Laws Of Chemical Combinations & Dalton's Atomic Theory - NEET Chemistry

The law of definite proportions, also known as the law of constant composition states that all pure samples of the same chemical compound contain the same elements combined in the same proportions by mass.
What this law emphasizes is that, if pure samples of the same chemical substance, wherever they may be found, are analyzed, it will be found that they all consist of the same elements, as well as having these elements combine in the same proportions by mass.
For examples, pure sample of copper(II) oxide is composed of copper and oxygen, in the proportion of 1:1 by mole, or 64 g of copper to 16 g of oxygen or 1 g of copper to 0.25 g of oxygen. 

Chemical reactions involve merely the combination, separation, or rearrangement of atoms and that during these processes atoms are not subdivided, created, or destroyed.

According to Gay Lussac's Law of Gaseous Volume, under same pressure and temperature, whenever gases react, they do so in volumes and volumes of reacting gases bear a simple ratio with each other and with product gases formed. 100 : 50 : 100 : : 2 : 1: 2 Hence, option B is correct.

Avogadro's law states that, "equal volumes of all gases, at the same temperature and pressure, have the same number of molecules". For a given mass of an ideal gas, the volume and amount (moles) of the gas are directly proportional if the temperature and pressure are constant.

Avogadro made the distinction between atoms and molecules, which today seems clear. However, Dalton rejected Avogadro's hypothesis because Dalton believed that atoms of the same kind could not combine. Since it was believed that atoms were held together by an electrical force, only unlike atoms would be attracted together, and like atoms should repel. Therefore it seemed impossible for a molecule of oxygen, O₂, to exist. Avogadro's work, even if it was read appears not to have been understood, and was pushed into the dark recesses of chemistry libraries and ignored. Avogadro continued to teach at the university of Turin, when it was not closed because of the political upheavals going on in Italy at the time, and died in 1854, an unknown figure. 

Law of Definite Proportions:

1. The law of definite proportions was given by French chemist Joseph Proust in 1799.

2. The law of definite proportions is also known as the law of definite or fixed proportion.

Avogadro’s Law
Amedeo Avogadro in 1811 combined the conclusions of Dalton’s Atomic Theory and Gay Lussac’s Law to give another important Gas law called the Avogadro’s Law. According to Avogadro’s law, at constant temperature and pressure, the volume of all gases constitutes an equal number of molecules. In other words, this implies that in unchanged conditions of temperature and pressure the volume of any gas is directly proportional to the number of molecules of that gas.
Mathematically, V ∝ n
Here, n is the number of moles of the gas. Hence, V= kn

Law of definite proportions or Proust's law states that a chemical compound always contain the same proportion of elements by mass.
Although the law holds good for a large no. of cases, it is not universally true. It's not true in the case of non - stoichiometric compounds.

The old ideas were put on a scientific scale by John Dalton in the form of a theory, known as Dalton’s atomic theory,
Main postulates of which are as follows :
• All matters are made of atoms. Atoms are indivisible and indestructible.
• All atoms of a given element are identical in mass and properties.
• Atoms of different elements differ in properties and have different masses and sizes.
• Compounds are formed by a combination of two or more different kinds of atoms.
• A chemical reaction is a rearrangement of atoms.These are neither created nor destroyed in a chemical reaction

Option a is incorrect about the Law of Conservation of Mass because it actually describes the Law of Definite Proportions, which states that a given compound always contains the same elements in the same proportion by weight. The Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction, meaning the mass of reactants is equal to the mass of products (as described in options b and c). This law was given by Antoine Lavoisier (option d). Therefore, only option a does not describe the Law of Conservation of Mass.

Atoms are composed of particles called protons, electrons and neutrons. Protons carry a positive electrical charge, electrons carry a negative electrical charge and neutrons carry no electrical charge at all. The protons and neutrons cluster together in the central part of the atom, called the nucleus, and the electrons 'orbit' the nucleus. A particular atom will have the same number of protons and electrons and most atoms have at least as many neutrons as protons.

According to the law of definite proportions a given compound always contains exactly the same proportion of elements by weight irrespective of its source.

Law of Definite Proportions
The discovery that mass was always conserved in chemical reactions was soon followed by the law of definite proportions, which states that a given chemical compound always contains the same elements in the exact same proportions by mass. As an example, any sample of pure water contains 11.19%11.19% hydrogen and 88.81%88.81% oxygen by mass. It does not matter where the sample of water came from or how it was prepared. Its composition, like that of every other compound, is fixed.

Gay Lussac’s law of gaseous volumes states that when gases combine or are produced in a chemical reaction, they do so in a simple ratio by volume provided all gases are at same temperature and pressure.

The Law of Conservation of Mass (or Matter) in a chemical reaction can be stated thus:
In a chemical reaction, matter is neither created nor destroyed.
It was discovered by Antoine Laurent Lavoisier (1743-94) about 1785. However, philosophical speculation and even some quantitative experimentation preceeded him. In addition, he was certainly not the first to accept this law as true or to teach it, but he is credited as its discoverer.

Law of multiple proportions states that when two elements combine to form two or more compounds, the masses of one element which combine with the fixed mass of another element, will always be in ratio of whole numbers. Going with the definition we will get C.

Hence C

2KClO₃​→2KCl+3O₂​ 

Mass of the reactant = total mass of the products (2.96+1.92=4.88 g)

According to the law of conservation of mass, the mass of the products in a chemical reaction must equal the mass of the reactants. Therefore it is the law of conservation of mass.

20 g of zinc sulphate

x = (20 * 22.65)/100

x = 4.53g