Lavoisier’s Law: Definition, Formulas, and Example Problems

Laviosier’s law is – The law of conservation of mass in chemistry is often referred to as Lavoisier’s law. What is the law of conservation of mass? Check out the following meaning.

Inventor of Lavoisier’s Law

Before discussing further about Lavoisier’s law and examples of problems, it would be better if we got to know the “Father of Modern Chemistry” who created Lavoisier’s law first.

The figure, whose full name is Antoine-Laurent Lavoisier, was a chemist originally from France and was born on August 26, 1743 and died at the age of 51 on May 8, 1794 .

Do you know, You , even though Antoine-Laurent Lavoisier has a title as a chemist, he actually studied at College Mazarin majoring in Law, you know.

His family belonged to the bourgeois social group, with his own father working as a lawyer. However, actually since he was young, Lavoisier was more interested in science.

Therefore, after graduating from law school, Lavoisier chose to start doing research in science, then chose to join the Academy of Sciences (community of natural philosophers in Paris) in 1768.

A book entitled Traité élémentaire de chimie , published in 1789, marked the beginning of a new era in chemical science which was accompanied by a historic event, namely the French Revolution.

Through the book, Lavoisier explained his findings regarding the processes of respiration and combustion that occur due to chemical reactions with substances in the air. Lavoisier stated that oxygen has a big role in the combustion process.

The results of this research are also the basis for the formation of the “Law of Conservation of Mass” or what is often referred to as “Lavoisier’s Law”.

Understanding Laviosier’s Law and Its History

As previously explained, the inventor of the Law of Conservation of Mass was a French chemist named Antoine Laurent Lavoisier (1743-1794). Lavoisier investigated between the weight (mass) of a substance before and after the reaction. He discovered the law of the conservation of mass in 1789.

Because of these discoveries, Lavoisier is known as the father of modern chemistry. Previously, Mikhail Lomonosov (1748) had also proposed a similar idea and had proven it in an experiment. However, in this idea the conservation of mass is still difficult to understand because of the buoyancy conditions of the earth’s atmosphere.

According to the research results of Lavoisier, the amount of substances before and after the reaction will always be the same as the mass of the substances after the reaction when in a closed system. Even so, material changes generally take place in an open system, so if a reaction product leaves the system or a substance from the bound environment, the mass of the substance before and after the reaction will be different.

The conclusion drawn by Lavoisier that the law of conservation of mass viz

“The mass of the substances before and after the reaction is the same” .

Therefore, the notion of the law of conservation of mass or what is also known as Lavoisier’s law is a law which states that the mass of a closed system will be constant even though various processes occur in the system where it is in a closed system. The mass of the substance before and after the reaction is constant (fixed/same).

Lavoisier’s Law of Conservation of Mass Experiment

In Lavoisier’s law experiments, Antoine Laurent Lavoisier has conducted experiments by heating mercury oxide (HgO) to produce metallic mercury (Hg) and also oxygen gas (O2) with the following reaction or Lavoisier’s law formula:


Furthermore, the two products are reacted again and form mercuric oxide. This shows that the mass of oxygen gas that has been produced in the mercury oxide combustion activity is the same as the mass of oxygen required to convert mercury metal into mercury oxide.

Lavoisier’s Law of Burning Wood

The law of conservation of mass applies in general. This means that all existing events obey the law of conservation of mass. However, the law of conservation of mass does not apply to wood burning events.

Burning wood is said to not fulfill the law of conservation of mass because it produces charcoal and also light ash. The mass of charcoal and ash produced by burning is much less than the mass of wood that is burned.

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However, actually burning wood until it turns into charcoal and ash obeys the law of conservation of mass. Why doesn’t the law of conservation of mass seem to apply to the burning of wood?

This is because burning wood is a chemical change that changes matter into another form. Thus, it is as if a mass of wood is crushed and all that is left is charcoal and light ashes.

However, actually the mass of wood is not destroyed, but changed into other forms.

