1.5 Laws of Chemical Combinations The combination of elements to form compounds is governed by the following five basic laws. 1.5.1 Law of Conservation of Mass This law was put forth by Antoine Lavoisier in 1789. He performed careful experimental studies for combustion reactions and reached to the conclusion that in all physical and chemical changes, there is no net change in mass duting the process. Hence, he reached to the conclusion that matter can neither be created nor destroyed. This is called ‘Law of Conservation of Mass’. This law formed the basis for several later developments in chemistry. Infact, this was the result of exact measurement of masses of reactants and products, and carefully planned experiments performed by Lavoisier.
-- NCERT Class 11 Chemistry, Ch. 1, p. 14Laws Chemical Combination
Laws Chemical Combination, explained for NEET
The laws of chemical combination are the experimental foundation on which modern chemistry rests. NEET questions on this topic test whether you can state, distinguish, and apply these laws — and the common confusion is mixing up which law applies to which observation.
Law of Conservation of Mass (Lavoisier, 1789). In a chemical reaction, total mass of reactants equals total mass of products. No mass is created or destroyed. NCERT Class 11 Chemistry Chapter 1, page 14 treats this as the first law. The trap: students forget this applies to the entire closed system — if gas escapes an open vessel, the apparent mass change does not violate the law.
Law of Definite Proportions (Proust, 1799). A given compound always contains the same elements in the same ratio by mass, regardless of source or method of preparation. Water from a river or a lab synthesis is always 1:8 by mass (H:O). Do not confuse this with the law of multiple proportions — definite proportions is about ONE compound, not two.
Law of Multiple Proportions (Dalton, 1803). 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 ratio of small whole numbers. Example: CO and CO₂ — for a fixed mass of carbon, the oxygen masses are in the ratio 1:2. NCERT Class 11 Chemistry Chapter 1, page 4.
Gay-Lussac's Law of Gaseous Volumes (1808). Gases react in simple ratios by volume (at the same T and P). Two volumes of hydrogen react with one volume of oxygen to produce two volumes of water vapour.
The high-frequency confusion in NEET: misidentifying which law a given experimental observation illustrates. When a stem describes two different compounds of the same two elements, the answer is multiple proportions — not definite proportions. Read the stem for "one compound, fixed ratio" versus "two compounds, ratio of ratios."
Can you answer these Laws Chemical Combination MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which law states that in a chemical reaction, the total mass of reactants equals the total mass of products?
Show answer and why every option is right or wrong
Answer: C. The Law of Conservation of Mass (Lavoisier) states that mass is neither created nor destroyed in a chemical reaction. NCERT Class 11 Chemistry Chapter 1, page 14.
Why A is wrong: A refers to a compound always having the same elemental mass ratio — it does not address total mass balance in a reaction.
Why B is wrong: B deals with the ratio of masses when two elements form multiple compounds — not mass conservation.
Why D is wrong: D describes volume ratios of reacting gases — not mass conservation.
The Law of Definite Proportions was proposed by:
Show answer and why every option is right or wrong
Answer: C. Joseph Proust proposed the Law of Definite (or Constant) Proportions. NCERT Class 11 Chemistry Chapter 1, page 4.
Why A is wrong: Dalton proposed the Law of Multiple Proportions and atomic theory — not the law of definite proportions.
Why B is wrong: Lavoisier proposed the Law of Conservation of Mass.
Why D is wrong: Gay-Lussac proposed the Law of Gaseous Volumes.
Gay-Lussac's Law of Gaseous Volumes states that gases react in:
Show answer and why every option is right or wrong
Answer: A. Gay-Lussac observed that gases combine in simple whole-number ratios by volume when measured at the same temperature and pressure. NCERT Class 11 Chemistry Chapter 1, page 15.
Why B is wrong: The law specifically refers to volume ratios, not mass ratios.
Why C is wrong: The ratios are simple (small whole numbers), not complex, and the condition is constant T and P.
Why D is wrong: The law is stated in terms of volumes of gases, not number of atoms.
