Laws of chemical combination

Correct answer: B. 1:3

The mass ratio of B is 14:42. Dividing both terms by 14 gives 1:3. The same mass of A is specified in both compounds, so this compares the masses of B that combine with a fixed mass of A.

Correct answer: C. Law of multiple proportions

The governing idea is 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 a small whole-number ratio. Here the mass of A is fixed at 4 g, while the masses of B are 7 g and 28 g. Their ratio is 7:28, which simplifies by dividing both terms by 7 to 1:4, a simple whole-number ratio. Therefore, option C is correct. Definite proportions concerns the fixed composition of one pure compound, conservation of mass concerns total mass before and after a reaction, and Gay-Lussac’s law compares reacting gas volumes, not these element masses.

Correct answer: A. Methane (CH₄) and ethane (C₂H₆)

Methane and ethane are different compounds made from the same two elements, carbon and hydrogen. For a fixed mass of carbon, the masses of hydrogen that combine with it are in a simple whole-number ratio, as required by the law of multiple proportions. The other options are mixtures, not two compounds of the same elements.

Correct answer: C. Law of multiple proportions

SnO and SnO₂ are two compounds made from tin and oxygen. For the same mass of tin, the oxygen masses are in the ratio 1:2, a simple whole-number ratio. This is the law of multiple proportions.

Correct answer: A. 2:3

At the same temperature and pressure, the ratio of volumes of reacting gases is obtained directly from their given volumes. Simplifying 16:24 by dividing both terms by 8 gives 2:3. The ratio 3:2 is the reverse ratio, so it is incorrect. Exam tip: always reduce gas-volume ratios to the simplest whole-number ratio using the greatest common factor.

Correct answer: C. 3:1

In the given order, the ratio is 18:6. Dividing both terms by their greatest common factor, 6, gives 3:1. The ratio 1:3 represents the reverse order, so it is not correct. Exam tip: simplify both terms by the same number while retaining the order given in the question.

Correct answer: D. 1:3

The ratio of gas A to gas B is 7:21. Dividing both terms by their greatest common factor, 7, gives 1:3. Thus, for every 1 part of gas A, there are 3 parts of gas B. The ratio 3:1 represents the reverse order, so it is incorrect. Exam tip: reduce a ratio by dividing both terms by their greatest common factor.

Correct answer: C. 1:2:1

The given volume ratio of gas A : gas B : gas C is 4:8:4. Dividing every term by their greatest common factor, 4, gives 1:2:1. Although 4:8:4 represents the same ratio, it is not in the simplest form. Exam tip: reduce every term of a volume ratio by the same common factor to obtain the smallest whole-number ratio.

Correct answer: A. At the same temperature and pressure

The law compares the volumes of reacting gases, or gaseous products, measured at the same temperature and pressure. These conditions are necessary for a meaningful volume comparison.

Correct answer: C. Treating gram masses of solids as a litre ratio

Gay-Lussac’s law of combining volumes applies to gases that react or are formed in a chemical reaction, with volumes compared at the same temperature and pressure. Thus, gas volumes may be measured in litres and reduced to a simple whole-number ratio. A gram is a unit of mass, whereas a litre is a unit of volume; the mass of a solid cannot be treated as a gas-volume ratio merely by changing the wording. Therefore, option C is the incorrect use. Options A, B, and D describe appropriate ways of handling gaseous volumes, although the condition of constant temperature and pressure is essential for a valid comparison.

Correct answer: B. 28%

Because the compound contains only A and B, their mass percentages add up to 100%. Therefore, B is 100% − 72% = 28%.

Correct answer: A. 3:7

The percentages of A and B are 30 and 70 respectively, so A:B = 30:70. Dividing both terms by 10 gives the simplest ratio, 3:7. The ratio 7:3 reverses the order and represents B:A. Exam tip: write the percentages in the stated order and simplify the ratio using their common factor.

Correct answer: B. 25 percent

The direct answer is B: 25 percent. Percentage by mass is calculated as \\(\\text{mass of part}/\\text{total mass}\\) multiplied by 100. The mass of A is 24 grams and the total mass of the compound is 96 grams. Therefore, percentage of A = \\(24/96\\times100=25\\%\\). Another quick observation is that 24 is one-fourth of 96, and one-fourth of 100 percent is 25 percent. Option A, 20 percent, does not come from the correct fraction. Option B is correct because it uses the mass of A as the numerator and the total compound mass as the denominator. Option C, 40 percent, is too large and would represent 38.4 grams in a 96-gram sample. Option D, 75 percent, is the percentage of the remaining 72 grams, not A: \\(72/96\\times100=75\\%\\). The denominator must always be the total mass when finding mass percentage. Remember: part divided by whole, then multiply by 100.

