# Lesson Video: Conservation of Mass Science

In this video, we will learn how to define, and explain the conservation of mass in chemical reactions.

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### Video Transcript

In this video, we will learn how to define and explain the conservation of mass in chemical reactions.

In the late 1700s, French chemist Antoine Lavoisier was studying the reactions of various chemicals. Lavoisier carefully measured the masses of substances before and after a chemical reaction. He found that when the reaction was carried out in a closed system, where all of the reactants and products were accounted for, the mass before and after the reaction was the same even though the reactants and products were different. His work led to the law of conservation of mass.

The law of conservation of mass states that mass is neither created nor destroyed during a chemical reaction. In other words, the total mass of the reactants before the reaction must equal the total mass of the products after the reaction. Let’s examine how the law of conservation of mass can be applied to a chemical equation.

This chemical equation describes the reaction between carbon and oxygen to produce carbon dioxide. Carbon and oxygen are the reactants, and carbon dioxide is the product. In this reaction, one atom of carbon reacts with one molecule of oxygen to produce one molecule of carbon dioxide. We should notice that a molecule of oxygen and a molecule of carbon dioxide each contain two atoms of oxygen. We also see that there’s one atom of carbon on the reactant and product sides of the equation. We can say that this reaction equation is balanced because there are the same number of each type of atom on both sides of the reaction arrow.

We know that according to the law of conservation of mass, the mass of the reactants must equal the mass of the product. We can see that this is true by using relative atomic mass. Each type of atom has its own relative atomic mass. One atom of carbon has a relative atomic mass of 12, and one atom of oxygen has a relative atomic mass of 16. So, the relative atomic mass of the single carbon atom in the reaction equation is 12.

We can determine the mass of a molecule by adding the masses of the atoms it contains. For example, a molecule of oxygen contains two oxygen atoms that each have a relative atomic mass of 16. Therefore, the relative mass of an oxygen molecule is 32. A molecule of carbon dioxide contains two oxygen atoms and one carbon atom. So, the relative mass of a molecule of carbon dioxide is 44. We call the relative mass of a molecule the relative formula mass.

If we add together the relative atomic mass of the carbon atom plus the relative formula mass of the oxygen molecule, we get a relative mass of 44. This is the total relative mass of the reactants. We can see that the total relative mass of the reactants is equal to the total relative mass of the products. This shows that the law of conservation of mass is obeyed.

Let’s take a look at another chemical equation and a different way to find the total mass of the reactants and products. Let’s separate the reactants and products at the reaction arrow and make a list of the different types of atoms on each side of the reaction.

Now, we can determine the number of atoms of each element on both sides of the reaction arrow. The number four written in front of Na is a coefficient. A coefficient is a number written in front of a species in a chemical equation that indicates the smallest number of particles present within the reaction. Coefficients should be seen as multipliers. So, on the reactant side, the number of sodium atoms is four and the number of oxygen atoms is two. On the product side, the number of sodium atoms is two times two, or four, and the number of oxygen atoms is two. This is another example of a balanced chemical equation, as the number of each type of atom is the same on both sides of the reaction arrow.

Now, we need to know the relative atomic mass of each type of atom. The relative atomic mass of sodium is 23, and the relative atomic mass of oxygen is 16. We can multiply the number of atoms of each element by its relative atomic mass to determine a total relative atomic mass for each element. If we add the results, we get the total relative mass of the reactants and products. Once again, the mass of the products is equal to the mass of the reactants, and the law of conservation of mass is obeyed. Both examples have shown that a balanced chemical equation obeys the law of conservation of mass.

Now that we’ve seen how we can explain the law of conservation of mass using relative atomic masses, let’s take a look at an experiment. In this experiment, a colorless liquid is added to a test tube, which is then carefully lowered into a flask containing a second colorless liquid. A stopper is added to the flask, and the entire apparatus is placed on an electronic balance. The liquid in the test tube contains the compound lead(II) nitrate. And the liquid in the flask contains the compound potassium iodide.

Before any reaction occurs, the mass of the chemicals, the flask, the stopper, and the test tube is 301.23 grams. The flask is then carefully tipped to allow both liquids to combine. As soon as the liquids combine, a reaction occurs, producing a yellow solid suspended in a clear liquid. The clear liquid contains the compound potassium nitrate, and the yellow solid is the compound lead(II) iodide. When we place the flask back on the balance, we find that the mass is the same as before the reaction took place. As the mass before and after the reaction is the same, this experiment shows that chemical reactions obey the law of conservation of mass.

Let’s take a look at another experiment. In this experiment, a ball of iron wool is placed on an electronic balance. We then light the iron wool with a match. As the iron wool burns, the mass begins to increase. The mass continues to increase as more of the iron wool burns. When the iron wool stops burning, the mass is greater than it was at the start of the reaction.