Carbon forms two oxides: CO and CO₂. In CO, 12 g of carbon combines with 16 g of oxygen. In CO₂, 12 g of carbon combines with 32 g of oxygen. The ratio of oxygen masses combining with a fixed mass of carbon is 16:32 = 1:2. This observation illustrates:
Show answer and why every option is right or wrong
Answer: B. When two elements form more than one compound, the different masses of one element combining with a fixed mass of the other are in a ratio of small whole numbers. This is the Law of Multiple Proportions (Dalton). NCERT Class 11 Chemistry Chapter 1, page 4.
Why A is wrong: Conservation of mass addresses total mass before and after reaction — not the ratio of combining masses across different compounds.
Why C is wrong: Definite proportions applies to ONE compound having a fixed composition — not to comparing two different compounds of the same elements. This is the classic confusion NEET exploits.
Why D is wrong: Gay-Lussac's law applies to volume ratios of gases, not mass ratios of elements in compounds.
A sample of pure water is decomposed and found to contain hydrogen and oxygen in the mass ratio 1:8. A second sample of water from a completely different source is also decomposed and gives the same 1:8 ratio. This observation is an illustration of:
Show answer and why every option is right or wrong
Answer: D. The same compound (water) always has the same elemental mass ratio regardless of source — this is the Law of Definite Proportions (Proust). NCERT Class 11 Chemistry Chapter 1, page 4.
Why A is wrong: Multiple proportions requires two or more DIFFERENT compounds of the same elements. Here only one compound (water) is involved.
Why B is wrong: Gay-Lussac's law applies to volume ratios of reacting gases, not mass composition of a single compound.
Why C is wrong: Conservation of mass addresses total mass in a reaction system, not the constancy of composition of a single compound.
Hydrogen and oxygen react as: 2H₂(g) + O₂(g) → 2H₂O(g). At constant temperature and pressure, 100 mL of hydrogen reacts completely with 50 mL of oxygen. Which law does this volume ratio (2:1) directly illustrate?
Show answer and why every option is right or wrong
Answer: B. Gases combining in simple whole-number volume ratios (here 2:1) at the same T and P is Gay-Lussac's Law of Gaseous Volumes. NCERT Class 11 Chemistry Chapter 1, page 15.
Why A is wrong: Definite proportions concerns constant mass composition of a compound, not volume ratios of reacting gases.
Why C is wrong: Multiple proportions requires two different compounds formed from the same pair of elements — not a single reaction's volume ratio.
Why D is wrong: Conservation of mass concerns total mass balance, not volume ratios of gaseous reactants.
Nitrogen forms several oxides: N₂O, NO, NO₂, N₂O₃, and N₂O₅. For a fixed mass of nitrogen, the masses of oxygen in these compounds are in the ratio 1:2:4:3:5. Which law is illustrated?
Show answer and why every option is right or wrong
Answer: A. Multiple compounds of the same two elements (N and O) with oxygen masses in a small whole-number ratio for a fixed mass of nitrogen — this is the Law of Multiple Proportions. NCERT Class 11 Chemistry Chapter 1, page 4.
Why B is wrong: Definite proportions applies to ONE compound, not to comparing five different compounds of the same elements.
Why C is wrong: Gay-Lussac's law deals with volume ratios of gases in a reaction, not mass ratios across a series of compounds.
Why D is wrong: Conservation of mass addresses mass balance in a reaction, not ratios across different compounds.
In a closed container, 10.0 g of calcium carbonate is heated and decomposes completely: CaCO₃ → CaO + CO₂. If the calcium oxide produced has a mass of 5.6 g, what is the mass of CO₂ produced?
Show answer and why every option is right or wrong
Answer: D. By the Law of Conservation of Mass, mass of products = mass of reactants. Mass of CO₂ = 10.0 g − 5.6 g = 4.4 g. NCERT Class 11 Chemistry Chapter 1, page 4.
Why A is wrong: 5.6 g is the mass of CaO, not CO₂. This error comes from confusing the two products.
Why B is wrong: 10.0 g is the mass of the reactant CaCO₃. The total product mass equals 10.0 g, but CO₂ is only part of that.
