Laws of Chemical Combinations – Class 9 Tutorial


LAWS OF CHEMICAL COMBINATIONS In an era of scientific advancement, scientists found themselves amidst questions pertaining to fundamental principles of chemical combinations.
Chemical combinations were studied and these laws were theorized as: The Law of Conservation of Mass The Law of Conservation of Mass was established by French Chemist Antoine Laurent Lavoisier in 1789. According to this Law, the mass of the substance
produced in a chemical reaction is always equal to the
mass of the reacting substances. Therefore, the Law of Conservation of Mass
states that mass can neither be created nor destroyed in a chemical reaction. Let us now consider the example in which magnesium
reacts with oxygen to form magnesium oxide. Here, mass of one mole Magnesium is 24 grams Total mass of 2 moles Magnesium is 48 grams Mass of one mole oxygen molecule is 32 grams So, the total mass of reactants is 48 + 32
which is 80 grams Mass of one mole Magnesium Oxide is 40 grams Total mass of products is 40 and 40 which
is 80 grams Therefore, it’s clear that the total mass
of reactants=total mass of products. Also, the number of magnesium atoms and number of oxygen atoms on the reactants and the products sides are equal. Let’s move on to the second law of chemical
combination which is the Law of Constant Proportion. This law was stated by a French chemist, Joseph
Louis Proust. It is also called the Law of Definite Proportion. According to the Law, In a chemical compound,
the elements are always present in definite proportion by mass. The Law of Definite Proportion applies when
elements react together to form the same product such as
pure water collected from different sources such as well, spring or river. As such, a water molecule contains two hydrogen
atoms, each of relative mass 1gm and one oxygen atom of relative mass of 16grams. This means
that there are 2grams of hydrogen and 16grams oxygen in 18grams of water. So in a water
molecule, hydrogen and oxygen are present in the ratio 1:8. The example shows the application of the Law
of Definite Proportion. We will now learn about the Law of Multiple
Proportions This Law was proposed by John Dalton around
1800. This Law states that when two elements combine
to form more than one compound, the different masses of one element that combine with the same
mass of another element are in the ratio of simple whole numbers. Let’s try to understand through an example. Carbon combines with high concentration of
oxygen to form carbon dioxide and low concentration of oxygen to form carbon monoxide. In carbon
monoxide, 12 parts by mass of carbon combines with 16 parts by mass of oxygen. In carbon dioxide,
12 parts by mass of carbon combines with 32 parts by mass of oxygen. The ratio of the masses of oxygen
that combine with a fixed mass of carbon is 16: 32 or 1: 2. Therefore, we can see that different masses
of one element that combine with the same mass of another element are in the ratio of simple whole numbers. Things to remember: Law of chemical combinations are : Law of
Conservation of Mass, Law of Constant Proportion and Law of Multiple Proportion The mass of the substance produced in a chemical
reaction is always equal to the mass of reacting substances. In a chemical compound, the elements are always
present in definite proportion by mass. Different masses of one element that combine
with the same mass of another element are in the ratio of simple whole numbers.




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