APCE Chemical Reactions & Stoichiometry

Question 1

What does the Law of Conservation of Mass state?

Accepted Answer

Mass is conserved in a chemical reaction.

Answer

The Law of Conservation of Mass states that in any closed system, the total mass of the reactants before a chemical reaction must equal the total mass of the products after the reaction. This means that mass cannot be created or destroyed, only rearranged, during a chemical change. It is a fundamental principle in chemistry and physics.

Question 2

What is stoichiometry used for in chemical reactions?

Accepted Answer

To calculate the amounts of reactants and products.

Answer

Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. It uses balanced chemical equations to determine the precise amounts (moles, mass, volume) of substances consumed or produced. This allows chemists to predict yields and optimize reaction conditions.

Question 3

What is the molar ratio in a balanced chemical equation?

Accepted Answer

The ratio of moles of reactants to moles of products.

Answer

In a balanced chemical equation, the coefficients in front of each chemical formula represent the molar ratio. This ratio indicates the relative number of moles of each reactant consumed and each product formed during the reaction. It is crucial for stoichiometric calculations, allowing conversion between the amounts of different substances in a reaction.

Question 4

What is the limiting reagent in a chemical reaction?

Accepted Answer

The reactant that is used up first.

Answer

The limiting reagent (or limiting reactant) is the substance that is completely consumed first in a chemical reaction. Its quantity determines the maximum amount of product that can be formed, as the reaction stops once it runs out. All other reactants are considered to be in excess.

Question 5

What is the mole concept used for in stoichiometry?

Accepted Answer

To convert between mass, volume, and number of particles in a reaction.

Answer

The mole concept provides a bridge between the macroscopic world (mass, volume) and the microscopic world (number of atoms/molecules). In stoichiometry, it allows chemists to convert between grams, liters (for gases), and the number of moles of a substance, which is essential for relating the quantities of different substances in a balanced chemical equation.

AP Practice Test

AP Practice Test

APCE Chemical Reactions & Stoichiometry