Laws of Chemical Combination: Conservation of Mass, Definite & Multiple Proportions

Laws of Chemical Combination: While you are writing chemical equations, how do you balance them? What happens if they don’t balance? What are the rules which will be used to balance these equations?

Chemistry deals with properties of matter, composition and structure. These aspects are best described and understood in terms of the fundamental constituents of matter, i.e. atoms and molecules. Therefore, it is essential to know about atoms and molecules to understand matter within and outside our body.

Laws of Chemical Combination

The three important laws of chemical combination are:

1. Law of conservation of mass (or matter),
2. Law of constant proportions or definite composition, and
3. Law of multiple proportions.

An Indian philosopher Maharishi Kanad(\(400\) BC), postulated that if matter keeps on dividing, then a stage will come when the particles obtained by division would no longer divide further, i.e. further division will not be possible. These particles were called paramanu by him. Democritus and Leucippus, the ancient Greek philosophers, also suggested the same thing around that period. Democritus first used the word atom (meaning indivisible) for these indivisible particles. But there was no experimental verification available for these considerations. Some essential laws of the chemical combination were given by Antoine L Lavoisier and Joseph L Proust. These laws have proved very helpful in the development of chemistry.

Law of Conservation of Mass

In the \({\rm{18th}}\) century, scientists noticed no change in mass when they were doing a chemical reaction in a closed container. This mass preservation in a chemical reaction led to the law of conservation of mass (or the law of conservation of matter. In \(1744\) Lavoisier gave this law).

According to the law of conservation of mass, during a chemical reaction, the matter is neither created nor destroyed but remains conserved. It means that in a chemical reaction, the total mass of the products is equal to the total mass of the reactants.

For example, in the reaction of hydrogen \(\left( {{{\rm{H}}_{\rm{2}}}} \right)\) and chlorine\(\left( {{{\rm{Cl}}_{\rm{2}}}} \right)\) represented below, \(2{\rm{g}}\) of \({{\rm{H}}_{\rm{2}}}\) reacts with \(71{\rm{g}}\) of \({\rm{C}}{{\rm{l}}_{\rm{2}}}\) to give \(73{\rm{g}}\) of \({\rm{HCl}}{\rm{.}}\)

\({{\rm{H}}_{\rm{2}}}{\rm{ + \;C}}{{\rm{l}}_2}\;\; \to 2{\rm{HCl}}\)

The above molecular mass data indicate that the mass of the reactants is equal to the mass of the products, i.e. the total mass is conserved in the reaction.

Experiment to Verify Law of Conservation of Mass

Prepare \(5\% \) solutions of sodium chloride and silver nitrate in two separate measuring flasks. First, take \({\rm{10}}\,{\rm{mL}}\) of sodium chloride \(\left( {{\rm{NaCl\;}}} \right)\) solution in a conical flask. Next, take \({\rm{10}}\,{\rm{mL}}\) of silver nitrate \(\left( {{\rm{AgN}}{{\rm{O}}_{\rm{3}}}} \right)\) solution in a test tube. Suspend the test tube in the conical flask carefully with the help of a thread so that the two solutions do not get mixed. Close the mouth of the conical flask with a rubber cork.

Weigh the conical flask along with its contents carefully. Now tilt the test tube so that the silver nitrate solution mixes with the sodium chloride solution. Let the reaction take place. Weigh the conical flask again and compare the masses of the conical flask before and after the chemical reaction.

\({\rm{NaCl + AgN}}{{\rm{O}}_3}\; \to \;{\rm{AgCl}}\left(  \downarrow  \right) + {\rm{NaN}}{{\rm{O}}_3}\)

It was observed that the mass of the conical flask along with its contents remains unchanged after the chemical reaction. Thus, the experiment proves that there is no change in the mass during a chemical reaction.

The mass of any product or reactant can utilise the law of conservation of mass.

Law of Constant Proportions

The law of constant proportions is also known as the law of definite proportions or law of fixed proportions. This law, proposed by Joseph Proust in \(1797\), states that whatever be the method of its formation or whatever may be its source, a chemical compound always consists of the same elements combined together in the same proportions by mass. This law means that a chemical compound will always have its elements combined in a fixed ratio by mass.

Let us consider two elements \(X\) and \(Y\), which combine to form a compound \({X_m}{Y_n}\), where \(m\) and \(n\) are integers. Let the mass of an atom of \(X\) be \(x\) and the mass of an atom of \(Y\) be \(y\). So, the mass of \(X\) in one molecule is \(xm\) and the mass of \(Y\) in one molecule is \(yn\). The ratio of the masses of \(A\) and \(B\) is given by \(xm:yn\). Since all the atoms are alike, \(x\) and \(y\) are constants. It follows from the formula of the compound that \(xm:yn\) is constant.