Burning Wood Produces Water Vapor

The mass of wood that undergoes the combustion process also changes form to water vapor. Adapted from Sciencing, water vapor is the most common substance released by wood when it is burned, especially young wood which still has a lot of water in its fibers. The water inside the wood evaporates due to the heat of combustion and is released into water vapor which is mixed in the combustion smoke.

Wood Burning Produces Carbon Dioxide

Adapted from Chemistry LibreTexts, when wood undergoes a burning process, the carbon in the wood reacts with oxygen and forms carbon dioxide gas. Carbon dioxide gas leaves burning wood by going through the form of smoke and leaving charcoal and ashes.

In addition to carbon dioxide, burning wood also produces nitrogen oxides and volatile organic compounds.

Wood Burning Produces Heat And Light

Adapted from Scientific American, fire is the conversion of chemical energy into heat and electromagnetic energy from wood molecules and oxygen in the air. That is, some of the mass of wood will change shape into fire which produces heat energy and also light.

Unlike ash and charcoal, it is difficult to know the mass of heat and light produced by combustion.

Wood Burning Produces Small Particles

Burning wood produces smoke which is formed from water vapor, carbon dioxide, and also small particles of ash. The mass of ash from burning is much different from the original mass of wood. This can happen because a lot of ash becomes smoke particles. Ash which has a very small size will blend with the smoke and be carried away by the wind.

That is, charcoal and ash are only part of the results of burning wood. Thus, the mass is much lighter than the original wood and makes it appear as if burning wood does not fulfill the law of conservation of matter.

Even though the burning wood material also turns into smoke which contains carbon dioxide gas, water vapor, small particles, nitrogen oxide gas and also volatile organic compounds. If all the products of combustion are totaled, the mass will be the same as the mass of wood before it is burned and also the oxygen used during combustion. From the previous explanation, it can be concluded that burning wood actually obeys the law of conservation of mass.

Examples of Lavoisier’s Law Questions

So, in order to understand more about Lavoisier’s law, has prepared several examples of complete Lavoisier’s law questions accompanied by discussion, check them out below:

Example Question 1

There is magnesium 12 (Mg) which reacts with element 16 sulfur or sulfur (S), then how much Magnesium Sulfide (MgS) is produced? Write down the reaction equation.


12gram Mg + 18gram S -> 28gram MgS.

Example Problem 2

There are 28 grams of calcium oxide (CaO) which reacts with 22 grams of Carbon Dioxide (CO2), what reaction will it produce?


28 grams of calcium oxide reacts with 22 grams of carbon dioxide to produce 50 Calcium Carbonate (CaCO3).

28 gr CaO + 22 gr CO2 -> 50 CaCO3

Example Problem 3

A total of 31.75 grams of copper (I) metal reacted with 8 grams of oxygen gas in a closed container to form copper (II) oxide 2 Cu(s) + O2 (g) -> 2 CuS (s), what is the mass of the substance generated?


2 Cu(s) + O2 -> 2 CuS

31.75 gr Cu(s) + 8 gr O2 –> 39.79 gr CuS

Example Problem 4

When burning magnesium in air, magnesium oxide is produced. If 0.098 grams of magnesium is burned it produces 0.162 grams of MgO (magnesium oxide). What mass of oxygen gas is used in the reaction?


Mg(s) + O2 (g) -> MgO(s)

0.098 gr + O2 (g) -> 0.162 gr

O2 (g) = 0.162 gr – 0.098 gr

O2 (g) = 0.064gr.

Example Problem 5

As much as 100 grams of limestone (CaCO3) was baked in the furnace, and it turned out that 56 grams of quicklime (CaO) and carbon dioxide gas were formed.

Write down the equation for the reaction and determine how many grams of CO2 gas (carbon dioxide) that comes out of the furnace?

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CaCO3 -> CaO + CO2

100 grams -> 56 grams + CO2

100 grams – 56 grams -> CO2

4 grams -> CO2.

Example Problem 6

It is known that a carbon burned with oxygen has a mass of 30 grams. At the end of the reaction, there is still carbon with a mass of 4 grams.