Correct answer: C. 25%

Mass percentage of B = (mass of B ÷ mass of compound) × 100 = (30 g ÷ 120 g) × 100 = 25%.

Correct answer: A. In a closed system, the total mass of reactants and products, including any gas formed, remains the same

According to the law of conservation of mass, the total mass before and after a chemical reaction remains the same in a closed system. Any gas formed is a product and its mass must be included in the total. In an open vessel, the measured mass of the vessel may appear to decrease if gas escapes, but the law is not false; the gas has left the system. Exam tip: in reactions involving gases, first check whether the system is open or closed.

Correct answer: A. Different samples of the same pure compound give same composition

The direct answer is A: different samples of the same pure compound give the same composition. The law of definite proportions states that a particular pure compound always contains the same elements in the same fixed proportion by mass, regardless of where the sample comes from or how large it is. For example, every pure sample of water contains hydrogen and oxygen in a fixed mass ratio. Option A correctly describes testing different samples and observing the same composition. Option B is wrong because mixtures can have variable composition; their components may be present in different amounts. Option C is wrong because the amount of sugar in tea can change from cup to cup, and tea is generally a mixture. Option D is wrong because soil contains many substances and is not always a pure compound. The key words are “same pure compound,” “different samples,” and “same composition.” The law concerns fixed chemical composition, not appearance or everyday recipes. Remember: pure compound, same elements, fixed mass ratio.

Correct answer: A. When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in a simple whole-number ratio

The law of multiple proportions applies when the same two elements form different compounds. For a fixed mass of one element, the masses of the other that combine with it are in a simple whole-number ratio.

Correct answer: A. Volumes of reacting gases and gaseous products, measured at the same temperature and pressure, are in simple whole-number ratios

Gay-Lussac’s law of combining volumes states that reacting gases and gaseous products have volumes in simple whole-number ratios when measured at the same temperature and pressure.

Correct answer: A. Conservation of mass concerns the equality of total reactant and product masses in a chemical reaction; definite proportions concerns the fixed composition of a pure compound

Conservation of mass says that total mass is unchanged during a chemical reaction. The law of definite proportions says that a pure compound has the same elements in a fixed mass ratio, regardless of the sample.

Correct answer: A. The law of multiple proportions compares masses; Gay-Lussac’s law of combining volumes compares gas volumes at the same temperature and pressure

Multiple proportions compares the masses of elements that form different compounds. Gay-Lussac’s law of combining volumes compares the volumes of reacting gases and gaseous products measured at the same temperature and pressure.

Correct answer: A. The law of definite proportions concerns the fixed mass ratio of elements in a pure compound; the law of multiple proportions concerns simple whole-number ratios of masses in different compounds made from the same two elements

A pure compound always contains its elements in the same fixed mass ratio, which is the law of definite proportions. If the same two elements form different compounds, the masses that combine follow simple whole-number ratios, which is the law of multiple proportions.

Correct answer: A. Conservation of mass

The direct answer is option A: the law of conservation of mass. This law says that in a closed system the total mass of substances before a chemical reaction equals the total mass after it. Step 1: notice the key words “total reactant mass” and “total product mass.” Step 2: recognize that these compare mass before and after a reaction. Step 3: apply conservation of mass: total reactant mass = total product mass. Option A exactly names this law. Option B, definite proportions, says that a pure compound always contains its elements in a fixed mass ratio; it does not mainly compare total reactant and product masses. Option C, multiple proportions, compares different compounds made from the same elements. Option D, reciprocal proportions, concerns relationships among combining masses of elements in different compounds. Thus A is the correct clue-based answer. Memory cue: total mass before and after means conservation of mass.

Correct answer: A. Law of definite proportions

The law of definite proportions states that a pure compound contains the same elements in a fixed ratio by mass, whatever its source or method of preparation. Therefore, its percentage composition by mass is constant.

Correct answer: C. Law of multiple proportions

The law of multiple proportions applies when the same two elements form different compounds. For a fixed mass of one element, the masses of the other element in those compounds are in simple whole-number ratios.

Correct answer: C. Law of gaseous volumes

Gay-Lussac’s law of gaseous volumes states that, at the same temperature and pressure, the volumes of reacting gases and gaseous products are in simple whole-number ratios.