1

Chemical Formula

• inventory of all atoms in a compound

NO2element symbol

for nitrogenelement symbol

for oxygen

number nitrogen atoms = 1

number oxygen atoms = 2

element more to left on PT is written first

ionic compounds form lattices

1.8 nm

1.0 mm 2.0 cm

• geometric arrangement of ions

• no definite size

sodium chloride = NaCl

ionic compounds electrically neutral

• charge positive ions = charge negative ions

NaCl = 1 Na+ and 1 Cl-

Li2S = 2 Li+ and 1 S2-

CaF2 = 1 Ca2+ and 2 F-

1-+1

2-+1 +1

2+ 1-1-

2

Formula Unit• Chemical formula of ionic compound called Formula Unit

• smallest whole number ratio of ions that will be electrically neutral

SrO = 1 Sr2+ and 1 O2-

64 Na+ and 64 Cl-

1019 Na+ and 1019 Cl-1022 Na+ and 1022 Cl-

CaCl2 = 1 Ca2+ and 2 Cl- Li2O = 2 Li+ and 1 O2-

NaCl = 1 Na+ and 1 Cl-

Na64Cl64

Na Cl1019 1019

Na Cl1022 1022

smallest whole number ratio is same for different size lattices

crossing over rule

Ca Ca2+ N3- N

3-

2+2+2+

3-

total + charge3 x 2

total -charge2 x 3

=

3 2

=

3 2=

periodictable

periodictable

What is the formula unit of calcium nitride?

why? b/c ionic compounds electrically neutral

Ca3N2

crossing over rule

Mg Mg2+ O2- O2 2

periodictable

periodictable

What is the formula unit of magnesium oxide?

why notMg2O2?

b/c the formula unit has smallest whole number ratioof ions that will be electrically neutral (1 to 1 smaller 2 to 2)

MgO

3

polyatomic ions

• groups of atoms bonded together that have a charge

Hg2 2+

PO43-NH4

+ CN-

OH-

SO42-

• acts as a single ion

H COO O

atoms

+ e-

H

CO O

O

-1

HCO3-

HCO3-

polyatomic ions form ionic compounds

Hg2 2+ PO4

3-

NH4+

CN-

OH-

SO42-HCO3

-

• positive polyatomic ions can substitute a metal ion• negative polyatomic ions can substitute a non-metal ion

memorize this chart

( )2

crossing over rule (polyatomic ions)

Ca Ca2+ PO43- phosphate

3

periodictable

table of

polyatomic ions

What is the formula unit of calcium phosphate?

why use ( ) ? Ca3(PO4)2

(not phosphide)

PO43- PO4

3-Ca2+ Ca2+ Ca2+

2 PO43- ions

not PO423-

4

counting atoms in chemical formula

Ca3(PO4)2

PO43- PO4

3-Ca2+ Ca2+ Ca2+

# Ca 2+ ions = 3# PO4

3- ions = 21 P + 4 O 1 P + 4 O

total = 2 P and 8 O2 Ca

naming ionic compounds

• metal name anion namefor polyatomic ions substitute either part with name � Table 3.5

sodium acetate

lithium dichromate

NaC2H3O2

Li2Cr2O7

ammonium sulfate

(NH4)2 SO4

(with polyatomic ions)

naming ionic compounds

• metal name anion name

metal ions that can have differentcharges use roman numerals

iron (III) chloride

cobalt (II) dichromate

FeCl3

CoCr2O7

(with ions not from main group)

(Fe3+ and 3Cl-)

(Co2+ and Cr2O72-)

remember formula unit must have zero overall charge

5

molecular compounds form molecules

• molecules are individual particles with definite size

• chemical formula does not have smallest whole number ratios

NO2

N2O4

different compoundsboth are molecules

N and O = non-metalsform molecular compounds

naming binary molecular compounds

• element further to left on PT first• if in same group lower element comes first• both elements get prefixes (mono, di, tri, tetra,

penta, hexa, hepta, octa, nona; mono optional)• second element gets ending -ide

N2O4

NOnitrogen oxide

dinitrogen tetroxide

NO2 nitrogen dioxide

N2Odinitrogen oxide

(2 elements)

Acids• H+ + anion

H+ -anion

• Acids are molecular compound because they candissolve without dissociating into ions

• Ionic compounds must separate into ions to dissolve

• Weak acids have a small percentage of moleculesseparated into H+ and an anion, the rest stay together as one particle

