Video: Application of the Law of Conservation of Mass

Antoine Lavoisier was one of the first scientists to state the law of conservation of mass. When he heated a sample of tin and air in an air-tight flask, tin reacted with oxygen in the air to produce tin oxide. Did the mass of the flask and its contents increase, decrease or stay the same because of this reaction?

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Video Transcript

Antoine Lavoisier was one of the first scientists to state the law of conservation of mass. When he heated a sample of tin and air in an air-tight flask, tin reacted with oxygen in the air to produce tin oxide. Did the mass of the flask and its contents increase, decrease or stay the same because of this reaction?

Antoine Lavoisier, the so-called father of modern chemistry, lived from 1743 to 1794 and is credited with the discovery of four different chemical elements. The law of conservation of mass is one of the foundational principles of modern chemistry. The law of conservation of mass states that, in an isolated system, mass is not created or destroyed by chemical reactions or physical processes. For chemists, what this means is that if we have a reaction where reactants are transforming into products, if the total mass of the reactants is 100 grams, the total mass of the products will also be 100 grams.

Now let’s have a look at the scenario mentioned in the question. We have an air-tight flask containing air and a sample of tin. This is being heated, and the tin is reacting with oxygen in the air to produce tin oxide. Air is only about 21 percent oxygen and the rest is mostly nitrogen. What the question is asking is, what happened to the mass of the flask and its contents because of this reaction?

Before, we have a flask containing tin and air. And after, we have a flask containing less oxygen and tin oxide. The whole set-up is an isolated system. Air from the outside cannot get in and substances inside cannot get out. Tin reacts with oxygen to form tin(IV) oxide, otherwise known as stannic oxide.

If, for instance, we were working with one liter of air to start with, we’d have about 0.28 grams of oxygen. To fully react with the oxygen, we’d need about a gram of tin. After the reaction, we would produce 1.28 grams of tin oxide. This is an example of a chemical reaction. So, the law of conservation of mass applies. Therefore, the total product mass is equal to the sum of the masses of the reactants.

So, how do you put this in writing? So, the mass lost from the air inside the flask was gained by the tin. The mass of the flask would stay the same. Therefore, the mass of the flask and its contents must’ve stayed the same too. So, here we have our answer.

Although, technically speaking, nowadays we have instruments that are so accurate that we can detect the miniscule changes in mass because of bonding energy. However, these are so so very small that these can be ignored. Also, bear in mind that the law of conservation of mass does not apply to nuclear reactions. For instance, in the conversion of hydrogen to helium in the sun, 0.7 percent of the mass is converted into energy. But that’s not what we’re dealing with here. In chemical reactions, we can assume that the difference in mass between the reactants and the products is zero.

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