CHAPTER 4

Stoichiometry

2

Calculations Based on Chemical Equations

How many CO molecules are required to react with 25 molecules of Fe2O3?

Fe O + 3 CO 2 Fe + 3 CO2 3 2

3

Calculations Based on Chemical Equations

What mass of CO is required to react with 146 g of iron (III) oxide?

Fe O + 3 CO 2 Fe + 3 CO2 3 2

4

Calculations Based on Chemical EquationsWhat mass of carbon dioxide can be produced by

the reaction of 0.540 mole of iron (III) oxide with carbon monoxide?

Fe O + 3 CO 2 Fe + 3 CO2 3 2

5

Calculations Based on Chemical Equations

How many iron atoms can be produced by the reaction of 2.50 x 105 molecules of iron (III) oxide with excess carbon monoxide?

Fe O + 3 CO 2 Fe + 3 CO2 3 2

6

Calculations Based on Chemical Equations

What mass of iron (III) oxide reacted with carbon monoxide if the carbon dioxide produced by the reaction had a mass of 8.65 grams?

You do it!You do it!

Fe O + 3 CO 2 Fe + 3 CO2 3 2

7

Limiting Reactant Concept

The reagent consumed first is the limiting reactant (or limited reagent)

– -A chemical reaction proceeds forward according to the stoichiometry of the reaction until one (or both) reactants are consumed

8

Limiting Reactant Concept

• Kitchen example of limiting reactant concept:

1 packet of muffin mix + 2 eggs + 1 cup of milk

makes 12 muffins

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Limiting Reactant Concept

• Mix Packets Eggs Milk1 1 dozen 1 gallon

limited by the muffin mix2 1 dozen 1 gallon3 1 dozen 1 gallon4 1 dozen 1 gallon5 1 dozen 1 gallon6 1 dozen 1 gallon7 1 dozen 1 gallon

limited by the amount of eggs

1 packet of muffin mix + 2 eggs + 1 cup of milk 12 muffins

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Limiting Reactant Concept

Suppose a box contains 87 bolts, 110 washers, and 99 nuts. How many sets, each consisting of one bolt, two washers, and one nut, can you construct from the contents of one box?

11

Limiting Reactant Concept

How much sulfur dioxide can be produced by the reaction of 95.6 g of carbon disulfide with 1.10 x102 g of oxygen?

mol 2 mol 1 mol 3 mol 1

SO 2 CO O 3 CS 2222

• Which is limiting reactant?

• What is maximum mass of sulfur dioxide that can be produced?

12

Percent YieldTheoretical yield is calculated by assuming

that the reaction goes to completion– -Determined from a limiting reactant calculation

Actual yield is the amount of a specified pure product made from the reactants– -In the laboratory, this is the amount of product

that is formed in your beaker, after it is purified and dried

Percent yield indicates how much of the product is obtained from the reaction

% yield = actual yield

theoretical yield100%

13

Percent Yield

A 10.0 g sample of ethanol, C2H5OH, was boiled with excess acetic acid, CH3COOH, to produce 14.8 g of ethyl acetate, CH3COOC2H5. What is the percent yield?

• 2. Calculate the percent yield

yieldal theoretic theCalculate 1.

O HHCOOCCH OH HC +COOH CH 2523523

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Types of Solutions

15

Concentrations of Solutions

• Common unit of concentration:

L

molessolution of liters ofnumber

solute of moles ofnumber molarity

M

=

g solute

MW solute

liters solution

16

Solution PreparationMaking a 1 M solution of sodium chloride

Then add addition water untilthe 1.0 L mark is reached

17

Concentrations of Solutions

• Calculate the molarity of a solution that contains 12.5 g of sulfuric acid in 1.75 L of solution.

18

Concentrations of Solutions

• Determine the mass of calcium nitrate required to prepare 3.50 L of 0.800 M Ca(NO3)2 .

You do it!You do it!

19

Diluting a Solution

A known amount of solvent is added to a known concentration and known amount of solution

• -number of moles of solute in the two solutions (concentrated and dilute) remains constant

• -mathematical relationship:• M1V1 = M2V2

• -Note: appropriate for dilutions, but not for chemical reactions

20

Diluting a Solution

• If 10.0 mL of 12.0 M HCl is added to enough water to give 100. mL of solution, what is the concentration of the solution?

21

Diluting a Solution

• What volume of 18.0 M sulfuric acid is required to make 2.50 L of a 2.40 M sulfuric acid solution?

22

Solutions in Chemical Reactions

• -chemical reactions are most often performed by using solutions of the reactants

• -Combine the concepts of molarity and stoichiometry

23

Solutions in Chemical Reactions

• What volume of 0.500 M BaCl2 is required to completely react with 4.32 g of Na2SO4?

NaCl 2 + BaSO BaCl + SONa 4242

24

Solutions in Chemical Reactions

• What volume of 0.200 M NaOH will react with 50.0 mL 0f 0.200 M aluminum nitrate, Al(NO3)3?

You do it!

3333 NaNO 3OHAlNaOH 3NOAl

25

Titrations

A method of determining the concentration of an unknown solution by reacting it with a known amount and concentration of a standard solution

Standard solution: solution whose concentration has been determined using a primary standard by standardization

Primary standard: chemical compound which can be used to accurately determine the concentration of another solution

Examples: include KHP and sodium carbonate

26

Primary Standard

• Potassium hydrogen phthalate (KHP) – molar mass of 204.2 g/mol.

CH

CHCH

C

CCH C

OH

O

C

OH

O

CH

CHCH

C

CCH C

O

O

C

OH

O+ KOH

-K+ + H2O

KHP

acidic H

27

TitrationsStandardization: process in which the

concentration of the standard solution is determined from the primary standard

Indicator: substance that exists in different forms with different colors depending on the concentration of the H+ in solution Examples: phenolphthalein and bromothymol blue.

Equivalence point: point at which stoichiometrically equivalent amounts of the acid (H+) and base (OH-) have reacted

End point: point at which the indicator changes color and the titration is complete

28

Titration

• Calculate the molarity of a NaOH solution if 27.3 mL of it reacts with 0.4084 g of KHP.NaOH + KHP NaKP + H O2

NaOH 0.0733NaOH L 0.0273

NaOH mol 0.00200 = NaOH ?

NaOH mol 0.00200KHP mol 1

NaOH mol 1

KHP g 204.2

KHP mol 1KHP g 0.4084 = NaOH mol ?

MM

NaOH mol 0.00200KHP mol 1

NaOH mol 1

KHP g 204.2

KHP mol 1KHP g 0.4084 = NaOH mol ?

KHP(aq) + NaOH(aq) NaKP(aq) + H2O(l)

29

Titrations

• What is the molarity of a KOH solution if 38.7 mL of the KOH solution is required to react with 43.2 mL of 0.223 M HCl?

KOH(aq) + HCl(aq) H2O(l) + KCl(aq)

30

Titrations

• What is the molarity of a barium hydroxide solution if 44.1 mL of 0.103 M HCl is required to react with 38.3 mL of the Ba(OH)2 solution?

OH 2 + BaCl HCl 2 + Ba(OH) 222

31

Titrations On Your Own

• Calculate the molarity of a sulfuric acid solution if 23.2 mL of it reacts with 0.212 g of Na2CO3.

H2SO4(aq) + Na2CO3(aq) Na2SO4(aq) + H2O(l) + CO2(g)

32

Transmittance and Absorption Spectrum

Transmittance (T): ratio of the amount of light that passes through a sample relative to the initial amount of light entering the sample

Absorbance (A) = -log T = -log (P/Po)

Beer-Lambert Law (Beer’s Law):A = bc