Laws of chemical combination

Correct answer: B. The percentage composition by mass of a pure compound is always the same

A pure compound has a fixed composition by mass, so its percentage composition remains the same from sample to sample. This is an implication of the law of definite proportions.

Correct answer: B. 32:48 = 2:3

The greatest common divisor of 32 and 48 is 16. Dividing both terms by 16 gives 32 ÷ 16 = 2 and 48 ÷ 16 = 3, so the simplified ratio is 2:3. The ratio 3:2 reverses the order and is therefore incorrect. Exam tip: simplify a ratio by dividing both terms by the same greatest common divisor.

Correct answer: D. 24:40 = 5:3

Dividing both terms of 24:40 by 8 gives 3:5. Therefore, 5:3 is incorrect because the order of the terms has been reversed. In option C, dividing 18 and 30 by 6 correctly gives 3:5. Exam tip: simplify both terms by the same number and never change their order.

Correct answer: C. Richter and the law of reciprocal proportions

The correct historical association is Richter with the law of reciprocal proportions. This law states that if two elements combine separately with a fixed mass of a third element, the masses of the first two elements are related to each other in the same or a simple multiple of the ratio in which they combine directly. Therefore, option C is correct. Lavoisier is chiefly associated with conservation of mass, not definite proportions. Proust is associated with definite proportions, and Dalton is associated with multiple proportions. Gay-Lussac is known for the law of combining gaseous volumes, so option D also mismatches the scientist and law.

Correct answer: D. Gay-Lussac — law of definite proportions

Gay-Lussac is associated with the law of gaseous volumes, not the law of definite proportions. The latter is associated with Proust, so option D is the incorrectly matched pair.

Correct answer: C. The total mass of the reactants equals the total mass of the products

In a closed system, matter is not lost or gained during a chemical reaction. Therefore, the total mass of the reactants equals the total mass of the products.

Correct answer: A. Check the units of the given masses and then simplify the ratio

Checking that the quantities have compatible mass units and then simplifying the ratio helps prevent unit and arithmetic errors. Gases still have mass, and a mixture should not be treated as a pure compound.

Correct answer: B. 50 grams

According to the law of conservation of mass, in a closed vessel the total mass of reactants equals the total mass of products. Therefore, total mass = 15 g + 35 g = 50 g. The value 20 g is only the difference between the two masses, not the total mass of the products. Exam tip: When a reaction occurs in a closed vessel, add the masses of all reactants.

Correct answer: A. 38 grams

By the law of conservation of mass, the total mass of reactants equals the total mass of products. Therefore, mass of the second reactant = 84 − 46 = 38 grams. The value 46 grams is the mass of the known reactant, and 130 grams cannot be the combined reactant mass because the product mass is 84 grams. Exam tip: In such questions, subtract the known reactant mass from the total product mass.

Correct answer: B. 12 g

By conservation of mass, the total product mass is 72 g. The third product has mass 72 − 41 − 19 = 12 g.

Correct answer: C. The escaped gas was not included in the measurement

The measured mass is lower because the escaped gas is outside the measured system. If the gas were collected and included, the total mass would be conserved; the gas has not been destroyed.

Correct answer: C. Law of conservation of mass

The total mass is the same before and after the reaction: 125 g. This supports the law of conservation of mass, which states that mass is conserved in a chemical reaction.

Correct answer: A. Definite proportions

The direct answer is A: the law of definite proportions. This law says that a pure compound always contains the same elements combined in a fixed proportion by mass. Pure barium sulfate, written as \\(BaSO_4\\), contains barium, sulfur, and oxygen joined in a definite composition. A sample taken from any source will have the same mass relationship among these elements if it is pure barium sulfate. Option A is correct because the clue is “pure compound” and “always fixed.” Option B, the law of multiple proportions, applies when two elements form more than one compound and their masses combine in simple whole-number ratios; that situation is not described here. Option C, the law of gaseous volumes, deals with combining volumes of gases under suitable conditions. Option D, reciprocal proportions, compares the combining masses of elements in different compounds and is not the direct idea in this statement. The safest exam clue is fixed composition of one pure compound. Remember: one pure compound plus fixed element ratio means definite proportions.

Correct answer: C. Salty water

Salty water is a mixture, so the amount of salt relative to water can vary. Pure compounds such as CO, H₂S, and Na₂O have a definite composition.

Correct answer: B. 15 g

The ratio has 10 + 3 = 13 total parts. Each part is 65 ÷ 13 = 5 g, so B has mass 3 × 5 = 15 g.

Correct answer: C. 8 g

The 11 parts corresponding to A have a mass of 44 g, so one part is 44 ÷ 11 = 4 g. B is 2 parts, so its mass is 2 × 4 = 8 g.

Correct answer: B. 3:4

The ratio A:B is 42:56. The greatest common divisor of 42 and 56 is 14; dividing both terms by 14 gives 3:4. Therefore, the simplest ratio is 3:4. The ratio 4:3 reverses the order, so it is not correct for A:B. Exam tip: keep the order stated in the question and divide both terms by their greatest common divisor.

Correct answer: A. They show the same ratio

Dividing both terms of 20:25 by 5 gives 4:5. Thus, the mass ratio of the constituents is the same in both samples, consistent with the law of definite proportions for a pure compound. Option B is incorrect because the ratios are not different. Exam tip: reduce ratios to their simplest form before comparing them.

Correct answer: A. 5:7

The greatest common divisor of 15 and 21 is 3. Dividing both terms by 3 gives 15:21 = 5:7, so option A is correct. The ratio 7:5 reverses the terms, while 3:7 and 5:3 are not equivalent to the given ratio. Exam tip: simplify a ratio by dividing both terms by their greatest common divisor.

Correct answer: A. 7:3

The ratio A:B is 63:27. Dividing both terms by their greatest common divisor, 9, gives 7:3. The ratio 3:7 reverses the order, so it is incorrect. Exam tip: simplify both terms by the same number and keep the stated order unchanged.

Correct answer: B. 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 lead, the masses of oxygen in PbO and PbO₂ are in the ratio 1:2, a simple whole-number ratio.

Correct answer: B. 1:3

The mass of A is the same, 13 g, in both compounds. Therefore, the required ratio of the masses of B is 16:48. Dividing both terms by 16 gives 1:3. The ratio 3:1 is the reverse ratio, so it is not correct. Exam tip: when the mass of one element is fixed, reduce the masses of the other element to their simplest ratio.

Correct answer: A. 1:5

The mass of A is the same, 6 g, in both compounds, so compare the masses of B: 5:25. Dividing both terms by 5 gives 1:5. The ratio 5:1 is the reverse ratio, so it is incorrect. Exam tip: in multiple-proportion questions, compare the masses of one element that combine with a fixed mass of the other and reduce them to the simplest whole-number ratio.

Correct answer: C. Law of multiple proportions

For a fixed 8 g mass of A, the masses of B are 6 g and 18 g. Their ratio is 6:18 = 1:3, a simple whole-number ratio. Therefore, this illustrates the law of multiple proportions: 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. The law of definite proportions applies to the fixed composition of one particular compound. Exam tip: fixed mass plus two compounds and a simple ratio usually indicates the law of multiple proportions.

Correct answer: A. Cl₂O and Cl₂O₇

Cl₂O and Cl₂O₇ are different compounds made from the same two elements, chlorine and oxygen. For a fixed amount of chlorine, the oxygen amounts in the two formulas are in the ratio 1:7, a simple whole-number ratio.