Strong acids HCl HNO3 H2SO4

H+ Cl- H+ NO3- H+ SO4

2-

hydrochloric acid nitric acid sulfuric acid

Weak acidsHC2H3O2 HF

H+ C2H3O2- H+ F-

acetic acid hydrofluoric acid

Hydrogenacts as anon-metal

HF ~ 95% H+ and F- 5%

Strong acids separate 100% into H+ and anion in waterHCl ~ 0 % H+ and Cl- ~ 100%

6

molecularcompounds

all other molecular

compoundsdissolve

Dissolved molecules

ioniccompounds

dissolve

cations(+ ions)

anions(- ions)

separated ionsH+ + F- ~5%(separated ions)

HCl � H+ + Cl-~ 100%

HF ~ 95%(molecules)

acids

dissolve

Acids aremolecular compounds

electrolytes

-anion

+cation

•molecular compounds = non-electrolytes

• ionic compounds and strong acids are called strong electrolytes- form ions in water- ions = conduct electricity

• weak acids = weak electrolytes- small % molecules � ions- conduct electricity poorly

water is a molecular compound and a non-electrolyte(remember H is an exception it acts like a non-metal)

tap water only conducts b/cions are dissolved in it

view quicktime movies on1) electrolytes2) aqueous acids

7

molecular vs ionic compounds

• chemical formula of ionic compound called formula unit has smallest whole number ratio of ions

• formula unit predicted from crossing over rule

• for each combination of ions there is only one formula unit

• in contrast different molecular compoundswith same elements are numerous, NO, NO2, N2O, N2O4 etc. chemical formulas of molecular compounds

predicted by Lewis Structures ( chapter 9)

8

1 mole NO2

molecules

6.02 x 1023

NO2 molecules

mole concept for compounds• as a counting unit the mole is also applied to

molecules, ions, and formula units

1 mole CaCl2formula units

6.02 x 1023

CaCl2 formula units

Ca2+

Cl- Cl-

1 CaCl2formula unit

9

counting moles ions in formula unit1 mole CaCl2= 6.02 x 1023 CaCl2 formula units

How many moles of Ca2+ ions are present?

1 mole Ca2+

How many moles of Cl- ions are present?2 moles Cl-

4 wheels per 1 car

4 dozen wheelsper 1 dozen cars

Ca2+

Cl- Cl-

2 Cl- ionsper CaCl2

formula unit

2 moles Cl- ionsper mole CaCl2formula units

why?

formula mass (aka molar mass)of a compound

• sum of all elements molar/atomic masses

• can be interpreted in amu per formula unit or grams per mole of formula units

what is the formula mass of CaCl2?

Ca 1 x 40.0782 Cl + 2 x 35.45

110.98

amuatom

grams mole atomor

amuformula unit

orgrams CaCl2mole CaCl2

molar mass (of a compound) is also a conversion factor

0.789 moles NO2 molecules x = 36.29 g NO2

old unitmoles

conversion factormolar mass

new unitgrams

46.00 g NO21 mole NO2

massmoles46.00 g/mol

NO2

N 1 x 14.02 g/molO 2 x 15.99 g/mol

multiplymolar mass

Example. Convert 0.789 moles of NO2 into mass of NO2

10

molar mass (of a compound) is also a conversion factor

6.66 grams C6H12O6 x 1 mole C6H12O6

180.16 grams C6H12O6

= 3.70 x 10-2 moles C6H12O6 molecules

old unitgrams conversion factor

1 / molar massnew unitmoles

mass molesC 6 x 12.01H 12 x 1.008O 6 x 15.99180.16 g/mol

C6H12O6

Example. Convert 6.66 grams of glucose C6H12O6 into moles of glucose

dividemolar mass

3 different units to describe amounts of a compound like length of an object measure in mm, cm, inches, etc.

masscompound

moles of formula units,

molecules, atoms, or ions

number formula units,

molecules, atoms, or ions

multiplymolar mass

dividemolar mass

divideAvogadro's

number

multiplyAvogadro's

number

percent composition from formula mass

Ca2+

Cl- Cl-

40.078 amu

35.45 amu35.45 amu

1 formula unitof CaCl2

formulamass = 110.98 amu

% by mass Cl = x 100%

= 63.88 % Cl

Ca2+

Cl-Cl-

Cl-Cl-mass 2 Cl-

mass CaCl2formula unit

2 x 35.45 amu110.98 amu

Ca2+

Cl-Cl-

110.98 amusample

CaCl2

Cl-Cl- Ca2+2 x 35.45 amu 40.078 amu

x 100%

11

masscompound

moles of formula units,

molecules, atoms, or ions

number formula units,

molecules, atoms, or ions

multiplymolar mass

dividemolar mass

divideAvogadro's

number

multiplyAvogadro's

number

12

moles of atoms vs molecules

How many moles of fluorine molecules (F2) are in 5.1 grams of molecular fluorine?

