Chemistry: Conservation of Mass in Reactions Worksheet

Question 1

State the law of conservation of mass in your own words.

Accepted Answer

The law of conservation of mass says that matter is not created or destroyed in a chemical reaction, so the total mass of the reactants equals the total mass of the products in a closed system.

Question 2

In a closed container, 12 grams of hydrogen reacts with 96 grams of oxygen to form water. What is the total mass of water produced?

Accepted Answer

The total mass of water produced is 108 grams because 12 grams plus 96 grams equals 108 grams.

Question 3

A student mixes two clear liquids in a sealed bag. The mass before mixing is 145 grams. After a reaction occurs, the mass is still 145 grams. Explain why the mass did not change.

Accepted Answer

The mass did not change because the bag was sealed, so no matter could enter or leave. The atoms rearranged during the reaction, but the total amount of matter stayed the same.

Question 4

A reaction in an open beaker starts with 50 grams of vinegar and 5 grams of baking soda. After the reaction, the beaker and contents have a mass of 52 grams. Why is the measured mass less than 55 grams?

Accepted Answer

The measured mass is less because gas formed during the reaction and escaped from the open beaker. If the reaction had happened in a closed container, the total mass would still be 55 grams.

Question 5

A closed container holds 30 grams of reactant A and 18 grams of reactant B. They react completely to form one product. What is the mass of the product?

Accepted Answer

The mass of the product is 48 grams because 30 grams plus 18 grams equals 48 grams.

Question 6

Magnesium burns in oxygen to form magnesium oxide. If 24 grams of magnesium combines with 16 grams of oxygen, what mass of magnesium oxide forms?

Accepted Answer

A total of 40 grams of magnesium oxide forms because the mass of the reactants is 24 grams plus 16 grams.

Question 7

Count the atoms in this reaction: 2H2 + O2 -> 2H2O. How many hydrogen atoms and oxygen atoms are on each side?

Accepted Answer

There are 4 hydrogen atoms and 2 oxygen atoms on the reactant side. There are also 4 hydrogen atoms and 2 oxygen atoms on the product side, so the atoms are conserved.

Question 8

A diagram shows 4 carbon atoms and 8 oxygen atoms before a reaction. After the reaction, the products must contain how many carbon atoms and oxygen atoms in total?

Accepted Answer

The products must contain 4 carbon atoms and 8 oxygen atoms in total because atoms are conserved during a chemical reaction.

Question 9

A sealed jar contains iron and oxygen. The total mass before rust forms is 200 grams. What should the total mass be after rust forms inside the sealed jar?

Accepted Answer

The total mass after rust forms should be 200 grams because no matter can enter or leave the sealed jar.

Question 10

A science class records the mass of a closed reaction system as 86.4 grams before a reaction. After the reaction, the mass is 86.1 grams. Give one possible reason for the small difference.

Accepted Answer

One possible reason is measurement error, such as reading the balance incorrectly or using a balance that was not calibrated. In a truly closed system, the mass should stay the same.

Question 11

A chemical reaction has 75 grams of reactants. The products are 30 grams of solid and an unknown mass of gas. In a closed system, what is the mass of the gas?

Accepted Answer

The mass of the gas is 45 grams because the total product mass must equal 75 grams, and 75 grams minus 30 grams equals 45 grams.

Question 12

The reaction C + O2 -> CO2 represents carbon reacting with oxygen to form carbon dioxide. Is the number of atoms conserved in this equation? Explain.

Accepted Answer

Yes, the number of atoms is conserved. There is 1 carbon atom and 2 oxygen atoms on the reactant side, and there is 1 carbon atom and 2 oxygen atoms on the product side.

Question 13

A student says, 'The candle wax disappeared when the candle burned, so mass was destroyed.' Explain why this statement is not correct.

Accepted Answer

The statement is not correct because the wax reacted with oxygen and formed new substances, such as carbon dioxide and water vapor. The atoms were rearranged, not destroyed.

Question 14

A tablet is dropped into water in a sealed bottle. The bottle expands slightly as bubbles form, but the cap stays on. The mass before the reaction is 132 grams. What is the mass after the reaction, and why?

Accepted Answer

The mass after the reaction is 132 grams because the bottle is sealed. The gas bubbles are still inside the bottle, so the total mass stays the same.

Question 15

A reaction uses 10 grams of substance X and 25 grams of substance Y. It produces 20 grams of substance Z and 15 grams of substance W. Does this data support the law of conservation of mass? Explain.

Accepted Answer

Yes, the data supports the law of conservation of mass because the reactants have a total mass of 35 grams and the products also have a total mass of 35 grams.

Chemistry: Conservation of Mass in Reactions

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