Q: Which equation correctly represents the Law of Conservation of Mass for a chemical reaction?

$$\text{Total mass of reactants} = \text{Total mass of products}$$

Question 1

Which statement best describes the Law of Conservation of Mass as originated by Antoine Lavoisier?

Accepted Answer

The total mass of reactants equals the total mass of products in a chemical reaction.

Question 2

Given the reaction $$2 \mathrm{H}_2 (g) + 1 \mathrm{O}_2 (g) \rightarrow 2 \mathrm{H}_2\mathrm{O} (g)$$, if you start with 25.0 grams of hydrogen and 12.0 grams of oxygen, what is the total mass of water vapor produced?

Accepted Answer

$$37.0$$ grams

Question 3

In the reaction $$\mathrm{N}_2 + 3 \mathrm{H}_2 \rightarrow 2 \mathrm{NH}_3$$, if 50.0 grams of hydrogen react to produce 92.5 grams of ammonia, what is the minimum amount of nitrogen required?

Accepted Answer

$$4.625$$ grams

Question 4

A student mixes 112.6 grams of calcium with 24.0 grams of oxygen to produce 126.6 grams of calcium oxide. According to the Law of Conservation of Mass, how much oxygen remains unreacted?

Accepted Answer

$$2.7$$ grams

Question 5

Which of the following is always true for a balanced chemical equation?

Accepted Answer

The number of atoms of each element is the same on both sides of the equation.

Question 6

If 10.0 grams of reactant A reacts completely with 20.0 grams of reactant B to form product C, what is the mass of product C?

Accepted Answer

$$30.0$$ grams

Question 7

In a chemical reaction, which term refers to the substances present before the reaction occurs?

Accepted Answer

Reactants

Question 8

A reaction is performed in a closed system. If the total mass of reactants is 100 grams, but only 95 grams of products are measured, what is the most likely explanation?

Accepted Answer

Some product was lost or not measured.

Question 9

Which equation correctly represents the Law of Conservation of Mass for a chemical reaction?

Accepted Answer

$$	ext{Total mass of reactants} = 	ext{Total mass of products}$$

Question 10

In the reaction $$\mathrm{C} + \mathrm{O}_2 \rightarrow \mathrm{CO}_2$$, if 12 grams of carbon reacts with 32 grams of oxygen, what is the mass of carbon dioxide produced?

Accepted Answer

$$44$$ grams

Question 11

A chemical reaction occurs in an open container and some gas escapes. How does this affect the measured mass of products compared to the reactants?

Accepted Answer

The measured mass of products will be less than the mass of reactants.

Question 12

Which of the following is NOT a direct consequence of the Law of Conservation of Mass?

Accepted Answer

Energy is always conserved in chemical reactions.

Question 13

If a reaction produces 200 grams of product from 80 grams of reactant A and an unknown amount of reactant B, what is the mass of reactant B?

Accepted Answer

$$120$$ grams

Question 14

In the reaction $$\mathrm{N}_2 + 3 \mathrm{H}_2 \rightarrow 2 \mathrm{NH}_3$$, if 28 grams of nitrogen and 6 grams of hydrogen are used, what is the expected mass of ammonia produced?

Accepted Answer

$$34$$ grams

Question 15

A reaction has the following equation: $$\mathrm{Ca} + \mathrm{O}_2 \rightarrow \mathrm{CaO}. If 40 $$grams of calcium reacts with 16 grams of oxygen, what is the mass of calcium oxide formed?

Accepted Answer

$$56$$ grams

Question 16

Which of the following best explains why chemical equations must be balanced?

Accepted Answer

To ensure the Law of Conservation of Mass is obeyed.

Question 17

If 15 grams of reactant X and 25 grams of reactant Y produce 35 grams of product Z, what is the mass of unreacted material?

Accepted Answer

$$5$$ grams

Question 18

In a reaction, 100 grams of reactants produce 80 grams of products. What could account for the missing mass?

Accepted Answer

A gas was released and not measured.

Question 19

Which scientist is credited with formulating the Law of Conservation of Mass?

Accepted Answer

Antoine Lavoisier

Question 20

A chemical reaction is performed in a sealed container. If the mass of reactants is 150 grams, what should be the mass of products?

Accepted Answer

$$150$$ grams

The Law of Conservation of Mass and Stoichiometry in Chemical Reactions

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