How many moles of fluorine atomsare in 5.1 grams of molecular fluorine?

5.1 grams fluorine molecules ×1 mole F2 molecules

38 grams F2 molecules= 0.13 moles F2 molecules

5.1 grams fluorine atoms ×1 mole F atoms

19 grams F atoms= 0.26 moles F atoms

F FFF

2 fluorine atoms 1 fluorine molecule

if every moleculedecomposed intoatoms

double2 ×

or

= 0.13 moles F2 molecules ×2 mole F atoms

1 mole F2 molecules= 0.26 moles F atoms

masscompound

moles of formula units,

molecules, atoms, or ions

number formula units,

molecules, atoms, or ions

multiplymolar mass

dividemolar mass

divideAvogadro's

number

multiplyAvogadro's

number

13

chemical equations

22.4 g Mg + ? g O2 � 38.4 g MgO

22.4 g Mg + 16 g O2 � 38.4 g MgO

(apply conservation of mass)

mass ratios

MgO

(mass reactants = mass products) and their“one overs”

a balanced chemical equation !(aka chemical reaction) conversion factors

‘balance equation”

reactants products

38.4 g MgO

16 g O2 38.4 g MgO

22.4 g Mg

38.4 g MgO

16 g O222.4 g Mg

16 g O2 22.4 g Mg

16 g O238.4 g MgO

22.4 g Mg

if 22.4 g Mg reacts with 16 g O, 38.4 g MgO form

no Mg or O remain

14

mass ratios as conversion factors

What mass of MgO will form if 5.0 grams of Mg reacts with O2 ? The balanced reaction is,

22.4 g Mg + 16 g O2 � 38.4 g MgO

5.0 g Mg x38.4 g MgO

22.4 g Mg

38.4 g MgO

22.4 g Mg= 8.57 g MgO=

5.0 x 38.422.4

Chemical reactions (mole interpretation)

22.4 g Mg + 16.0 g O2 � 38.4 g MgO

convert all massesto moles

mole ratios (conversion factors)

MgO

1 mole Mg + ½ mole O2 � 1 mole MgO

mass molesdividemolar mass

also a balanced chemical equation !

1 mole Mg

½ mole O2 1 mole MgO

1 mole Mg

1 mole MgO

½ mole O2

1 mole Mg

½ mole O21 mole MgO

½ mole O2

1 mole MgO

1 mole Mg

notation in chemical reactions(mole interpretation)

• state of matter abbreviated in ( )s = solidl = liquidg = gasaq = dissolved in water

Na2CO3 (s) + 2 HCl (aq) � CO2 (g) + H2O (l) +2 NaCl (aq

reactants

ClH

ClH

2HCl =

2HCl mean 2 moles of HClnot same as H2Cl2 or HCl2 or H2Cl

products

numbers in front of compounds are called the stoichiometric numbers

15

balancing a chemical reaction (mole interpretation)

FeCl3 (aq) + NaOH � Fe(OH)3 (s) + NaCl (aq)162.2 g/molor grams per 1 mole

39.99 g/molor grams per 1 mole

106.84 g/molor grams per 1 mole

58.44 g/molor grams per 1 mole

165.28 grams202.19 grams≠

not balanced !

FeCl3 (aq) + NaOH � Fe(OH)3 (s) + NaCl (aq)

by inspection 1 OH on left 3 on right

3now 3 Na on left 1 on right

3162.20 g 3 x 39.99 g 106.85 g 3 x 58.44 g

282.17 grams 282.17 grams= balanced !

balance this!

Fe3O4 + CO � Fe + CO2

Fe3O4 + CO � 3Fe + CO2

3 Fe on left 1 on right so try

C and Fe are balanced but O is notif we change the stoichiometric numbers anywhereto balance O then Fe and C will become unbalanced

there are special methods for balancing some types of reactions

Fe3O4 + 2 CO � 3Fe + CO2Now O is balanced but C is not !

combustion reactionscompound (C H O) + O2 (g) � CO2 (g) + H2O (l)

Example: Write the balanced combustion reaction of ethanol, C2H6O.

