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Law of Conservation of Mass

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The law of conservation of mass refers to the total mass in chemical reactions. When we study the combining mass ratios in chemical reactions other interesting relationships emerge.

The law of conservation of mass and conservation of energy must therefore be combined into a single law of conservation of mass plus energy. The law of conservation of mass is an approximation. However, in chemical transformations, the amount of mass lost or gains is always too small to measure.

Law of Conservation of Mass Definition

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Definition of law of conservation of mass

If the total mass involved in a chemical reaction is precisely measured before and after the reaction takes place, the most sensitive balances cannot detect any change. This generalization is known as the law of conservation of mass.

"No detectable change in the total mass occurs during a chemical reaction."

To define the law of conservation of mass it is considered that if a chemical reaction takes place in a sealed vessel that permits no matter to enter or escape, the mass of the vessel and its contents after the reaction will be identical to its mass before.

State the Law of Conservation of Mass

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The law of conservation of mass states that no change is observed in the total mass of the substances involved in a chemical change. This law tested by extensive laboratory experimentation, is the basis for the quantitative mass relationships among reactants and products.

In a chemical reaction
mass of reactants = mass of products


The law of conservation of mass (from Lavoisier) states that mass can be neither created nor destroyed (but it may change its form into something that is not desired, such as heat with an engine) or that in a chemical reaction, the total mass of the reacting substances is equal to the total mass of products formed.

Law of Conservation of Mass Examples

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This law was discovered by Lavoisier, "the father of chemistry" in 1774. He performed the first quantitative experiments that help to prove the conservation of mass. Law of conservation of mass is verified by performing the following experiment.

Example

The complete burning of 4.09g of carbon in oxygen produces 15.00g of carbon dioxide as the only product. How many grams o oxygen gas must have reacted?

Answer: The total mass before and that after reaction must be equal. Because carbon dioxide was the only product and weighed 15.00g, the total mass of carbon and oxygen that combined to produce it must have been 15.00g. Because carbon's mass was 4.09g, the oxygen that reacted with it must have contributed 15.00g minus 4.09g or 10.91g.

Law of Conservation of Mass

When a candle is burned in an airtight container with oxygen, there is no detectable change in the candle's mass, as illustrated by the balance pointer being in the same place.

Explain the Law of Conservation of Mass

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In any chemical reaction, the total mass of the reacting substances is equal to the total mass of the products of the reaction provided masses are measured under similar conditions.

Explanation

If A and B react to form C and D then according to Law of conservation of mass

Mass of A + Mass of B = Mass of C + Mass of D

For example, in the formation of iron II sulphide by heating iron and sulfur.
56 parts of Fe and 32 parts of sulfur chemically combine to form 88 parts of FeS.

Mass of iron (56) + Mass of sulfur (32) = mass of iron II sulphide (88)

According to the law of conservation of mass, the total mass of reactants before the reaction and mass of products after the reaction is equal.

Antoine Lavoisier Law of Conservation of Mass

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Antoine Lavoisier made chemistry a modern science, just as Galileo and Newton had done for physics more than a century earlier. Lavoisier performed the first quantitative chemical experiments to explain combustion and to settle the question of whether mass was gained, lost or unchanged during a chemical reaction.

Lavoisier understood the nature of gases and took care to do his experiments in closed containers so that no substances could enter or leave. In addition he used the most precise balance that had ever been built.

After many experiments he was able to formulate the following law: "no detectable change in the total mass occurs during a chemical reaction (Lavoisier law of conservation of mass)."

Law of Conservation of Mass Equation

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Law of conservation of mass states that matter cannot be created nor destroyed. Or in other words, the matter or mass is constant. For one dimensional steady flow, the mass per second = $rho$AV

Where

$\rho$ = Mass density
A = Area of cross section
V = velocity

As mass or mass per second is constant according to law of conservation of mass. Hence

$\rho$AV = Constant

Differentiating and dividing the equation by $\rho$AV we get,

$\frac{dV}{V} + \frac{dA}{A} + \frac{d\rho}{\rho}$ = 0

This equation is called law of conservation of mass formula also called continuity equation.

Law of Conservation of Mass Experiment

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The law of conservation of mass may be verified by simple experiment.

Experiment
  • A sealed tube containing phosphorus was weighed ans heated carefully to initiate complete burning of phosphorus.
  • When the reaction was completed the tube was cooled and reweighed.
Law of Conservation of Mass Experiment
Conclusion

No change in weight was detected before and after the experiment.

Law of Conservation of Mass and Energy

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Mass is a measure of an objects energy, we need to re-evaluate our statements of the law of conservation of mass and the law of conservation of energy.

Based on Einstein's discovery, however mass and energy are two concepts effectively describing the same thing, therefore we could more appropriately combine these two laws into a single law: the law of conservation of mass and energy. This law states that "mass energy cannot be created nor destroyed."
The law of conservation of energy states that energy can be neither created not destroyed and expresses the fact that the total amount of energy remains constant as it changes from one form to another. This law is one of the cornerstones of science and helps us to develop a better understanding of the world around us.
→ Read More The term atomic mass unit (amu) is used in place of unified mass unit. amu is defined by assigning a mass of 12amu to C-12 isotope of carbon.

6.023 $\times$ 1023 carbon atoms weigh = 12.00gram
→ Read More A compound of two or more elements is always formed by the union of certain definite and unalterable proportions of its constituent elements. This is called law of definite proportions.
→ Read More
The law of multiple proportions deduced by Dalton from his atomic theory states that "when two elements form more than one compound the masses of one element in these compounds for a fixed mass of the other element are in ratios of small whole numbers."
→ Read More

Law of Conservation of Mass Problems

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Solved problems based on law of conservation of mass are given below.

Solved Examples

Question 1: 16.8g NaHCO3 is treated with 12.0g CH3COOH. The residue was found to weigh 20.0g. Determine the mass of CO2 escaped in the reaction.
Solution:
 
According to the law of conservation of mass
Mass of reactants = Mass of products

CH3COOH + NaHCO3 $\rightarrow$ CH3COONa + H2O + CO2

CH3COOH = 12.0g
NaHCO3 = 16.8g
CH3COONa = 20.0g

Let the mass of CO2 = xg

(12.0 + 16.8)g = (20.0 + x)g

x = 28.8 - 20.0 = 8.8g
 

Question 2: 6.4g of MgCO3 on heating gave 2.88g MgO and 3.52g CO2. Show that these observations are in agreement with law of conservation of mass.
Solution:
 
Mass of the reactant = 6.4g
Mass of the products = (2.88 + 3.52)g = 6.4g

Since mass of reactant is equal to the mass of the products therefore these results are in agreement with law of conservation of mass.