Exercises: Law of conservation of mass (Lavoisier)

Complete the sentence with the missing words: "In any chemical reaction, the total mass of the $\dots$ is equal to the total mass of the $\dots$: mass is neither created nor $\dots$, it is only transformed."

Olivia weighs two solutions in a sealed beaker: before mixing them, the balance reads $185\ \text{g}$. When mixed, a white precipitate appears. The figure shows the sealed beaker on the balance, before and after mixing.

What will the balance read after the reaction? Justify your answer using Lavoisier's law (the law of conservation of mass).

We leave a burning candle on a balance, in the open air, and see that the reading gradually decreases, as shown in the figure.

Does this mean that matter is being destroyed? Where does the mass go that the balance no longer detects?

In a reaction, $12\ \text{g}$ of carbon combine exactly with $32\ \text{g}$ of oxygen, with nothing left over, as shown in the diagram.

What mass of carbon dioxide is formed?

When heating $10\ \text{g}$ of copper in the presence of air, $12.5\ \text{g}$ of copper oxide are formed, as shown in the diagram.

Why does the oxide weigh more than the copper it comes from? What mass of oxygen has combined?

When cooking with butane gas, the cylinder gradually loses mass. Does this mean that matter disappears? Explain where the mass lost by the cylinder goes.

An ice cube of $30\ \text{g}$ melts completely inside a tightly covered glass. How much will the resulting liquid water weigh? Has a chemical reaction taken place? Explain your reasoning.

Mateo bakes a cake: the raw batter weighs $800\ \text{g}$ and, when taken out of the oven, the cake weighs $750\ \text{g}$. What has happened to the missing $50\ \text{g}$? Has that matter been destroyed?

We dissolve an effervescent tablet of $4\ \text{g}$ in an open glass with $150\ \text{g}$ of water. When the bubbling stops, the glass with its contents weighs $152\ \text{g}$. The figure shows the open glass on the balance.

1. a) What mass of gas has escaped into the air?
2. b) How much would the whole assembly weigh if the experiment had been carried out in a sealed container?

A magnesium ribbon of $6\ \text{g}$ burns in air, and $10\ \text{g}$ of powdered magnesium oxide are collected.

1. a) What mass of oxygen from the air has combined with the magnesium?
2. b) If we burned $12\ \text{g}$ of magnesium with excess oxygen, what mass of oxide would we obtain?

Inside an airtight glass bell jar, there are $250\ \text{g}$ of air and a candle of $20\ \text{g}$. We light it, and after a while, the candle weighs $14\ \text{g}$. The figure shows the sealed bell jar.

1. a) What is the total mass of the contents of the bell jar at that moment?
2. b) By how much has the mass of the air mixed with the gases produced increased?

In a sealed beaker, we mix $100\ \text{g}$ of one solution with $50\ \text{g}$ of another, forming a solid precipitate of $8\ \text{g}$.

1. a) What is the total mass of the beaker's contents after the reaction?
2. b) If we then filter and remove the dry precipitate, what mass remains in the liquid?

An iron nail of $28\ \text{g}$ is left outdoors, and when it has completely rusted, the iron oxide formed weighs $40\ \text{g}$.

1. a) What mass of oxygen has been bound from the air?
2. b) Express that mass of oxygen in milligrams.

In a camping stove, $58\ \text{g}$ of butane are burned, consuming $208\ \text{g}$ of oxygen from the air.

1. a) What total mass of products (carbon dioxide and water vapor) is formed?
2. b) If the water produced is $90\ \text{g}$, what mass of carbon dioxide has been emitted?

An open container contains $200\ \text{g}$ of hydrogen peroxide, which decomposes slowly, releasing oxygen gas. When the decomposition finishes, $184\ \text{g}$ of water remains.

1. a) What mass of oxygen has escaped from the container?
2. b) Explain why this result does not contradict the law of conservation of mass.

A motorcycle's tank loses $5\ \text{kg}$ of gas on a trip, but $22\ \text{kg}$ of gases come out of the exhaust pipe.

1. a) Where does the extra mass expelled by the exhaust pipe come from?
2. b) Calculate the mass of oxygen from the air that took part in the combustion.

In a lime kiln, $100\ \text{kg}$ of limestone are heated to produce $56\ \text{kg}$ of quicklime.

1. a) A worker claims: "Mass has been lost, so Lavoisier was wrong." Explain to him why this is not the case.
2. b) Calculate the mass of carbon dioxide that has escaped through the smokestack.
3. c) How would you repeat the experiment to test the law of conservation of mass directly?

Lavoisier heated mercury for twelve days in a retort (a sealed glass vessel) containing air. The mercury became covered with a red powder and its mass increased, while the air inside lost part of its volume. The figure shows the sealed retort.

1. a) Why did the mass of the mercury increase?
2. b) Where did the gained mass come from?
3. c) Explain why the total mass of the sealed retort did not change during the experiment.

Design an experiment using a bottle, a balloon, vinegar and baking soda to demonstrate the law of conservation of mass even when a gas is given off in the reaction. Describe the setup step by step, predict what the balance will read before and after the reaction, and point out two experimental errors that could ruin the verification.

A log of $2\ \text{kg}$ burns in the fireplace, and when the fire goes out, only $100\ \text{g}$ of ash remains.

1. a) Does it mean that $1.9\ \text{kg}$ of matter have disappeared?
2. b) Explain what products have been formed and where they are now.
3. c) What would you need to measure to verify that mass is conserved?

We heat a roll of steel wool of $50\ \text{g}$ on a balance until it burns; the remaining residue weighs $68\ \text{g}$. The figure shows the balance before and after burning.

1. a) Has the mass increased or decreased? Where did the difference come from?
2. b) Predict what the balance would read if we could also weigh the air that reacted.
3. c) Explain why this experiment showing a "gain" in mass confirms Lavoisier's law just as those showing a "loss" do.

In a sealed spacecraft, an astronaut consumes about $840\ \text{g}$ of oxygen per day and exhales about $1000\ \text{g}$ of carbon dioxide and about $400\ \text{g}$ of water vapor.

1. a) The mass of what he exhales exceeds that of the oxygen he breathes: where does the difference come from?
2. b) Does the total mass of the spacecraft and all its contents change? Justify your answer.
3. c) If mass is conserved, why is a system needed to purify the cabin air?

A group of students checks the conservation of mass in a precipitation reaction and notes down: mass before mixing, $164.8\ \text{g}$; mass after, $164.6\ \text{g}$.

1. a) Do these data refute Lavoisier's law?
2. b) Calculate the difference between the two measurements and express it in milligrams.
3. c) Give two possible experimental causes for this slight discrepancy.

At a party, a sparkler of $80\ \text{g}$ burns completely, and the solid residue left on the stick weighs $52\ \text{g}$.

1. a) How much mass has the sparkler lost on burning?
2. b) A friend says: "Since it has lost mass, the law of conservation of mass does not apply here." Correct him.
3. c) What role does oxygen from the air play in the mass of the smoke emitted?