C2H6O (l) + O2 (g) � CO2 (g) + H2O (l)

to write a standard combustion reactiondo not change the stoichiometric number of the compound

2 34 moles O + 3 moles O

7 moles O

1 moles O need 6 moles O

3 moles O2 has 6 mole O!!

3

notice all the C on right is in CO2 so if we do not change C2H6O 2 will balance C

notice all the H on right is in H2O so if we do not change C2H6O 3 will balance H

You must remember this

You must remember this

16

Empirical Formula• experimentally determined chemical formula• has smallest whole number ratio if atoms

-for ionic compounds this is the formula unit-for molecular compounds this is not the chemical formula

Examples:

NO2 and N2O4 have different chemical formula but same empirical formula NO2

C2H2 (acetylene )and C6H6 (benzene) different chemical formulas same empirical formula CH

NO2nitrogen dioxide N2O4

dinitrogen tetroxide

C C HHC C HH

C CHH

C CHH

C6H6benzene

C2H2acetylene

empirical formula

chemical formulaethane = C2H6

HH

H

HH C

HC

HH

H

HH C

HCH

H CHC

HH

H

HH C

HC

HH

H

HH C

HCH

H CHC H

HH

HH C

HC

HH

H

HH C

HCH

H CHC

HH

H

HH C

HC

HH

H

HH C

HCH

H CHC

HH

H

HH C

HC

HH

H

HH C

HCH

H CHC

HH

H

HH C

HC

HH

H

HH C

HCH

H CHC

HH

H

HH C

HC

HH

H

HH C

HCH

H CHC

HH

H

HH C

HC

HH

H

HH C

HCH

H CHC

50.0 g ethane C H

40.0 g C 10.0 g H

convert to moles

3.33 moles C 10.0 moles H

empirical formula is the chemical formula with the smallest whole number ratio of the elements as they are in the compound

C3.33 H10.0 mole ratio of elements in ethane

converted to smallest whole number ratio

C1 H3 or CH3

divide all subscriptsby the smallest one

empirical formuladetermined experimentallyby decomposing a compound

empirical formulaof ethane

there is no way to determine how many of each atom were in the molecule from

decomposing and measuring massesall we can do is measure the ratio of atoms

3.33 3.33

chemical formulas �conversion factors mass description

CO2

C O O

44.0 gramsCO2

32.00 gramsO atoms

12.00 gC atoms

C / O mass ratio

% massO in CO2

% massC in CO2

44.00 g CO2 � 12.00 g C + 32.00 g O

12.00 g C

32.00 g O 44.00 g CO2

32.00 g O

44.00 g CO2

12.00 g C

chemical equation for decomposition of CO2 into its elements/atoms

written in moles

CO2 � C + 2 O

same equation re-written in mass

mass ratios =conversion

factors

(mass 1 mole CO2)

(mass 1 mole C) (mass 2 mole O)

17

chemical formulas �conversion factors CO2

C O O

44.0 gramsCO2

32.00 gramsO atoms

12.00 gC atoms

mass CO2 g x44.00 g CO2

12.00 g C44.00 g CO2

12.00 g C= mass C g

% massC in CO2

% massC in CO2

converts mass of CO2 into mass of C

masscompound

C O H

CO2mass Cmass O

mass H

empirical formula

moles C moles O moles H

done experimentally

calculateddivide by

atomic mass

masselement

moles of element

divide by atomic mass

masselement

moles of element

divide by atomic mass

C 1.97 H 6.01 O 1

C 1.390 H 4.251 O 0.707

C2 H6 O

1) chemical formula with ratios of moles

2) convert smallest whole number ratios

3) round to integers

divide all by smallest

chemical formula expressed with smallest whole numberratio of atoms

masscompound

C O H

CO2mass Cmass O

mass H

moles C moles O moles H

Calculateddivide by

molar mass

C 1.97 H 6.01 O 1

C 1.390 H 4.251 O 0.707

C2H6O

18

Combustion analysis

masscompound

C O H

combustion

CO2CO2

H2O

mass C mass O mass H

use %C in CO2as conversion

factor

use % H in H2Oas conversion

factorby subtraction

mass compound = mass C + mass H + mass O

mass O = mass compound – mass carbon– mass hydrogen

separates C H

Combustion analysis

masscompound

C O H

combustion

CO2CO2CO2

H2OH2O

mass C mass O mass H

use %C in CO2as conversion

factor

use % H in H2Oas conversion

factorby subtraction

mass compound = mass C + mass H + mass O

mass O = mass compound – mass carbon– mass hydrogen

separates C H