Chapter 7 part 2: Stoichiometry

Question 1

What is Stoichiometry?

Answer 1

• Stoichiometry is the relationship between the number of moles of different substances involved in a chemical reaction.

Question 2

What is the mole ratio?

Answer 2

• The mole ratio can be the ratio of the amount/the number of moles of substances that are required for a complete reaction/formed at the end of the reaction.

Question 3

How can we find the mole ratio?

Answer 3

• The mole ratio can be determined from the Stoichiometric ratios of different substances in a balanced chemical equation

Question 4

What are one of the functions/uses of mole ratio(s)?

Answer 4

• The mole ratio of a substance can be used to determine the number of moles of the substances.

Question 5

What is the volumetric analysis?

Answer 5

• The volumetric analysis usually involves the use of 2 solutions, namely (usually) the analyte and the titrant.
• This method is known as titration.

Question 6

What is Titration?

Answer 6

• Titration is the process that is a volumetric analysis that involves the use of the analyte and the titrant.

Question 7

What is an analyte?

Answer 7

• An analyte is a solution with an unknown concentration.
• A fixed volume of analyte is placed into a conical flask using a pipette.

Question 8

What is a titrant?

Answer 8

• A titrant is a solution with a known concentration.
• It is placed in a burette and is added to the analyte during titration.

Question 9

What is the substance that is added into the analyte in the conical flask before the titrant is added?

Answer 9

• An indicator is added into the analyte in the conical flask before the titrant is added
• The indicator then changes colour when the titrant that was added has reacted completely with the analyte.
• This shows that the titration has reached its end-point.

Question 10

What is a limiting reactant?

Answer 10

• A limiting reactant is the reactant that is completely used up in a reaction.

Question 11

What is the dependent variable of the amount of product that is formed?

Answer 11

• The amount of the product formed depends on the amount of the limiting reactant.

Question 12

How is the limiting reactant presented?

Answer 12

• The limiting reactant is presented in a smaller amount/has a smaller number of moles, compared to other reactants.

Question 13

What does the mole ratio of reactants show?

Answer 13

• The mole ratio of the reactants obtained from the chemical equation of the reaction shows the Stoichiometric ratio of reactants that should be used.

Question 14

What happens if the reactants are not used in the correct Stoichiometric ratio?

Answer 14

• If the reactants are not used in the correct Stoichiometric ratio, then the limiting reactant must be determined.
• E.g. of how a reactant can be determined:
  1. 16g of sulfur with 20g of oxygen to form sulfur dioxide gas, SO2.
  (a) Write a balanced chemical equation:
• S + O2 –> SO2

(b) Calculate the number of moles of sulfur and oxygen used.
- Number of moles of sulfur
= 16/32
= 0.500 mol
- Number of moles of oxygen
= 20/(2x16)
= 20/32
= 0.625 mol

(c) Obtain the mole ratio of sulfur to oxygen from the equation and use it to determine the number of moles of reactants required.
- From the equation, 1 mol of sulfur reacts with 1 mol of oxygen.
- The number of moles of sulfur
= The number of moles of oxygen = 0.500 mol

(d) Determine the limiting reactant based on the number of moles of reactants required.
- From the mole ratio of sulfur to oxygen, 0.500 mol of oxygen is required, but 0.625 mol of oxygen is present.
- Thus, oxygen is in excess and sulfur is the limiting reactant.

Question 15

What happens to the products in a reversible reaction?

Answer 15

• In a reversible reaction, the products can decompose to form reactants.

Question 16

What happens to the conversion of reactants into products when an impure reactant is used?

Answer 16

• When an impure reactant is used, not all of the reactants are converted inot products.

Question 17

How can we represent yield?

Answer 17

• Yield can be in the form of the mass/volume/number of moles of a substance.

Question 18

What is the theoretical yield?

Answer 18

• the theoretical yield is the amount of product formed when all the reactants react completely in a reaction.

Question 19

What is the actual yield?

Answer 19

• The actual yield is the actual amount of product formed in a reaction.
• This value that is formed is always smaller than the theoretical yield.

Question 20

How can we determine the percentage yield?

Answer 20

• The percentage yield can be determined using:
• Percentage yield
  = Actual yield/theoretical yield x 100%

Question 21

How can we calculate the percentage purity?

Answer 21

• We can calculate the percentage purity using mass/volume/number of moles of a substance.

Question 22

Is the number/amount of a pure substance always smaller/bigger than the amount of the same substance that is present in the reaction?

Answer 22

• YES, the amount of a pure substance is ALWAYS smaller than the amount of the same substances present in the reaction.