Laws of chemical combination

Correct answer: A. Law of definite proportions

Definite proportions applies to pure compounds so impurity can create confusion. Exam tip: look for pure sample.

Correct answer: A. Gay-Lussac's law of gaseous volumes

Gay-Lussac’s law of gaseous volumes compares the volumes of reacting gases, or gaseous products, when temperature and pressure are kept the same. Under these common conditions, the volumes show simple whole-number relationships. The phrase “same conditions” is the important clue in the question. It prevents an unfair comparison in which temperature or pressure changes the volume independently of the chemical reaction.

Thus option A is correct. The law of definite proportions concerns the fixed mass composition of a compound, and the law of multiple proportions compares mass ratios in different compounds formed by the same elements. Neither is specifically a law about reacting gas volumes under equal conditions. The supplied explanation is accurate, although the more complete statement is that both temperature and pressure must be constant, not merely generally similar.

Correct answer: C. Air

Air is a physical mixture of several gases such as nitrogen, oxygen, argon, carbon dioxide, and water vapour, so it is the correct answer. \(NaCl\), \(H_2O\), and \(CO\) are pure compounds with fixed compositions, not mixtures. Exam tip: a substance whose composition can vary is generally a mixture.

Correct answer: A. Sugar water

The direct answer is A, sugar water. The law of definite proportions says that a pure chemical compound always contains the same elements in the same fixed mass ratio, regardless of where or how much of it is taken. Sugar water is not one new compound; it is a mixture or solution. We can put a little sugar or a lot of sugar into water, so its composition changes. Therefore it cannot obey one fixed ratio. Option A is correct because sugar water has variable composition. Option B, pure HCl, is a compound made of hydrogen and chlorine in a fixed composition, so it is suitable. Option C, pure MgO, contains magnesium and oxygen in a fixed chemical ratio, so it is suitable. Option D, pure CaO, contains calcium and oxygen in a fixed ratio, so it is also suitable. A common mistake is to call every clear liquid a compound. In this question, the word pure and the fixed formula are important clues. Remember: mixture can vary, but a pure compound has a definite composition.

Correct answer: A. Two identical samples of pure water

Two samples of the same compound are not suitable for multiple proportions. Exam tip: different compounds are necessary.

Correct answer: C. It gives masses of solids in litres

This law is about gas volumes not masses of solids. Exam tip: read negative questions carefully.

Correct answer: C. Mass is destroyed in an ordinary reaction

Mass is not destroyed in an ordinary chemical reaction. Exam tip: watch for the word destroyed in wrong options.

Correct answer: A. Masses occur in simple whole number ratios in different compounds of two elements

Multiple proportions is based on simple whole number mass ratios. Exam tip: identify simple whole number ratio.

Correct answer: A. The mass ratio of elements in a pure compound is fixed

The governing concept is Proust’s law of definite proportions. It states that a particular pure compound always contains the same elements combined in the same fixed ratio by mass, regardless of where the sample comes from or how it is prepared. For example, pure water contains hydrogen and oxygen in a mass ratio of 1:8. Therefore option A is correct because it refers specifically to a pure compound and a fixed mass ratio. A mixture, in contrast, can have variable composition, so option B is false. Elements need not be present in equal amounts, making option C false. Option D is unrelated and scientifically incorrect because gases occupy volume. The words “pure compound” and “fixed mass ratio” identify the correct principle.

Correct answer: A. Law of conservation of mass

Equal total mass before and after is conservation of mass. Exam tip: look for before-after mass.

Correct answer: A. Law of definite proportions

Samples of the same pure compound have the same composition. Exam tip: look for same compound samples clue.

Correct answer: A. Law of multiple proportions

Multiple proportions compares masses of the same two elements in different compounds. Exam tip: look for different compounds.

Correct answer: A. Conservation of mass

The law is checked by weighing total mass of reactants and products. Exam tip: look for balance and mass clue.

Correct answer: A. Definite proportions

Percentage composition is a form of definite proportions. Exam tip: treat percentage composition as mass ratio.

