Download Unit 9 - Chemical Quantities: The Mole

Molecular and Formula Mass
• How much does a compound weigh?
Unit 9 – Chemical Quantities: The
Mole
Essential Questions:
• What is a mole?
• How is the mole used in chemical calculations?
• How is an empirical formula different from a
molecular formula?
• How is a molecular formula calculated?
• What is percent composition?
The Mole
• Avogadro’s Number
• A counting number
• Like a dozen
• 6.02 x 1023
• Named for Amedeo Avogadro
• Mole
• 1 mole = 6.02 x 1023 particles
• Relates atomic mass to grams
• 1 atom of 12C – 12 amu
• 1 mole of 12C atoms – 12 g
Molar Mass
• Molar Mass (MM)
– Mass of one mole of a substance
– Unit is grams/mole
• g/mole or g/mol
– Same as molecular mass with different unit
• molecular mass of water = 18.02 amu
• Molar Mass of water = 18.02 g/mol
– Add up masses of elements to find out
• Molecular Mass
– Mass in a molecule
– Covalent Bonds
• Formula Mass
– Mass in a formula unit
– Ionic Bonds
• Unit is amu for both
The Mole
1 Mole = 6.02 x 1023 particles
• 1 mole C
– 6.02 x 1023 C atoms
• 1 mole H2O
– 6.02 x 1023 H2O molecules
• 1 mole NaCl
– 6.02 x 1023 NaCl formula units
– 6.02 x 1023 Na+ ions and 6.02 x 1023 Cl– ions
Mole – Particle Conversions
• Mole to Particle Conversions
– Particle could be atom, molecule, or formula unit
– Convert using Avogadro’s number
– You MUST use dimensional analysis for conversions!
– Watch your Exponents!!!
• You will NEVER have 1046 particles
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Mole – Mass Conversions
Mass – Particle Conversions
• Mole to Mass Conversions
– Convert using molar mass of the substance
Molar Volume
• Mass to Particle Conversions
– Find molar mass first
– Require 2 step conversion
– Mole is intermediate – in the middle
Percent Composition
• STP
 Percent Composition
– Standard Temperature and Pressure
– 0 °C and 1 atm of pressure
• Molar Volume
– All gases have the same number of molecules
present in the same volume
• At STP: 1 mole of a gas = 22.4 L of gas
–(Element mass) x (# of atoms)
Percent Composition
Empirical Formula
 Formula for calculating % composition:
Total amu of the element in the compound
Total formula mass (amu)
Sum of all elements should add up to 100 %
– Percentage by mass of each element in a
compound
– Need:
• Mass of compound
• Mass of element in compound
• Empirical Formula
x 100%
– Smallest whole number ratio of the atoms in
compound
– Ionic formulas are always empirical formulas
• Molecular Formula
– Actual number of each kind of atom found in
one molecule of the compound
– A multiple of the empirical formula
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Empirical Formula
To Calculate Empirical Formula:
1. Assume the percent is out of 100 grams (change the %
to g).
2. Convert each element to moles.
3. Divide each by the smallest answer from step 2.
4. If whole numbers are not obtained in step 3, multiply
everything by a factor to make the ratios whole #s.
– If .5, multiply everything by 2. If .33, multiply
everything by 3.
–
Empirical/Molecular Formula
To Determine Empirical Formula from
Molecular Formula:
To Determine Molecular Formula from
Empirical Formula:
CH
P4O10
–
•
1. Calculate molar mass of the empirical formula
2. Divide molar mass of the Molecular Formula by
molar mass of the Empirical Formula
Molecular Mass
Empirical Mass
P2O5
BaCl2
–
Percent to Mass
Mass to Mole
Divide by Small
Multiply 'til Whole
Empirical/Molecular Formula
All you need to do is reduce!!
• C6H6
•
Empirical Formula
3. Multiply each subscript by answer from step 2
BaCl2
Hydrates
• Hydrate
– H2O is chemically bonded to compound
– Use prefixes to indicate how many H2O
are bonded to the compound
• Ex: Copper (II) Sulfate Pentahydrate
• CuSO4  5 H2O
• Anhydrous
Hydrates
• Hydrate Molar Mass
–Include mass of water molecules with
compound MM
–MM of water – 18.02 g/mol
–Ex: MM of Na2CO3  10 H2O
• 286.19 g/mol
– No water bonded to compound
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Hydrates
• Percent Composition of Hydrates
– Use Hydrate Molar Mass
– Find percentage of water and anhydrous
compound
• Add up to 100%
Hydrates
• Empirical Formula of Hydrates
– Same as anhydrous, but include water
– Ex: A hydrate is found to contain 48.9 %
MgSO4 and 51.1 % water. What is its empirical
formula?
• MgSO4 ∙ 7 H2O
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