Law Of Constant Proportions

What is the Law of Constant Proportions?

The law of constant proportions states that chemical compounds are made up of elements that are present in a fixed ratio by mass. This implies that any pure sample of a compound, no matter the source, will always consist of the same elements that are present in the same ratio by mass. For example, pure water will always contain hydrogen and oxygen in a fixed mass ratio (a gram of water consists of approximately 0.11 grams of hydrogen and 0.88 grams of oxygen, the ratio is 1:8).

The law of constant proportions is often referred to as Proust’s law or as the law of definite proportions. An illustration describing the mass ratio of elements in a few compounds is provided below. The ratio of the number of atoms of each element is provided below the mass ratio. For example, in a nitrogen dioxide (NO₂) molecule, the ratio of the number of nitrogen and oxygen atoms is 1:2 but the mass ratio is 14:32 (or 7:16).

In the year 1794, the French chemist Joseph Proust formulated the law of constant proportions from his work on sulphides, metallic oxides, and sulfates. This law was met with a lot of opposition in the scientific community in the 18th century. The introduction of Dalton’s atomic theory favoured this law and a relationship between these two concepts was established by the Swedish chemist Jacob Berzelius in the year 1811.

What are the Exceptions to the Law of Constant Proportions?

Despite being a building block in the development of chemistry, the law of constant proportions does not hold true for all chemical compounds. Some exceptions to this law are listed below.

• Some non-stoichiometric compounds have varying compositions of elements between samples. These compounds obey the law of multiple proportions instead.
• One such example is wustite, an oxide of iron with the chemical formula FeO. The ratio of iron and oxygen atoms can range from 0.83:1 to 0.95:1.
• This is caused by the crystallographic vacancies in the samples caused by a disorderly arrangement of atoms.
• Various samples of a compound may vary in the isotopic composition of its constituent elements. This can lead to fluctuations in the mass ratios.
• The differences in the mass ratios between samples are very useful in the process of geochemical dating, due to the preferential concentration of isotopes in many deep Earth and crustal processes.
• This also occurs in many oceanic, atmospheric and even astronomical processes. Despite the effects being quite small, the challenges in the measurement of the effects have been overcome by modern instrumentation.
• Since natural polymers can vary in their compositions, various samples can show different mass proportions.

Frequently Asked Questions on the Law of Constant Proportions

What is the Statement of the Law of Definite Proportions?

The law of definite proportions, also known as the law of constant proportions, states that the individual elements that constitute a chemical compound are always present in a fixed ratio (in terms of their mass). This ratio does not depend on the source of the chemical compound or the method through which it was prepared.

What are the Exceptions to the Law of Constant Proportions?

The ratio of elements in non-stoichiometric compounds varies from sample to sample. Therefore, these compounds are an exception to the law of constant proportions. Samples of elements that vary in their isotopic composition can also defy the law of definite proportions since the masses of two different isotopes of an element are different. Natural polymers are also known to disobey the law of constant proportions.

Who theorized the law of definite proportions?

The law of definite proportions was first put forward by the French chemist Joseph Louis Proust in the year 1779. This is the reason why this law is also known as Proust’s law. The observations associated with this law were first made by the French chemists Antoine Lavoisier and Joseph Priestley.

Give some examples of compounds that obey the law of definite proportions.

Water molecules feature the combinations of hydrogen and oxygen atoms in a 2:1 ratio. Since they are present in a fixed ratio of mass, water molecules obey the law of constant proportions. Another example of a chemical compound that obeys the law of constant proportions is methane. To form one methane molecule, 4 hydrogen atoms combine with 1 carbon atom.

What is the significance of the law of definite proportions?

Although this law is easily understandable today, it was of great use in the late 18th century when chemical compounds did not have any proper definition. The law of definite proportions also contributed to the development of Dalton’s atomic theory.

Is the law of constant proportion true?

