Laws of chemical combination

This question applies the law of conservation of mass to a closed reaction system. Since the vessel is closed, no reactant or product can escape, so the total mass of all products must equal the total mass of the reactants. Add the given masses: 8 g oxygen + 1 g hydrogen = 9 g. Thus, the total product mass is 9 g and option C is correct. The question asks for total mass, not the mass of oxygen, hydrogen, or a particular product. The chemical identity of the product is not needed for this calculation. Options A and B result from omitting one part of the reactant mass, whereas option D incorrectly adds an extra gram. The closed-vessel condition is essential because it makes the measured system complete.

By conservation of mass, the total product mass equals the total reactant mass: 12 g + 32 g = 44 g.

The total reactant mass is 15 g + 5 g = 20 g. In a closed system, conservation of mass means the products also have a total mass of 20 g.

In a closed system, the total reactant mass equals the product mass. The other reactant therefore has a mass of 18 g − 6 g = 12 g.

In the 2:3 ratio, 2 parts correspond to 4 g, so 1 part is 2 g. Thus, B has a mass of 3 × 2 g = 6 g.

The ratio 1:8 means the oxygen mass is 8 times the hydrogen mass. Thus, 2 g × 8 = 16 g of oxygen.

A:B = 5:2, so the total number of parts is 5 + 2 = 7. One part has mass 14/7 = 2 g. Therefore, the mass of A is 5 × 2 = 10 g. 7 g is half of the total mass, not the mass represented by 5 parts of the ratio. Exam tip: In ratio questions, first add the parts and find the value of one part.

The governing idea is the law of definite proportions expressed through a fixed mass ratio. The ratio 3:4 contains 3 + 4 = 7 equal parts. Since the total mass is 21 g, one part has a mass of 21 ÷ 7 = 3 g. The second element occupies four parts, so its mass is 4 × 3 = 12 g. Therefore, option B is correct. The first element would have 3 × 3 = 9 g, and 9 + 12 = 21 g, which checks the result. Option A is the mass of the first element, while 14 g and 16 g do not satisfy the stated 3:4 division of the total mass. The ratio must be applied to the total rather than treated as a direct gram value.

The law of definite proportions states that a given pure compound always contains its elements in the same fixed mass ratio. The matching 1:2 ratios illustrate this law.

By the law of definite proportions, a pure compound has the same composition by mass in every sample. Therefore, the A:B mass ratio remains 4:1.

The mass ratio A:B is 6:9. Dividing both terms by their greatest common divisor, 3, gives 2:3. Although 6:9 represents the original ratio, it is not in simplest form; 3:2 reverses the order. Exam tip: simplify a ratio by dividing both terms by their greatest common divisor.

The mass ratio of metal to non-metal is 10:5. Dividing both terms by their greatest common factor, 5, gives 2:1, so option A is correct. The ratio 5:10 reverses the order and represents 1:2. Exam tip: reduce a ratio to its simplest form by dividing both terms by their greatest common factor.

The mass of the other element is the same, 12 g, in both compounds. Therefore, the oxygen-mass ratio is directly 16:32. Dividing both terms by 16 gives 1:2. Option 1:1 is incorrect because the masses of oxygen in the two compounds are not equal. Exam tip: before comparing masses, check whether the mass of the other element is kept fixed.

For a fixed mass of one element (5 g), the masses of the other element are 10 g and 20 g. Their ratio is 10:20 = 1:2, a simple whole-number ratio, illustrating the law of multiple proportions.

The mass of carbon is fixed at 3 g in both compounds, so compare the masses of oxygen: \(4:8=1:2\). Therefore, the correct ratio is 1:2. The ratio 2:1 is simply the reverse order. Exam tip: when one element has a fixed mass, reduce the masses of the other element to the simplest whole-number ratio.

The ratio of the given masses is 9:27. Dividing both terms by their greatest common factor, 9, gives 1:3. The ratio 3:1 is the reverse order, so it is not correct for 9 g : 27 g. Exam tip: retain the given order and reduce both terms by the greatest common factor.

The correct answer is B, the law of gaseous volumes (Gay-Lussac’s law). It states that, at the same temperature and pressure, the volumes of reacting gases and gaseous products are in simple whole-number ratios; here the reacting-gas ratio is 2:1.

The correct answer is A, the law of gaseous volumes. At the same temperature and pressure, the volumes of hydrogen, oxygen and water vapour are 2 L, 1 L and 2 L, giving the simple whole-number ratio 2:1:2.

The relevant principle is Gay-Lussac’s law of combining volumes, which states that reacting gas volumes, measured at the same temperature and pressure, are in simple whole-number ratios. The question already gives 1 volume of nitrogen and 3 volumes of hydrogen. Therefore, writing nitrogen first and hydrogen second, their ratio is 1:3, so option A is correct. The condition of equal temperature and pressure matters because gas volume changes when either condition changes. Option B reverses the order, option C incorrectly treats the two volumes as equal, and option D combines numbers in a way that does not represent the stated ratio. This is a volume comparison, not a mass or molecular-mass calculation.

The volume ratio of gas A to gas B is 4:2. Dividing both terms by their greatest common factor, 2, gives the simplest ratio, 2:1. The ratio 1:2 reverses the order of the gases, so it is incorrect. Exam tip: always retain the order of substances given in the question while writing a ratio.

The correct answer is B, the law of conservation of mass. In a closed system, the total mass remains unchanged during a chemical reaction; here both totals are 30 g.

To find the total mass of products, add the masses of both the solid and gaseous products: 18 grams + 7 grams = 25 grams. Therefore, 25 grams is correct. The 18 grams refers only to the solid product, not to all products. Exam tip: in conservation-of-mass questions, include the mass of every product.

The correct answer is C, 12 g. By conservation of mass, the total product mass is 40 g. Gas mass = 40 g − 28 g = 12 g.

The ratio A:B is 7:21. Dividing both terms by their highest common factor, 7, gives 1:3. Although 7:21 represents the same ratio, it is not in simplest form. Exam tip: simplify a ratio by dividing both terms by their HCF.

The ratio A:B is 9:6. Dividing both terms by their greatest common factor, 3, gives 3:2, so option B is correct. Although 9:6 is equivalent, it is not in simplest form. In exams, keep the order A:B unchanged.