Why C is wrong: 15.6 g exceeds the reactant mass (10.0 g), which would violate the Law of Conservation of Mass.
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How do you solve a Laws Chemical Combination question? A worked example
- 1
Given
• Oxide I: 32 g S combines with 32 g O• Oxide II: 32 g S combines with 48 g O• Mass of sulphur is fixed at 32 g in both compounds
- 2
Required
Verify that the masses of oxygen combining with a fixed mass of sulphur form a ratio of small whole numbers.
- 3
Concept
The Law of Multiple Proportions (Dalton) states: when two elements form two or more compounds, the masses of one element that combine with a fixed mass of the other are in a ratio of small whole numbers. NCERT Class 11 Chemistry Chapter 1, page 4.
- 4
Formula
No algebraic formula needed. The procedure is: fix the mass of one element, take the ratio of the other element's masses across the two compounds, and simplify.
- 5
Substitution
Ratio of oxygen masses = mass of O in Oxide I : mass of O in Oxide II = 32 : 48
- 6
Calculation
32 : 48 = 32/16 : 48/16 = 2 : 3
Note on exact values: 32 g and 48 g are given values in this problem context. The ratio 2:3 follows by simple division and involves no significant-figure ambiguity. - 7
Final answer
The oxygen masses are in the ratio 2:3, which is a ratio of small whole numbers. The data obey the Law of Multiple Proportions.
- 8
Common trap
Confusing this with the Law of Definite Proportions. Definite proportions applies to the fixed composition of ONE compound (e.g., Oxide I is always 50% S and 50% O by mass). Multiple proportions compares the composition ACROSS two or more different compounds of the same two elements. When a stem gives you data for two compounds, the answer is multiple proportions.
- 9
Similar NEET-style question
Carbon forms two chlorides. In one, 12 g of carbon combines with 71 g of chlorine. In the other, 12 g of carbon combines with 142 g of chlorine. What is the ratio of chlorine masses for a fixed mass of carbon, and which law does this illustrate?
*(Answer: 71:142 = 1:2, Law of Multiple Proportions.)*
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What to remember before solving Laws Chemical Combination questions
1.5.2 Law of Definite Proportions This law was given by, a French chemist, Joseph Proust. He stated that a given compound always contains exactly the same proportion of elements by weight. Proust worked with two samples of cupric carbonate — one of which was of natural origin and the other was synthetic. He found that the composition of elements present in it was same for both the samples as shown below: % of copper, % of carbon, % of oxygen — Natural Sample: 51.35, 9.74, 38.91; Synthetic Sample: 51.35, 9.74, 38.91 Thus, he concluded that irrespective of the source, a given compound always contains same elements combined together in the same proportion by mass. The validity of this law has been confirmed by various experiments. It is sometimes also referred to as Law of Definite Composition.
-- NCERT Class 11 Chemistry, Ch. 1, p. 151.5.3 Law of Multiple Proportions This law was proposed by Dalton in 1803. According to this law, if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers. For example, hydrogen combines with oxygen to form two compounds, namely, water and hydrogen peroxide. Hydrogen + Oxygen → Water 2g 16g 18g Hydrogen + Oxygen → Hydrogen Peroxide 2g 32g 34g Here, the masses of oxygen (i.e., 16 g and 32 g), which combine with a fixed mass of hydrogen (2g) bear a simple ratio, i.e., 16:32 or 1: 2.
-- NCERT Class 11 Chemistry, Ch. 1, p. 151.5.5 Avogadro’s Law In 1811, Avogadro proposed that equal volumes of all gases at the same temperature and pressure should contain equal number of molecules. Avogadro made a distinction between atoms and molecules which is quite understandable in present times. If we consider again the reaction of hydrogen and oxygen to produce water, we see that two volumes of hydrogen combine with one volume of oxygen to give two volumes of water without leaving any unreacted oxygen.
-- NCERT Class 11 Chemistry, Ch. 1, p. 15More in Some Basic Concepts of Chemistry: 7 exam traps and mistakes · 3 formulas · 1 question pattern from its other lessons.
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