4B2 Stoichiometric Calculations

Question 1

Define:

mole ratio

Answer 1

A ratio that shows the relationship between the number of moles of substances in a chemical reaction.

Mole ratios are derived from the coefficients in a balanced chemical equation.

Question 2

How do mole ratios help in stoichiometry problems?

Answer 2

They act as conversion factors to calculate quantities of reactants or products.

For example, the ratio 2H₂ / 1O₂ allows conversion from moles of H₂ to moles of O₂.

Question 3

What is the mole ratio formula?

Answer 3

Coefficient of the desired substance divided by the Coefficient of the given substance.

For example, in 2H₂ + O₂ → 2H₂O, the ratio of H₂O to O₂ is 2/1.

Question 4

What is the mole ratio of Ca to Ca₃N₂ in the reaction 3Ca + N₂ → Ca₃N₂?

Answer 4

3 moles Ca / 1 mole Ca₃N₂

The coefficients in the balanced equation (3 and 1) give the ratio.

Question 5

How much calcium is required to produce 5.4 moles of Ca₃N₂?

The molar ratio is 3 moles Ca / 1 mole Ca₃N₂.

Answer 5

16.2 moles Ca

The mole ratio used is 3 moles Ca / 1 mole Ca₃N₂.

Question 6

Which unit must be included in a mole ratio?

Answer 6

Moles

Mole ratios always refer to quantities of substances measured in moles.

Question 7

How many moles of NH₃ are produced from 5 moles of H₂ in the reaction N₂ + 3H₂ → 2NH₃?

Answer 7

3.33 moles NH₃

Using the mole ratio 2NH₃ / 3H₂, multiply 5 moles H₂ by the ratio (2/3).

Question 8

True or False:

Mole ratios can be used in reverse.

Answer 8

True

The reciprocal of a mole ratio is also valid (e.g., 2H₂ / 1O₂ = 1O₂ / 2H₂).

Question 9

Fill in the blanks:

In stoichiometry, mole ratios are used to convert moles of a ______ substance to moles of a ______ substance.

Answer 9

given; desired

This conversion allows solving for unknown quantities in a reaction.

Question 10

How many moles of O₂ are needed to react with 2 moles of CH₄ in the equation CH₄ + 2O₂ → CO₂ + 2H₂O?

Answer 10

4 moles O₂

The mole ratio O₂ to CH₄ is 2:1, so multiply 2 moles CH₄ by 2.

Question 11

List one example where mole ratios are used in real-world applications.

Answer 11

Calculating the amount of reactants needed for industrial chemical production.

Mole ratios are essential in designing reactions for efficiency and cost-effectiveness.

Question 12

Define:

stoichiometry

Answer 12

The study of the quantitative relationships between substances in a chemical reaction.

It uses mole ratios to calculate quantities of reactants or products.

It is based on balanced chemical equations.

Question 13

Which law underpins stoichiometric calculations?

Answer 13

The law of conservation of mass.

This law states that mass is neither created nor destroyed in a chemical reaction.

Question 14

Fill in the blank:

The periodic table provides the _______ of elements, which is used to calculate molar mass.

Answer 14

atomic mass

The atomic mass allows conversion between grams and moles for stoichiometric calculations.

Question 15

Define:

molar mass

Answer 15

The mass of one mole of a substance, measured in grams per mole (g/mol).

Molar mass is calculated by summing the atomic masses of all atoms in the molecule.

Question 16

What information is needed to find the mass of a substance from its moles?

Answer 16

The molar mass of the substance.

Question 17

How is the number of moles calculated from mass?

Answer 17

Divide the mass of the substance by its molar mass.

The formula is: moles=mass/molar mass.

Question 18

What is the purpose of using mole ratios in stoichiometry?

Answer 18

To relate the amounts of reactants and products based on the balanced chemical equation.

Mole ratios are derived from the coefficients in the balanced equation.

Question 19

List the three steps in solving a mass-to-mass stoichiometry problem.

Answer 19

1. Convert mass of A to moles of A.
2. Use the mole ratio to find moles of B.
3. Convert moles of B to mass of B.

massA → molesA → molesB → massB.

These steps ensure proper use of mole ratios and molar masses.

Question 20

What is the mass of water formed when 74 moles of H₂ react completely?

Answer 20

74×18=1332 grams

The calculation uses the stoichiometric ratio 2H₂:2H₂O and the molar mass of water.

The molar mass of H₂O = 18 g/mol.

Question 21

Define:

molarity

Answer 21

The number of moles of a solute per liter of solution.

Molarity quantifies the concentration of a solute in a solution, expressed in moles per liter.

Question 22

How do you calculate molarity?

Answer 22

Divide the moles of solute by the volume of solution in liters.

The formula is: Molarity=moles of solute/liters of solution.

Question 23

How do you calculate the number of moles from molarity and volume?

Answer 23

Multiply the molarity by the volume in liters.

The formula is Moles=Molarity×Volume (L).

Question 24

True or False:

Molarity is the ratio of moles of solute to moles of solvent.

Answer 24

False

Molarity is defined as the moles of solute per liter of solution, not per mole of solvent.

Question 25

What is the molarity of a solution with **48 grams of sucrose in 0.2 liters**?