No, for all forms of substances, the law of definite proportion is not valid. Elements with a stable isotope mixture often form a non-stoichiometric product. The role of certain elements in the crystal structure is replaced by their isotopes which induces the crystal’s internal composition to vary.

Which postulate of Dalton’s theory was correct?

An atom’s indivisibility has been proven wrong: it is possible to further subdivide an atom into protons , neutrons and electrons. However, the smallest particle that occurs in chemical reactions is an electron. The atoms of the same product are identical in all respects, according to Dalton.

Explain the law of constant proportion in a simple way

Law of constant proportion states that a chemical compound always contains exactly the same proportion of elements by mass. For a given unique chemical compound, its elemental composition is same for any sample that exists. We all know the chemical formula of water is H₂O. That is two Hydrogens and one Oxygen atom, which adds up to 18 u. 

Let us take one mole of water. Therefore one mole of water will weight 18 grams. So weight of hydrogen in 18gms or 1 mole water is 2 grams and that of oxygen is 16 grams. This ratio is fixed. If we take some mass of water, 8/9 of its mass will be oxygen and 1/9 will be hydrogen. This ratio will never change for water. It does not matter if we take sea water or river water or lake water, this ratio is constant. The stoichiometric ratio of individual elements is constant for a given compound regardless of whatever physical change we bring to it.

Related Questions

State the law of constant proportions. Give one example to illustrate this law.

2.75 g of cupric oxide was reduced by heating in a current of hydrogen and the weight of copper that remained was 2.196 g. Another experiment, 2.358 g of copper was dissolved in nitric acid and the resulting copper nitrate converted into cupric oxide by ignition. The weight of cupric oxide formed was 2.952 g. Show that these results illustrate the law of constant composition.

1.375 g of cupric oxide was reduced by heating in a current of hydrogen and the weight of copper obtained was 1.098 g. In another experiment, 1.179 g of copper was dissolved in nitric acid and the resulting solution was evaporated to dryness. The residue of copper nitrate when strongly heated was converted into 1.4476 g of cupric oxide. The result of this process show:
(A) law of reciprocal proportion
(B) law of multiple proportion
(C) law of constant proportion
(D) none of these

2.8 g of calcium oxide (CaO) prepared by heating limestone was found to contain 0.8 g of oxygen. When one gram of oxygen was treated with calcium, 3.5 g of calcium oxide was obtained. Show that the results illustrate the law of definite proportions.

As per the law of constant proportion; carbon and oxygen combine in the ratio of 3:8. Compute the mass of oxygen that would be required to completely react with 6.9g of carbon.

Choose the most correct option.
A sample of pure water, whatever the source always contains ___________ by mass of oxygen and 11.1 % by mass of hydrogen.
a) 88.8
b) 18
c) 80
d) 16

5.06 g of pure CuO on complete reduction by heating in a current of hydrogen gave 4.04 g of metallic copper. 1.3 g of pure metallic copper was carefully dried and ignited 1.63 g CuO were produced in the process. Show that these results illustrate the law of constant proportions.

Law of Conservation of Mass

A Mass cannot be generated or destroyed in an isolated system, but it can be converted from one form to another.

The mass of the reactants must equal the mass of the products in a low-energy thermodynamic process, according to the law of conservation of mass. It’s thought that mass conservation is defined by a few assumptions from classical mechanics. With the help of quantum mechanics and special relativity, the law of conservation of mass was later amended to the point where energy and mass are now one conserved quantity. The conservation of mass was discovered by Antoine Laurent Lavoisier in 1789.

Formula of Law of Conservation of Mass

In fluid mechanics and continuum mechanics, the law of conservation of mass can be stated in differential form using the continuity equation as:

∂ρ∂t +▽ (ρv) = 0

where;

• ρ is the density,
• t is the time,
• v is the velocity, and
• ▽ is the divergence.

