Chapter 27 - Stoichiometry

Question 1

Define a compound:

Answer 1

A pure substance composed of 2 or more elements in a fixed proportion

Question 2

How are compounds broken down? What does this produce?

Answer 2

Chemically to produce their constituent elements or other compounds

Question 3

What elements can react with other elements to form new compounds?

Answer 3

All elements except noble gases

Question 4

Define a molecule:

Answer 4

A combination of 2 or more atoms held together by covalent bonds and is the smallest unit of a compound that displays the properties of that compound

Question 5

Do ionic compounds form true molecules?

Answer 5

No

Question 6

Define molecular mass:

Answer 6

The sum of the atomic masses of the atoms that make up a molecule

Question 7

Define formula mass:

Answer 7

The sum of the atomic masses of each constituent atom according to the empire formula of the substance

Question 8

Define a mole:

Answer 8

Amount of a substance that contains the same number of particles found in a 12g sample of carbon-12
- Avogadros Number: 6.002 x 10^23

Question 9

What is the mass of one mole of a compound called? What is it usually expressed in?

Answer 9

Molar mass or molar weight

- g/mol

Question 10

What is the formula for “number of moles”?

Answer 10

Mols = Weight of Sample (g) / Molar Weight (g/mol)

Question 11

How many moles are in 9.52g of MgCl2 (page314)?

Answer 11

0.10 mol of MgCl2

Question 12

Why is it important to define a measure of reactive capacity for some substances? What is this called?

Answer 12

Expresses the fact that some molecules are more potent than others in performing certain reactions

This difference is expressed by using the term equivalent

Question 13

How do you determine equivalents? What is the equation?

Answer 13

By a new measure of weight called gram-equivalent weight (GEW):

Equivalents = Weight of Compound / GEW

AND

GEW = molar mass / n

*n = number of hydrogen atoms that could be donated/accepted

Question 14

What is the Law of Constant Composition?

Answer 14

States that all samples of a given compound will contain the same elements in identical mass ratios

Question 15

What are the 2 ways to express a formula for a compound?

Answer 15

1) Empirical Formula: Gives simplest whole number ratio of the elements in the compound

2) Molecular Formula: Gives the exact number of atoms of each element in the compound and is a multiple of the empirical formula
- Simplifying this formula will give the empirical formula

Question 16

Define percent composition:

Answer 16

The mass percent of the element in a specific compound

Question 17

How do you determine percent composition?

Answer 17

Percent Composition = [Mass of X in Formula / Formula Weight of Compound] x 100

Question 18

Can you determine the percent composition with the empirical and molecular formulas?

Answer 18

Yes

• If the percent composition is known, the empirical formula can be derived
• If both percent composition and molecular mass is know, the molecular formula can be derived

Question 19

What is the percent composition of chromium in K2Cr2O7 (page 316)?

• K = 39g/mol
• Cr = 52g/mol
• O = 16g/mol

Answer 19

35.4%

Question 20

What are the empirical and molecular formulas of a compound that contains 40.9% C, 4.58% H, and 52.52% O by mass and has a molecular mass of 264g/mol (page316/317)?

Answer 20

Molecular formula = C9H12O9

Empirical formula = C3H4O3

Question 21

What is the Law of Conservation of Mass? Why is this important? What is used to demonstrate this?

Answer 21

• The mass of reactants in a reaction must be equal to the mass of the products
• Chemical equations must be balance so that there are same number of atoms of each element in the product as there are in the reactants
• Stoichiometric coefficients are used to indicate number of moles of a species involved in the reaction

Question 22

What steps are used in stoichiometry to achieve a balanced equation?

Answer 22

1) Balance carbon reactant/products
2) Balance H reactant/products
3) Balance O reactant/products
4) Achieve whole numbers by multiplying/dividing, etc

Question 23

What is an application of stoichiometry?

Answer 23

Once an equation is balance, it can be used to solve many types of problems:

• Limiting reagent
• Yield

Question 24

Application of Stoichiometry: How many grams of CaCl2 is needed to prepare 72g of AgCl according to the following equation (page 318)? CaCl2 + 2AgNO3 —-> Ca(NO3)2 + 2AgCl

• Ca = 40g/mol
• Cl = 35.5g/mol
• Ag = 108g/mol

Answer 24

27.5g of CaCl2 is needed to produce 72g of AgCl

Question 25

What is a limiting reagent? What is an excess reactant?

Answer 25

In most reactions, one reactant will be confused first (limiting reagent) which limits the amount of product formed The excess reactant is what remains after the limiting reagent is used up

Question 26

If 28g of Fe are reacted with 24g of S to produce FeS, what is the limiting reactant? How many grams of excess reactant are present in the vessels at the en of the reaction (page 319)?

Answer 26

8g of S

Question 27

What is the yield of the reaction? Name the 3 types:

Answer 27

The amount of product predicted or obtained when the reaction is carried out 1) Theoretical: Product predicted from a balanced equation 2) Actual: Product isolated from reaction experimentally 3) Percent Yield: Express relationship between actual and theoretical which is expressed by Percent Yield = [Actual / Theoretical] x 100

Question 28

What is the percent yield for a reaction in which 27g of Cu is produced by reacting 32.5g of Zn in excess CuSO4 solution (page 319)? - Cu = 63.5g/mol - Zn = 65g/mol - S = 32g/mol - O = 16g/mol

Answer 28

84%