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The The MoleMole
Calculating Formula/Molar MassCalculating Formula/Molar MassCalculate the molar mass of carbon Calculate the molar mass of carbon dioxide, COdioxide, CO22..
12.01 g + 2(16.00 g) =12.01 g + 2(16.00 g) =44.01 44.01 gg
One mole of CO2 (6.02 x 1023 molecules) has a mass of 44.01 grams
Mole RelationshipsMole Relationships
Mole Mole
Atomsor
molecules
Atomsor
molecules
MassMass
6.02 X 1023 Atoms1 Mole
1 Mole
6.02 X 1023 Atoms
1 Mole
Molar Mass (g)
Molar Mass (g)
1 Mole
How would you convert,Atoms Moles Moles Atoms
Moles Mass (g) Mass (g) Moles
Mass (g) Atoms Atoms Mass (g)
atoms
atoms
mole
6.02 x 1023
1 moles atoms
mole
6.02 x 1023
1moles= = atoms
moles
mole1
gg=
g
g
mole1moles=
g
g
1 mole
1 mole
atoms6.02 x 1023
=atoms
atoms
atoms6.02 x 1023
1 mole
1 mole
gg=
Calculations with Moles:Calculations with Moles:Converting moles to gramsConverting moles to grams
How many grams of lithium are in 3.50 moles of lithium?
3.50 mol Li= g Li
1 mol Li
7 g Li 24.524.5
Calculations with Moles:Calculations with Moles:Converting grams to molesConverting grams to moles
How many moles of lithium are in 18.2 grams of lithium?
18.2 g Li= mol Li
7 g Li
1 mol Li2.602.60
Calculations with Moles:Calculations with Moles:Using Avogadro’s NumberUsing Avogadro’s Number
How many atoms of lithium are in 3.50 moles of lithium?
3.50 mol
= atoms1 mol
6.02 x 1023 atoms 2.11 x 102.11 x 102424
Calculations with Moles:Calculations with Moles:Using Avogadro’s NumberUsing Avogadro’s Number
How many atoms of lithium are in 18.2 g of lithium?
18.2 g Li
= atoms Li
1 mol Li 6.02 x 1023 atoms Li
1.57 x 1024
7 g Li 1 mol Li
(18.2)(6.02 x 1023)/6.94
1 mol NaOH
40.00 g NaOH
• How many moles is 5.69 g of NaOH?
Na = 22.99 g/mol
O = 16.00 g/mol
H = 1.01 g/ mol 40.00 g/mol
5.69 g NaOH = 0.142 mol NaOH
• How many grams are in 9.45 mol of dinitrogen trioxide?
N2O3 = 2(14.01) + 3(16.00) = 76.02 g/mol
9.45 mol N2O3 76.02 g N2O3
1 mol N2O3
= 718.2 =718 g N2O3
Find the number of sodium ions, Na+, in 3.00 mol of Na2CO3
3.00 mol Na2CO3
2 mol Na+ 6.022 x 1023 ions
1mol Na2CO3 1 mol Na+
= 3.61 x 10 24 Na+ ions
Find the number of sodium ions, Na+, in 3.00 mol of Na4P2O7
3.00 mol Na4P2O7
4 mol Na+ 6.022 x 1023 ions
1mol Na4P2O7 1 mol Na+
= 7.23 x 1024 Na+ ions
Percent Composition, Empirical and Percent Composition, Empirical and Molecular FormulasMolecular Formulas
Percent Composition, Empirical and Percent Composition, Empirical and Molecular FormulasMolecular Formulas
Courtesy www.lab-initio.com
7.3: Calculating Percent Composition of a Compound
• Like all percent problems: Part
whole
• Find the mass of each component,
• then divide by the total mass (assume one mole).
x 100
100cmpd of mass
element of masselement of mass %
• What is the % composition of water.– Find MM: 2(1.01) + 16.00 = 18.02 g/mol
• Double check: %’s should add up to 100.
100 =
Percent Composition
%H =2.02 g H
18.02 g H2O 100 =
%O =16.00 g O
18.02 g H2O
11.2 % H
88.8 % O
What is the percent carbon in C5H8NO4
(MSG monosodium glutamate), a
compound used to flavor foods and
tenderize meats?
1. Find mass of C
1. 5 x 12.0 g = 60.0 g C
2. Find mass of MSG
1. 5(12.0) + 8(1.0) +14.0 +4(16.0)=146.0 g
3. Mass of element/ mass of cmpd x 100
1. 60.0 g C/146.0 g x100 = 41.1% C
Practice Problem
%Fe =28 g
36 g 100 =78% Fe
%O =8.0 g
36 g 100 =22% O
• Find the percentage composition of a sample that is 28 g Fe and 8.0 g O.– Determine total mass:
Percent Composition
28 g + 8.0 g = 36 g
Calculating Percentage CompositionCalculating Percentage CompositionCalculate the percentage Calculate the percentage composition of magnesium composition of magnesium carbonate, MgCOcarbonate, MgCO33..Formula mass of magnesium carbonate:Formula mass of magnesium carbonate:24.31 g + 12.01 g + 3(16.00 g) = 24.31 g + 12.01 g + 3(16.00 g) = 84.32 g84.32 g
100.00 100.00
%83.2810032.84
31.24
g
gMg
%24.1410032.84
01.12
g
gC
%93.5610032.84
00.48
g
gO
• How many grams of copper are in a 38.0-gram sample of Cu2S?
