Chemical Formula - Nassau Community College
Transcript of Chemical Formula - Nassau Community College
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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-
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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
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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
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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-
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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
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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%
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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
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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)
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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
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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
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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%
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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
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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
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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
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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
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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!!
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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
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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)
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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
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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