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Molecules, Moles, and Chemical Equations

Chapter 3

Law of Conservation of Matter

Matter is neither created nor destroyed.

In chemistry the number of atoms going into a

chemical reaction must be the same as the

number of atoms coming our of the reaction.

Reactions must be balanced.

Chemical Equation or Reaction: Statement in

formulas depicting a chemical change.

Reactants � Products

(initial) (final)

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electricity

electricityMg Mg

2MgO(s)

O2

O2–

O2–

Mg2+

Mg2+

+ O2(g)2Mg(s)

MACROSCOPICVIEW

ATOMIC-SCALEVIEW

BALANCEDEQUATION

translate the statement

balance the atoms

specify states of matter

adjust the coefficients

check the atom balance

Write a balanced chemical equation for the following: If aqueous silver nitrate is mixed with

aqueous sodium phosphate a chemical reaction occurs and solid silver phosphate and aqueous sodium nitrate are formed.

Take out and complete Exercise 5 pg 26 of Lab

Manual.

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Interpreting Equations and the Mole

Common Terms that Represent Specific

Quantities

1 pair = 2

1 dozen = 12

1 baker’s dozen = 13

1 case = 24

1 gross = 144

1 mole = 6.022 X 1023

mole(mol) - the amount of a substance that contains the same number of entities as there are atoms in exactly 12 g of carbon-12.

This amount is 6.022x1023. The number is called Avogadro’s number and is abbreviated as N.

One mole (1 mol) contains 6.022x1023 entities

(to four significant figures)

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Water

18.02 gCaCO3

100.09 g

Oxygen

32.00 g

Copper

63.55 g

One mole of

common substances.

Summary of Mass Terminology

Term Definition Unit

Isotopic mass Mass of an isotope of an element amu

Atomic mass

Molecular

(or formula) mass

(also called molecular weight)

Molar mass (M)

(also called

atomic weight)

(also called gram-

molecular weight)

amu

amu

g/mol

Average of the masses of the naturally occurring isotopes of an element weighted

according to their abundance

Sum of the atomic masses of the atoms (or ions) in a molecule (or formula unit)

Mass of 1 mole of chemical entities (atoms, ions, molecules, formula units)

Let’s Consider 1.00 mole of Si

1 Si atom = 28.09 amu

1 mole Si atoms = 28.09 g

Here’s the Proof!

1.00 mol Si X 6.022X1023 Si X 28.09amu X 1.66054X10-24 g = 28.09 g Si

1 mol Si 1 Si atom 1 amu

Now you have two unit factors depending upon whether you are talking about individual atoms or a group of atoms.

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Information Contained in the Chemical Formula of Glucose C6H12O6 ( M = 180.16 g/mol)

Oxygen (O)

Mass of atomsper mole of

compound

6 atoms

96.00 g

Carbon (C) Hydrogen (H)

Atoms per moleculeof compound

Moles of atoms permole of compound

Atoms per mole ofcompound

Mass of atomsper molecule

of compound

6 atoms 12 atoms

6 moles

of atoms

12 moles

of atoms

6 moles

of atoms

6(6.022 x 1023)

atoms

12(6.022 x 1023)

atoms

6(6.022 x 1023)

atoms

6(12.01 amu)

=72.06 amu

12(1.008 amu)

=12.10 amu

6(16.00 amu)

=96.00 amu

72.06 g 12.10 g

Molar Mass of Compound: 72.06g + 12.10g + 96.00g = 180.16 g/mol

Interconverting Moles, Mass, and Number of Chemical Entities

Mass (g) = no. of moles x no. of grams

1 mol

No. of moles = mass (g) x

no. of grams

1 mol

No. of entities = no. of moles x6.022x1023 entities

1 mol

No. of moles = no. of entities x 6.022x1023 entities

1 mol

g

n

Another Example:

What is the formula unit mass of K2S?

What is the molar mass of K2S?

What is the mass of 0.250 moles of K2S?

