24 g. Hence option C is correct. Option A would be correct only for 1 g of hydrogen, option B does not preserve the given ratio, and option D has no basis in the 1:8 relationship. The calculation can also be written as H:O = 1:8, so 3:O = 1:8; cross-multiplication gives O = 3 × 8 = 24 g. The resulting total water mass would be 27 g.

Correct answer: A. 5 grams

Total 5 parts are 25 grams so 1 part is 5 grams and B is 5 grams. Exam tip: use total parts method.

Correct answer: A. 2:3

The mass ratio of A to B is 16:24. Dividing both terms by their greatest common factor, 8, gives 2:3. The ratio 3:2 reverses the order, while 4:3 and 8:3 are not obtained by dividing both terms by the same number. Exam tip: simplify a ratio by dividing each term by the greatest common factor.

Correct answer: A. Law of conservation of mass

The total mass of products is 5.6 + 4.4 = 10.0 g, equal to the mass of the reactant. In a closed vessel, mass is neither created nor destroyed. Definite proportions concerns composition, not total mass. Exam tip: add all product masses first.

Correct answer: B. 15 grams

7 parts of A are 35 grams so 1 part is 5 grams and B is 15 grams. Exam tip: calculate parts carefully.

Correct answer: A. 1:2

The mass of A is fixed at 6 g in both compounds, so compare the masses of B: 8:16 = 1:2. Therefore, the correct option is 1:2. The ratio 2:1 is the reverse order and is incorrect. Exam tip: identify the element with fixed mass, then simplify the ratio of the masses of the other element.

Correct answer: A. 1:2

In the first compound, 12 g of B combines with 10 g of A, while in the second, 36 g of B combines with the same 10 g of A. Thus, the ratio of the masses of B is 12:36. Dividing both terms by 12 gives 1:3. Therefore, among the given options, 1:3 should be option B; the original answer label is inconsistent. Exam tip: in multiple-proportion questions, when the mass of one element is fixed, reduce the masses of the other element to the smallest whole-number ratio.

Correct answer: B. Multiple proportions

Masses of B are 15:30 or 1:2. Exam tip: think multiple proportions when two compounds are given.

Correct answer: B. 1:3

The mass of element A is the same in both compounds, i.e. 4 g. Therefore, the ratio of the masses of B is 9:27. Dividing both terms by 9 gives 1:3. The ratio 1:2 is incorrect because 27 g is three times 9 g, not twice. Exam tip: identify the element with fixed mass and reduce the masses of the other element to their simplest ratio.

Correct answer: A. 1:2

For the fixed 2 g of A, the masses of B are 5 g and 10 g. Their ratio is 5:10, which simplifies to 1:2 on dividing both terms by 5. Option 2:1 is the reverse ratio, so it is incorrect. Exam tip: always reduce a ratio by dividing both terms by their greatest common factor.

Correct answer: B. Multiple proportions

Sulfur and oxygen form two different compounds. Exam tip: look for same two elements different compounds.

Correct answer: A. Law of definite proportions

In pure water, the mass ratio H:O is always 1:8, regardless of its source. This is the law of definite proportions; conservation of mass concerns total mass in a reaction. Exam tip: “constant composition” points to this law.

Correct answer: B. 1:3

The volume ratio of gas A to gas B is 5:15. Dividing both terms by 5 gives 1:3, so option B is correct. The ratio 3:1 is the reverse order. Exam tip: simplify gas-volume ratios by dividing both quantities by their highest common factor.

Correct answer: A. In a pure compound, the constituent elements are always present in a fixed ratio by mass.

The law states that a pure compound has a constant composition by mass; for example, water always contains hydrogen and oxygen in the mass ratio 1:8. Option B is incorrect even under conservation of mass. Exam tip: look for “fixed mass ratio.”

Correct answer: A. 1:2:1

The given volumes are gas A : gas B : gas C = 3:6:3. Dividing every term by their common factor, 3, gives the simplest ratio 1:2:1. The ratio 1:1:2 is incorrect because the volume of gas B is twice that of gas A. Exam tip: retain the given order of reactants and product while writing a ratio.

Correct answer: A. 2:1

The volume ratio of gas A to gas B is 8:4. Dividing both terms by 4 gives the simplest ratio, 2:1. The ratio 1:2 represents the reverse order, so it is not correct. Exam tip: retain the order of substances stated in the question when writing a ratio.

Correct answer: B. 2:1

The volume ratio of gas A to the gaseous product is 12:6. Dividing both terms by their greatest common factor, 6, gives 2:1. The ratio 1:2 is the reverse order, so it is not correct. Exam tip: simplify a ratio by dividing both terms by the same common factor.

Correct answer: A. 2:3

The mass percentages of A and B are 40 and 60 respectively, so their ratio is 40:60. Dividing both terms by 20 gives 2:3. The ratio 3:2 is the reverse ratio, so it is incorrect. Exam tip: write the percentages directly as a ratio and simplify it using a common factor.

Correct answer: B. 1:3

The ratio A:B is 25:75. Dividing both terms by their highest common factor, 25, gives 1:3. In 1:2, B would be only twice A, whereas here B is three times A. Exam tip: write the percentages as a ratio and divide both terms by their HCF to obtain the simplest form.

Correct answer: B. 30 percent

In a pure compound containing only A and B, their percentages add up to 100%. Therefore, percentage of B = 100 - 70 = 30%. The 70% option is the given percentage of A, not that of B. Exam tip: in percentage-composition questions, begin by taking the total composition as 100%.

Correct answer: B. 3:2

The ratio of the percentages of A and B is 60:40. Dividing both terms by their greatest common factor, 20, gives 3:2. The ratio 2:1 would imply 30% B rather than 40%, so it is incorrect. Exam tip: simplify the ratio in the same order as given, A:B.

Correct answer: A. Yes, because 12:18 simplifies to 2:3

Yes. Dividing both terms of 12:18 by 6 gives 2:3. Thus, the mass ratio of the components is the same in both samples, which agrees with the law of definite proportions. Different total amounts do not violate the law; the simplest ratio must remain the same. Exam tip: reduce ratios to their simplest form before comparing them.

Correct answer: A. Both ratios are equivalent to 2:3

Dividing 6:9 by 3 gives 2:3, and dividing 8:12 by 4 also gives 2:3. Therefore, the components are present in the same ratio in both pure samples. Option B incorrectly reverses the second ratio. Exam tip: reduce ratios to their simplest form before comparing them.