The Law of Conservation of Mass-Energy

The law of mass-energy conservation, which states that the total mass and energy of a system remain constant. The knowledge that mass and energy can be converted from one to the other is incorporated in this revision. because the amount of energy produced or used in a normal chemical reaction is so small In a reaction, the total number of atoms stays the same. 

This assumption allows us to formulate a chemical reaction as a balanced equation, in which both sides of the equation have the same number of moles of each element. Another significant application of this law is determining the masses of gaseous reactants and products. Any residual mass can be attributed to gas if the sums of the solid or liquid reactants and products are known.

Although it may appear like burning destroys matter, the same amount (or mass) of the matter remains after a campfire. When wood burns, it combines with oxygen and transforms into ashes, carbon dioxide, and water vapour, among other things. The gases float away into the air, leaving only the ashes behind.

The Law of Constant Proportions

According to the law of constant proportions, chemical compounds are made up of elements that are present in a stable mass ratio. This means that regardless of the source, each pure sample of a chemical will always have the same elements in the same mass ratio. 

Pure water, for example, will always have a constant mass ratio of hydrogen to oxygen (a gram of water consists of approximately 0.11 grams of hydrogen and 0.88 grams of oxygen, the ratio is 1:8). Chemical compounds are composed up of elements that have a constant mass ratio, according to the law of constant proportions. This indicates that any pure sample of a chemical will always have the same elements in the same mass ratio, regardless of the source.

From his work on sulphides, metallic oxides, and sulphates, the French scientist Joseph Proust created the law of constant proportions in 1794. In the 18th century, this regulation was faced with a lot of hostility from the scientific community. The introduction of Dalton’s atomic theory favoured this law, and the Swedish chemist Jacob Berzelius demonstrated a relationship between the two notions in 1811.

Exceptions to the Law of Constant Proportions

The law of constant proportions does not apply to all chemical substances, despite its importance in the evolution of chemistry. This law has a few exceptions, which are described below.

• The composition of components in some non-stoichiometric compounds varies from sample to sample. Instead, the law of multiple proportions governs these compounds.
• Wustite, an iron oxide with the chemical formula FeO, is one such example. The proportion of iron to oxygen atoms can vary between 0.83 and 0.95.
• This is due to crystallographic voids in the samples, which are created by a chaotic atom arrangement.
• The isotopic composition of a compound’s constituent elements may differ between samples. The mass ratios may fluctuate as a result of this.
• Due to the preferential concentration of isotopes in many deep Earth and crustal processes, changes in mass ratios across samples are highly valuable in the process of geochemical dating.
• Many marines, atmospheric, and even celestial processes are affected by this. Despite the fact that the impacts are minor, modern instrumentation has solved the obstacles of measuring them.
• Because natural polymers have a wide range of compositions, different samples may have varied mass proportions.

Dalton’s Law

In his 1804 publication, A New System of Chemical Philosophy, English chemist and meteorologist John Dalton introduced the concept of multiple proportions, often known as Dalton’s law. It’s a stoichiometric rule. The law asserts that when elements form compounds, the proportions of the components in those chemical compounds can be stated in small whole-number ratios. It was based on Dalton’s observations of atmospheric gas reactions.

The atoms carbon and oxygen, for example, can react to produce both carbon monoxide (CO) and carbon dioxide (CO₂) (CO₂). The amount of oxygen compared to the amount of carbon in CO₂ has a fixed ratio of 1:2, which is a ratio of whole numbers. The ratio in CO is 1:1.

Dalton proposed the idea that all matter is made up of diverse combinations of atoms, which are the indivisible building blocks of matter, in his theory of atomic structure and composition. These rules lay the foundation for our current understanding of atomic structure and composition, as well as concepts such as molecular and chemical equations.

Sample Questions

Question 1: State the Law of Conservation of Mass and Energy.

Answer: 

Although mass and energy cannot be converted, their total remains constant during any physical or chemical transformation.