Correct answer: A. Confusion between mixture and pure compound

Mixtures have variable composition while pure compounds have fixed composition. Exam tip: identify category first.

Correct answer: A. Identify the unit and type of substance

The unit tells mass or volume and the substance tells compound or mixture. Exam tip: catch clues first.

Correct answer: B. 25 grams

Total reactant mass is 10 plus 15 equal to 25 grams. Exam tip: keep total mass equal in a closed vessel.

Correct answer: C. 16 grams

The governing concept is conservation of mass in a chemical reaction. Assuming the reaction occurs in a closed system and there are only two reactants and one product, the sum of the reactant masses must equal the product mass. Let the unknown reactant mass be x. Then 12 g + x = 28 g, so x = 28 − 12 = 16 g. Therefore option C is correct. Option A incorrectly repeats the known mass, option B results from an incorrect subtraction, and option D treats the entire product mass as the unknown reactant while ignoring the 12 g already present. The calculation is straightforward, but the stated assumptions matter: no material should escape and no additional product or reactant should be omitted.

Correct answer: C. 16 grams

The direct answer is option C: 16 grams. In a closed system, matter is not created or destroyed during an ordinary chemical reaction. Therefore, when the reactants are completely considered, their total mass equals the total mass of the products. Step 1: mass of the first reactant = 7 g. Step 2: mass of the second reactant = 9 g. Step 3: total reactant mass = 7 + 9 = 16 g. Thus, product mass = 16 g. Option A, 2 g, is the difference between the two masses, not their total. Option B, 9 g, counts only one reactant. Option C correctly applies the law of conservation of mass. Option D, 18 g, results from an incorrect addition. Memory cue: in a closed system, total mass before reaction equals total mass after reaction.

Correct answer: B. 16 grams

According to the law of conservation of mass, the total mass remains unchanged in a chemical reaction. Therefore, mass of gas = 40 g − 24 g = 16 g. The 24 g value is the mass of the solid product, not of the gas. Exam tip: the combined masses of all products must equal the total mass of the reactants.

Correct answer: C. 30 grams

Total reactant mass is 30 grams. Exam tip: number of products does not matter check total mass.

Correct answer: C. 32 grams

The total mass of the products is 50 grams, and product A accounts for 18 grams of that total. If the question assumes that the products listed are A and B, the mass of B is found by removing A’s mass from the total. In symbols, product B mass is \\(50-18=32\\) grams. This uses conservation of total mass within the stated two-product calculation.

Therefore option C, 32 grams, is correct. Adding 18 grams and 32 grams gives the stated total of 50 grams, which checks the result. The values 28 and 30 do not complete the total, while 68 grams would be larger than the total and cannot be the remaining amount. The supplied answer is consistent. The calculation depends on the natural reading that “total products” means only A plus B; under that intended school-MCQ reading, no revision is needed.

Correct answer: B. 6 grams

The governing concept is conservation of mass. If the reaction is treated as a closed system and all products are included, the total mass of the products must equal the total mass of the reactants, which is 36 g. The first two products together have mass 13 g + 17 g = 30 g. If the third product has mass x, then 30 g + x = 36 g, giving x = 36 − 30 = 6 g. Thus option B is correct. Option A subtracts incorrectly, option C may result from using only one given product, and option D does not satisfy the total. Checking the final sum confirms the answer: 13 + 17 + 6 = 36 g, exactly matching the reactant mass.

Correct answer: C. Law of multiple proportions

This is the law of multiple proportions. In CO and CO₂, oxygen masses combining with the same carbon mass are 16 g and 32 g; 16:32 = 1:2. Exam tip: look for the phrase “more than one compound.”

Correct answer: C. 15 grams

The mass ratio A:B is 2:5. If 2 parts of A have a mass of 6 g, then 1 part = 3 g. Therefore, the mass of 5 parts of B is 5 × 3 = 15 g. For example, 12 g would give the ratio 6:12 = 1:2, not 2:5. Exam tip: multiply both terms of a ratio by the same factor.