– Multiply mass of cmpd with % element
(38.0 g Cu2S)x(0.79852) = 30.3 g Cu
Using Percent as a Conversion Factor
Cu2S is 79.8% Cu (Sample 7-10 p 227)
B. Empirical Formula
C2H6
CH3
reduce subscripts
• Smallest whole number ratio of atoms in a compound
EF vs MF
• Empirical Formula (EF)
• Def’n: lowest whole # ratio
• H2O
• HO• CH
• CH2O
• Molecular Formula (MF)
• Def’n: actual ratio
• H2O
• H2O2
• C6H6
• C6H12O6
Waterperoxid
ebenzenesugar
Calculating Empirical Formula
• It is not just the ratio of atoms, it is also the ratio of moles of atoms.
• In 1 mole of CO2 there is 1 mole of carbon and 2 moles of oxygen.
Steps for Calculating the Empirical Formula
We get a ratio from the percent composition.
Assume you have a 100 g sample.1. % = g
2. Convert grams to moles (÷ atomic mass)
3. Find lowest whole number ratio.
(÷ by smallest # of moles)
(if this step gives a decimal, multiply by 2, 3, or 4 to get whole #’s)
4. Use ratio to write the EF
• Calculate the empirical formula of a compound composed of 38.67 % C, 16.22 % H, and 45.11 %N.
Part 1 Calculate the number of moles
Assume 100 g• 38.67 g C x 1mol C = 3.220 mole C
12.01 g C • 16.22 g H x 1mol H = 16.09 mole H
1.01 g H• 45.11 g N x 1mol N = 3.219 mole N
14.01 g N
Part 2 calculate the formula!
3.220 mol C
16.09 mol H
3.219 mol N
3.219
3.219
3.219
=
=
=
1 mol C
5 mol H
1 mol N
Divide by the smallest number of moles
CH5N
Empirical Formula• Find the empirical formula for a
sample of 25.9% N and 74.1% O.
25.9 g 1 mol
14.01 g = 1.85 mol N
74.1 g 1 mol
16.00 g = 4.63 mol O
1.85 mol
1.85 mol
= 1 N
= 2.5 O
Empirical Formula
N1O2.5
Need to make the subscripts whole numbers multiply by 2
N2O5
Try this!Try this!Adipic acid contains 49.32% C, 43.84% O, and 6.85% H by mass. What is the empirical formula of adipic acid?
carbonmolcarbong
carbonmolcarbong107.4
01.12
132.49
hydrogenmolhydrogeng
hydrogenmolhydrogeng78.6
01.1
185.6
oxygenmoloxygeng
oxygenmoloxygeng74.2
00.16
184.43
1. Convert to grams, calculate # of moles
2. Divide each value of moles by the 2. Divide each value of moles by the smallest of the values.smallest of the values.
Carbon:Carbon:
Hydrogen:Hydrogen:
Oxygen:Oxygen:
50.174.2
107.4
mol
carbonmol
47.274.2
78.6
mol
hydrogenmol
50.174.2
74.2
mol
oxygenmol
3. Multiply each number by an 3. Multiply each number by an integer to obtain all whole numbers.integer to obtain all whole numbers.
Carbon: 1.50Carbon: 1.50 Hydrogen: 2.50Hydrogen: 2.50 Oxygen: 1.00Oxygen: 1.00x 2 x 2 x 2
33 55 22
Empirical formula:C3H5O2
Molecular Formula• “True Formula” - the actual number
of atoms in a compound
CH3
C2H6
empiricalformula
molecularformula
?
Steps to calculating Molecular Formula1. Find the empirical formula.2. Find the empirical formula mass.3. Divide the molecular mass by the empirical mass.4. Multiply each subscript by the answer from step 3.
nmass EF
mass MF nEF
Molecular Formula• The empirical formula for ethylene is CH2. Find the
molecular formula if the molecular mass is 28.1 g/mol?
28.1 g/mol
14.03 g/mol = 2.00
empirical mass = 14.03 g/mol
(CH2)2 C2H4
1. Find the empirical formula and its mass
2. Divide the molecular mass by the empirical mass.
3. Multiply each subscript by the answer from step 3.
Finding the Molecular FormulaFinding the Molecular FormulaThe empirical formula for adipic The empirical formula for adipic acid is Cacid is C33HH55OO22. The molecular . The molecular mass of adipic acid is 146 g/mol. mass of adipic acid is 146 g/mol. What is the molecular formula of What is the molecular formula of adipic acid?adipic acid?
1. Find the formula mass of C1. Find the formula mass of C33HH55OO22
3(12.01 g) + 5(1.01) + 2(16.00) = 3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g73.08 g
Finding the Molecular FormulaFinding the Molecular Formula
3(12.01 g) + 5(1.01) + 2(16.00) = 3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g73.08 g
273
146
2. Divide the molecular mass by the empirical mass.
3. Multiply each subscript by the answer from step 3.
(C(C33HH55OO22) x ) x 22 = = CC66HH1010OO44
Example
• A compound is known to be composed of 71.65 % Cl, 24.27% C and 4.07% H. Its molar mass is known (from gas density) to be 98.96 g. What is its molecular formula?–EF: CClH2
–MF: C2Cl2H4