The mass of the products in a nuclear reaction is slightly less than that of the reactants. The reason for this is that the lost mass is transformed into energy using the following equation:

Here,

m = mass lost

c = velocity of light

Question 2: If 4.2 g of KClO3 is heated, the result is 1.92 g of O and 2.96 g of KCl. Demonstrate that this result follows the Mass Conservation Law.

Answer: 

KClO3 →    KCl + 3/2 O₂

Sum of masses of products

1.92  +  2.96  =  4.88 g

The difference between the total mass of reactants and products and the sum of products is calculated as follows:

0.02 g = 4.9 – 4.88

Question 3: The value must have been zero in this case; yet, for experimental errors, the law of conservation of mass still applies.

What is the Ultimate Source of Energy if it can neither be created nor destroyed?

Answer: 

The Big Bang is the source from which we obtain energy. At the beginning of time, all energy was produced. As the universe expanded, it created a variety of materials, which in turn produced energy.

Question 4: 5.3 g sodium carbonate and 6 g ethanoic acid were combined in a process. 2.2 g carbon dioxide, 0.9 g water, and 8.2 g sodium ethanoate were the end products. Demonstrate that these findings are consistent with the law of mass conservation.

Sodium ethanoate + carbon dioxide + water sodium carbonate + ethanoic acid sodium ethanoate + carbon dioxide + water

Answer: 

Sodium carbonate interacts with ethanoic acid to generate sodium ethanoate, carbon dioxide, and water in the given reaction. 5.3 gram of sodium carbonate (Given)

Mass of sodium carbonate = 5.3 g (Given)

Mass of ethanoic acid = 6 g (Given)

Mass of sodium ethanoate = 8.2 g (Given)

Mass of carbon dioxide = 2.2 g (Given)

Mass of water = 0.9 g (Given)

Now, total mass before the reaction = (5.3 + 6) g= 11.3 g

After the reaction, the total mass is (8.2 + 2.2 + 0.9) g.(11.3 g)

Before the reaction, the total mass was = the total mass after the reaction.

As a result, the observations are consistent with the law of mass conservation.

Question 5: The law of conservation of mass results in which assumption of Dalton’s atomic theory?

Answer:

‘The relative number and types of atoms are constant in a given compound,’ according to Dalton’s atomic theory, which is based on the rule of conservation of mass. In a chemical reaction, atoms cannot be generated or destroyed.’

Postulates of Dalton’s atomic theory

1. Atoms are the indestructible building blocks of matter..
2. All of the atoms of a given element have the same properties, including mass. This can alternatively be expressed as an element’s atoms all having the same mass, whereas other elements have varying masses.
3. Compounds are formed when atoms of different elements mix in specific proportions.
4. Atoms do not make or destroy themselves. This means that no atoms are generated or destroyed during chemical processes.
5. The rearrangement of existing atoms results in the production of new products (compounds) (reactants).
6. The mass, size, and many other chemical and physical properties of an element’s atoms are all the same.
7. Even so, the mass, size, and many other chemical and physical properties of atoms from two different elements differ.

Question 6: Water is formed when hydrogen and oxygen mix in a mass ratio of 1:8. How much oxygen gas would it take to totally react with 3 g of hydrogen gas?

Answer:

It is given that the ratio of hydrogen and oxygen by mass to form water is 1:8.

Then, the mass of oxygen gas required to react completely with 1 g of hydrogen gas is 8 g.

Therefore, the mass of oxygen gas required to react completely with 3 g of hydrogen gas is 8 × 3 g = 24 g.

Question 7: The law of conservation of mass results in which assumption of Dalton’s atomic theory?

Answer:

The following is the postulate of Dalton’s atomic theory, which is based on the rule of mass conservation:

In a chemical reaction, atoms are indivisible particles that cannot be formed or destroyed.

Class 9 Chemistry Chapter 3 | Laws of Constant Proportion – Atoms and Molecules

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