The combustion results produce carbon monoxide with a mass of 58 grams. Calculate the mass of carbon initially?


a). The total mass of the products of the reaction = mass of carbon reaction + mass of oxygen

58 grams = mass of reacted carbon + 30 grams

Reaction carbon mass = 58 grams – 30 grams

The mass of the reacted carbon = 28 grams.

b). Initial mass of carbon = mass of reaction carbon + mass of residual carbon

Initial mass of carbon = 28 grams + 4 grams

The initial mass of carbon = 32 grams.

So, the initial mass of carbon is 32 grams.

Example Problem 7

It is known that a reaction between nitrogen and hydrogen gas is as follows.

Nitrogen gas which has a mass of 10 grams reacts perfectly with hydrogen gas with a mass of X. The reaction then produces ammonia which has a mass of 18 grams. What is the mass of hydrogen gas at the start of the reaction?


Mass + mass = mass

10 grams + X = 18 grams

X = 18 grams – 10 grams

X = 8 grams.

So, the total mass of hydrogen gas at the beginning of the reaction is 8 grams.

Example Problem 8
10 grams of sulfur is reacted with 10 grams of oxygen to form sulfur dioxide gas. If the reaction takes place perfectly, the mass of sulfur dioxide produced is………
A. 10 grams
B. 12 grams
C. 15 grams
D. 17 grams
E. 20 grams

Discussion :
According to Lavoisier’s law, the mass of the substance before and after the reaction is the same. This can be proven by carrying out the reaction in a closed room.
Based on Lavoisier’s law, we can make an equation for the reaction in this problem, which is as follows.

Sulfur + oxygen gas ⇒ sulfur dioxide gas

The mass of sulfur + the mass of oxygen = the mass of sulfur dioxide
10 grams + 10 grams = the mass of sulfur dioxide
the mass of sulfur dioxide = 20 grams

Answer: E

Example Problem 9

Look at the reaction equation below
SO2(g) + O2(g) ⇒ SO3(g)
In order for the above reaction to comply with the law of conservation of mass, the coefficients of SO2 and SO3 respectively are ……..
A. 1 and 2
B. 2 and 1
C. 2 and 2
D. 1 and 3
E. 3 and 2

Discussion :
According to the law of conservation of mass or Lavoisier’s law, in a reaction, no new substances are created and no substances are lost or destroyed. What happens is the formation of a new substance from the atoms that are the same as the reactants but have different properties.

Therefore, in his atomic theory, Dalton concluded that chemical reactions are reactions of termination, rearrangement and recombination of atoms.

In order for a reaction equation to comply with the law of conservation of mass, the equation of the reaction must be balanced. The point is that the number of atoms on the left must equal the number of atoms on the right.

The following is the result of equalizing the above reaction.
2SO2(g) + O2(g) ⇒ 2SO3(g)

So it can be seen that the coefficients of SO2 and SO3 in the equivalent reactions are 2 and 2.

Answer: C

Example Problem 10

Which of the following reactions does not conform to Lavoisier’s law?

  1. 2H2SO4(aq) ⇒ 2SO2(g) + 2H2O(l) + O2(g)
    B. 2Fe(s) + 3Cl2(aq) ⇒ 2FeCl3
    C. NH3(g) + HClO(aq) ⇒ NH3Cl(aq) + H2O (l)
    D. SiO2(s) + NaOH(l) ⇒ Na2SiO3(s) + H2O(l)
    E. 4NH3(aq) + 3O2(g) ⇒ 2Na(s) + 6H2O(l)

Discussion :
As explained in the previous problem, the reaction that fulfills Lavoisier’s law is the same or equivalent reaction.
Because in question number 10 what is being asked is a reaction that is not in accordance with Lavoisier’s Law, so we are looking for a reaction that is not yet equal.

Among the above reaction equations, reaction D is an unbalanced reaction. Supposedly, to balance the reaction, the NaOH coefficient is changed to 2.

SiO2(s) + 2NaOH(l) ⇒ Na2SiO3(s) + H2O(l)

Answer: D

Thus a brief explanation of the law of conservation of mass or also known as Lavoisier’s law. Hopefully, the example of the law of the conservation of mass and its discussion can be easily understood by You . Thanks for reading and hopefully useful!