Answer 25

0.7 M ## Footnote Use the formula M=moles of sucrose/volume (L). Calculate the moles using the formula: moles=mass/molar mass. The molar mass of sucrose is 342.2965 grams per mole.

Question 26

What is the resulting **molarity of ammonia** when 4.46 moles are dissolved in 5 liters of water?

Answer 26

0.89 M ## Footnote Calculation: 4.46 moles / 5 L = 0.89 moles/L.

Question 27

# Fill in the blank: The \_\_\_\_\_\_\_ are listed on the **left side** of a chemical equation.

Answer 27

reactants ## Footnote Reactants undergo transformation to produce products, which are on the right side of the equation.

Question 28

In the reaction 2HCl + Zn → ZnCl₂ + H₂, **what are the reactants**?

Answer 28

HCl and Zn ## Footnote Reactants are the starting substances in a chemical reaction.

Question 29

# Define: limiting reactant

Answer 29

The reactant in a chemical reaction that is **completely consumed**. ## Footnote Limiting reactants set the maximum yield of a reaction by limiting the amount of other reactants that can combine.

Question 30

# Define: excess reactant

Answer 30

A reactant that is **not completely consumed** after a chemical reaction is complete. ## Footnote It is present in greater quantities than required by the stoichiometric ratio.

Question 31

# True or False: The product formed **depends only** on the excess reactant.

Answer 31

False ## Footnote The amount of product formed is determined by the limiting reactant.

Question 32

What is the purpose of an **excess reactant**?

Answer 32

To ensure that the **limiting reactant is fully used** during the reaction. ## Footnote Excess reactants remain unreacted because they are in quantities greater than needed.

Question 33

List the steps to find the **limiting and excess reactants**.

Answer 33

1. Write a balanced equation. 1. Convert quantities to moles. 1. Compare mole ratios. 1. Identify the limiting reactant and excess reactant. ## Footnote Each step ensures accuracy in determining which reactants are limiting or in excess. The reactant with a lower quantity relative to its stoichiometric ratio is the limiting reactant.

Question 34

What **analogy** is used to explain limiting and excess reactants?

Answer 34

**Assembling bicycles**: * Two wheels and one frame that make a bicycle represent the limiting reactants. * Leftover frames represent the excess reactant. ## Footnote The analogy simplifies the concept by comparing chemical reactions to tangible objects.

Question 35

How do you **find the excess reactant**?

Answer 35

* Determine the limiting reactant. * Calculate how much of the excess reactant is consumed. * Subtract it from the initial quantity. ## Footnote Identifying the excess reactant requires knowing the limiting reactant first.

Question 36

What are the steps to **calculate the amount of product** formed?

Answer 36

* Identify the limiting reactant. * Use the balanced equation to determine the product's stoichiometric ratio. * Convert moles of the limiting reactant to grams of product. ## Footnote Accurate calculations depend on these systematic steps.

Question 37

# Fill in the blank: In the reaction 2H₂ + O₂ → 2H₂O, oxygen is the \_\_\_\_\_\_ reactant when **10 moles of H₂ react with 7 moles of O₂**.

Answer 37

excess ## Footnote Based on stoichiometry, only 5 moles of O₂ are needed, leaving 2 moles unreacted.

Question 38

# Define: theoretical yield

Answer 38

The maximum amount of **product that could be formed** in a reaction under perfect conditions. ## Footnote Theoretical yield is determined using stoichiometry and a balanced chemical equation.

Question 39

# Define: percent yield

Answer 39

The ratio of the **actual yield to the theoretical yield**, multiplied by 100. ## Footnote Percent yield = (actual yield / theoretical yield) × 100. The result is expressed as a percentage.

Question 40

What does **percent yield tell us**?

Answer 40

The **efficiency of a chemical reaction** by comparing actual and theoretical yields. ## Footnote It helps identify how much product was lost due to errors or inefficiencies. A good percent yield, typically above 80%, indicates efficient recovery of product.

Question 41

List *three* reasons why a percent yield might **exceed 100%**.

Answer 41

1. Contamination 1. Incorrect measurements 1. Competing reactions ## Footnote A percent yield over 100% often indicates errors in experimental procedures or unintended reactions.

Question 42

List *two* reasons why percent yield might be **less than 100%**.

Answer 42

1. Human error 1. Product loss during recovery ## Footnote Other reasons include incomplete reactions, competing side reactions, or contamination.

Question 43

How do you **calculate theoretical yield**?

Answer 43

Theoretical yield = (given mass of reactant × molar mass of product) / molar mass of reactant. ## Footnote Stoichiometric ratios from the balanced equation are used in the calculation.

Question 44

# Fill in the blank: Theoretical yield is calculated **based on** the \_\_\_\_\_\_\_ reactant.

Answer 44

limiting ## Footnote The limiting reactant determines the maximum amount of product that can be formed.

Question 45

# True or False: Theoretical yield **can be greater** than the actual yield.

Answer 45

True ## Footnote Theoretical yield represents the maximum possible product, while actual yield is typically less due to inefficiencies or errors.

Question 46

What is the **first step** in calculating theoretical yield?

Answer 46

Balance the chemical equation. ## Footnote A balanced equation ensures the law of conservation of mass is upheld during stoichiometric calculations.