So, did this reaction disobey the law of conservation of mass? Let’s consider the chemical reaction that took place. When a substance burns in the open, it reacts with oxygen gas in the air. So, during the reaction, the iron wool reacted with oxygen gas that we cannot see to produce the compound iron(III) oxide. Therefore, the mass increased because oxygen gas reacted and combined with the iron. So, even though we couldn’t see all of the reactants involved, the reaction still obeys the law of conservation of mass.

We can even determine the mass of the oxygen that reacted. We know from the law of conservation of mass that the mass of the reactants must equal the mass of the products. Therefore, the mass of the iron wool plus the mass of the oxygen must equal the mass of the iron(III) oxide produced. By subtracting 14 grams from both sides of the equation, we can determine that the mass of oxygen that reacted is 6.02 grams.

Let’s take a look at one final experiment. In this experiment, we’ve placed two beakers on a balance. One beaker contains vinegar, and the other contains baking soda. When the vinegar and baking soda are combined, lots of bubbles are formed. As this occurs, the mass begins to decrease. When the reaction stops bubbling, a liquid will remain in the beaker and the mass will be lower than before the reaction began.

So, did this reaction disobey the law of conservation of mass? We know that this reaction produces a liquid, which is still in the beaker. We also saw bubbles during the reaction, indicating that a gas was produced. This gas was carbon dioxide that bubbled away during the reaction. So, the mass the balance displays after the reaction does not include the mass of the carbon dioxide gas produced.

This reaction, like all chemical reactions, still obeys the law of conservation of mass. Using the law of conservation of mass, we can determine the mass of carbon dioxide produced by subtracting the mass after the reaction from the mass before the reaction. Therefore, the mass of carbon dioxide produced is 2.62 grams. It’s important that we recognize that all chemical reactions obey the law of conservation of mass even though it may initially appear as if they do not.

Now let’s practice what we’ve learned.

Which of these five particle diagrams correctly shows the law of conservation of mass being followed?

The law of conservation of mass states that mass is neither created nor destroyed during a chemical reaction. In order to abide by this law, a chemical equation must be balanced. A chemical equation is balanced when the number of each type of atom is the same on both sides of the reaction arrow.

So, to answer this question, we need to determine which of the five particle diagrams has the same number of black and green particles on both sides of the reaction arrow. In diagram (A), there’s one black particle on both sides of the equation. But there are two green particles on the reactant side and one green particle on the product side. Therefore, this equation isn’t balanced and does not show the law of conservation of mass being followed.

In diagram (B), the black particles are balanced, but the green particles are not. So, diagram (B) cannot be the correct answer.

In diagram (C), the green particles are balanced, but the black particles are not. So, diagram (C) isn’t the correct answer either.

In diagram (D), the number of black and green particles is the same on both sides of the reaction arrow. This equation is balanced and is likely the answer to the question. But just to be sure, let’s look at diagram (E).

In diagram (E), neither the black nor the green particles are balanced. Therefore, diagram (E) is not the correct answer to the question.

In conclusion, the diagram that correctly shows the law of conservation of mass being followed is the diagram in answer choice (D).

Mercury oxide can be heated to separate the compound into the two elements mercury and oxygen. If 4.30 grams of mercury oxide is heated, leaving only 4.00 grams of mercury, how much oxygen must have been released? Give your answer to two decimal places.

We are told in the question that mercury oxide is heated to separate the compound. So, in this reaction, mercury oxide is the only reactant. Upon heating, mercury oxide separates into the elements mercury and oxygen. So, mercury and oxygen are the products. We know the starting amount of the mercury oxide and the amount of mercury produced. We need to determine the mass of oxygen released during the reaction.

To do this, we need to recall the law of conservation of mass. This law states that mass is neither created nor destroyed during a chemical reaction. In other words, the total mass of the reactants before the reaction must equal the total mass of the products after the reaction. So, the mass of the mercury oxide must equal the mass of the mercury plus the mass of the oxygen. We can use the variable 𝑚 to represent the mass of oxygen. We can solve for the mass by subtracting 4.00 grams from both sides of the equation. This gives us a value of 0.30 grams for 𝑚. Therefore, the amount of oxygen that must have been released is 0.30 grams.

Now let’s review what we’ve learned. The law of conservation of mass states that mass is neither created nor destroyed during a chemical reaction. Balanced chemical equations obey the law of conservation of mass. Mass is conserved during all chemical reactions even though it may appear as if this is not the case when gases are involved in the reaction. As mass is conserved during a chemical reaction, the total mass of the reactants must equal the total mass of the products. We can use experimental data and the law of conservation of mass to calculate the mass of a reactant or product.