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Conversion Problem Solving

1. How many atoms of lead are present in 15 g of lead?

2. What is the mass of 1.36X 1021 atoms of silver?

3. How many moles of Ba(NO3)2 are in 20.0 grams of Ba(NO3)2?

4. How many atoms of oxygen are present in 20.0 grams of Ba(NO3)2?

5. How many moles of nitrate ions are present in 20.0 grams of Ba(NO3)2?

EXERCISE 4

Work in Groups and complete this exercise.

Empirical and Molecular Formulas

Empirical Formula -

Molecular Formula -

The simplest formula for a compound that agrees with the elemental analysis and gives

rise to the smallest set of whole numbers of atoms.

The formula of the compound as it exists, it may be a multiple of the empirical formula.

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Example

Acetylene C2H2

Benzene C6H6

What is the empirical formula for each of the above compounds?

Note: Two completely different compounds can have the same empirical formula.

Determination of Empirical and Molecular

Formulas

Example: A compound of nitrogen and oxygen is

analyzed and a sample weighing 1.587 grams

is found to contain 0.483 grams of nitrogen

and 1.104 grams of oxygen. Determine the

empirical formula of this compound.

Example: A compound contains 68.8% C, 5.0%

H, and 26.2 % O by mass. The molecular mass

of the compound is determined to be 122.13

amu. Determine the empirical and molecular

formulas of the compound.

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Example: A vitamin is analyzed and found to

contain 40.9%C, 4.58%H, and 54.5% O. The

molar mass of the compound is 176.13 g/mol.

Determine the empirical and molecular

formula.

Combustion Analysis: a procedure used to determine the chemical composition of organic

compounds.

General Equation for Combustion Analysis Reactions

Compound + n O2(g) ���� n CO2(g) + n H2O(g) + other elements

Example: A compound is combusted and the products are massed. Using the information provided below, determine the empirical and molecular formulas of the compound.

Products: 6.21 mg CO2 2.54 mg H2O

Compound Mass: 4.24 mg

Molar Mass of Compound: 60.6 g/mol

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Some Compounds with Empirical Formula CH2O

(Composition by Mass: 40.0% C, 6.71% H, 53.3% O)

Name

Molecular Formula

Whole-Number Multiple (g/mol) Use or Function

formaldehyde

acetic acid

lactic acid

erythrose

ribose

glucose

CH2O

C2H4O2

C3H6O3

C4H8O4

C5H10O5

C6H12O6

1

2

3

4

5

6

30.03

60.05

90.09

120.10

150.13

180.16

disinfectant; biological preservative

acetate polymers; vinegar(5% soln)

part of sugar metabolism

sour milk; forms in exercising muscle

component of nucleic acids and B2

major energy source of the cell

CH2O C2H4O2 C3H6O3 C4H8O4 C5H10O5 C6H12O6

Solutions

Solute + Solvent = Solution

x

x x

x x

x x

x

x

x

x x

x

x

More Dilute More Concentrated

x = solute particles

Molarity

Molarity is a form of representing concentration

of a solution.

Molarity (M) = moles of Solute

liters of Solution

Units of Molarity: moles/L

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Calculating Molarity

Example: A sample of sodium nitrate weighing

0.38 grams is dissolved in 50.0 mL of

solution. Determine the molarity of the

solution.

Laboratory preparation of molar solutions.

A•Weigh the solid needed.

•Transfer the solid to a

volumetric flask that contains

about half the final volume of

solvent.B Dissolve the solid

thoroughly by swirling.

C Add solvent until the solution

reaches its final volume.

Problem Solving with Molarity

1. A student must prepare 250.0 mL of a 0.250

M sodium hydroxide solution. What mass of

solute is required?

2. How many mL of 0.163 M sodium chloride

solution are required to contain 0.0958

grams of solute?

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Converting a concentrated solution to a dilute solution.

M1V1 = M2V2

Or

MdVd = McVc

Dilution Problem Solving

Explain how you would prepare 100.0 mL of a

0.100 M ammonia solution from 14.8 M

ammonia.