For example, Carbon dioxide gas \(\left( {{\rm{C}}{{\rm{O}}_{\rm{2}}}} \right)\) always found to contain only carbon and oxygen. Moreover, the ratio in which carbon and oxygen are present in carbon dioxide is permanently fixed at \(3:8\) (actual is \(12:32\)) by mass independent of the source of carbon dioxide. Thus, if \({\rm{3}}{\rm{.0}}\,{\rm{g}}\) of carbon is burnt, it will always combine with \({\rm{8}}{\rm{.0}}\,{\rm{g}}\) of oxygen to form \({\rm{11}}{\rm{.0}}\,{\rm{g}}\) of carbon dioxide.

Water \(\left( {{{\rm{H}}_{\rm{2}}}{\rm{O}}} \right)\) always found to contain hydrogen and oxygen combined in the ratio of \(1:8\) (actual is \(2:16\) by mass independent of the source of water. Thus, if \({\rm{9}}{\rm{.0}}\,{\rm{g}}\) of water decomposes, \({\rm{3}}{\rm{.0}}\,{\rm{g}}\) of hydrogen and \({\rm{8}}{\rm{.0}}\,{\rm{g}}\) of oxygen are always formed independently of the water source.

Let us solve a problem based on the law of constant proportions.

Example Problem: Oxygen and hydrogen combine in the ratio of \(8:1\) by mass to form water. What mass of oxygen gas would be required to react completely with \({\rm{3}}\,{\rm{g}}\) of hydrogen gas?

Solution: Given, hydrogen and oxygen always combine in the fixed ratio of \(1:8\) by mass. It means that:

\({\rm{8}}\,{\rm{g}}\) of oxygen gas requires \( = 1\,{\rm{g}}\) of hydrogen gas

So, \({\rm{3}}\,{\rm{g}}\) of hydrogen gas requires \({\rm{ = 8 \times 3\;}}\,{\rm{g}}\) of oxygen gas \( = 24\,{\rm{g}}\) of oxygen gas.

Hence, \({\rm{24}}\) grams of oxygen gas would be required to react completely with \({\rm{3}}\) grams of hydrogen gas.

Law of Multiple Proportions

This law, proposed by Dalton in \({\rm{1803}}\), states that when two elements combine to form two or more compounds, then the different masses of one element, which combine with a fixed mass of the other, bear a simple ratio to one another.

Oxygen combines with hydrogen to form two oxides. The oxygen content in one oxide (water) is \(88.89\% \), while that in the other oxide (hydrogen peroxide) is \(94.12\% \).

For \({{\rm{H}}_{\rm{2}}}{\rm{O}}\):
Mass \(\% \) of oxygen \( = 88.89\)
Mass \(\% \) of hydrogen \( = 100 – 88.89 = 11.11\)
Thus,
\({\rm{11}}{\rm{.11 g}}\) of hydrogen reacts with \({\rm{88}}{\rm{.89 g}}\) of oxygen
\({\rm{1 g}}\) of hydrogen reacts with \(\frac{{88.89}}{{11.11}}\;{\rm{g}}\) of oxygen
\({\rm{ = 8}}{\rm{.00 g}}\) of oxygen.

For \({{\rm{H}}_2}{{\rm{O}}_2}\):
Mass \(\% \) of oxygen \( = 94.12\)
Mass \(\% \) of hydrogen \( = 100 – 94.12 = 5.88\)

Thus,
\({\rm{5}}{\rm{.88 g}}\) of hydrogen reacts with \({\rm{94}}{\rm{.12 g}}\) of oxygen
\({\rm{1 g}}\) of hydrogen reacts with \(\frac{{94.12}}{{5.88}}\;{\rm{g}}\) of oxygen
\({\rm{ = 16}}{\rm{.00 g}}\) of oxygen
The ratio of oxygen masses which combine with \({\rm{1 g}}\) of hydrogen \( = 8.00:16.00 = 1:2\)
Since \(1:2\) is a simple ratio, the law of multiple proportions is supported by the above data.

Summary

From this article, we concluded that the importance of balancing a chemical reaction and the laws of conservation of mass, law of definite proportions and the law of multiple proportions.

FAQs on Laws of Chemical Combination

Q.1. State Law of Multiple Proportion?
Ans: This law, proposed by Dalton in 1803, states that when two elements combine to form two or more compounds, then the different masses of one element, which combine with a fixed mass of the other, bear a simple ratio to one another.

Q.2. State Law of conservation of mass?
Ans: According to the law of conservation of mass, during a chemical reaction, the matter is neither created nor destroyed but remains conserved. It means that in a chemical reaction, the total mass of the products is equal to the total mass of the reactants.

Q.3. What are chemical reactions according to the law of conservation of mass?
Ans: According to the law of conservation of mass, chemical reactions involve only the exchange of patterns in the reactants to form products. That is why there is no change in mass.

Q.4. State Law of Multiple Proportion?
Ans: This law, proposed by Dalton in 1803, states that when two elements combine to form two or more compounds, then the different masses of one element, which combine with a fixed mass of the other, bear a simple ratio to one another.

We hope this article on ‘Laws of Chemical Combination’ has helped you. If you have any queries, drop a comment below and we will get back to you.