The Role of Water as a Solvent

The Universal Solvent: H2O

Let’s consider the

(1) shape of the water molecule.

(2) bond angle.

(3) unequal sharing of electrons.

(4) partial charges.

(5) dipole moment.

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Electron distribution in molecules of H2 and H2O.

The dissolution of an ionic compound

in water.

• The separation of an ionic compound caused

by water molecules results in a solution

containing solvated ions.

–

+

––

––

+

++

+–––

–

–+

+

+

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The electrical conductivity of ionic solutions.

Solvation of Covalent Compounds by

H2O

Water does not dissociate some covalent compounds (nonelectrolytes).

Examples: sucrose, ethanol, acetone, etc.

Water can slightly dissociate some covalent compounds (weak electrolytes).

Examples: HC2H3O2 HC7H5O2 etc.

Note: Why is there one H in the front of the chemical formula?

Water can completely dissociate some covalent

compounds (strong electrolytes) such as

strong acids.

HCl HBr HI HNO3

H2SO4 HClO3 HClO4

Water dissociates soluble ionic compounds

completely into ions (strong electrolytes).

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Useful Tables

Page 17 of lab manual: Summary of Electrolyte

Rules

Page 16 of lab manual: Solubility Rules

Do not memorize. However, be prepared to use

similar tables on the exam.

Problem Solving

When one mole of the following compounds

dissociates in H2O, how many moles of each ion are

present?

Aluminum chloride

Copper (II) nitrate

Sodium phosphate

Problem Solving

If 150.00 mL of 0.200 M iron (III) sulfate are

present in a beaker, how many moles of each

ion are present?

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Acid-Base Reactions (Neutralization)

Acids

Release H+

Turn blue litmus paper pink

pH < 7

Electron pair acceptors

React with some metals to produce H2(g)

Taste Sour

Bases

Release OH-

Turn pink litmus paper blue

pH > 7

Electron pair donors

React with many metal ions to produce

precipitates (ppt’s)

Taste Bitter

Dissociation of Strong Acids in H2O

HCl(g) + H2O(l) � H3O+ (aq) + Cl-(aq)

(hydronium ion)

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The hydrated proton.

Dissociation of Weak Acids in H2O

HF(g) + H2O(l) � H3O+(aq) + F- (aq)

F- = Anion of a weak acid

GENERAL Neutralization Reaction

Acid + Base � H2O(l) + Salt

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

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An aqueous strong acid-strong base reaction on the atomic scale.

Determining the Molarity of H+ Ions in Aqueous Solutions of Acids

Nitric acid is a major chemical in the fertilizer and explosives industries. Nitric acid is a strong

acid/strong electrolyte. What is the molarity of H+(aq) in 0.900M nitric acid?

Writing Equations for Aqueous Ionic Reactions

The molecular equation: shows all of the reactants and

products as intact, undissociated compounds.

The total ionic equation: shows all of the soluble ionic substances

dissociated into ions.

The net ionic equation: eliminates the spectator ions and shows the

actual chemical change taking place.

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How to Begin Writing Net Ionic Equations

Write the molecular equation and balance.

Determine the solubility of each compound.

Write the total ionic equation. Dissociate ionic compounds that are soluble (Solubility Rules) . Dissociate strong acids and bases. Do not dissociate weak acids, weak bases, (Electrolyte Rules) solids, or gases.

Determine the spectator ions.

Write the balanced net ionic equation (w/o spectators).

A precipitation reaction and steps to writing its net ionic equation.

Problem Solving

Aqueous barium chloride reacts with aqueous

sodium phosphate to produce aqueous

sodium chloride and solid barium phosphate.

Write the net ionic equation for this reaction.

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Another Example: The reaction of Pb(NO3)2 and NaI.

double displacement reaction (metathesis)

Write a net ionic reaction

Another Example: An acid-base reaction that forms a gaseous

product.

Sodium carbonate reacts with acetic acid

Write a net ionic equation for this reaction

Practice Writing Net Ionic Equations

Exercise 7 of Lab Manual, page 30.