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3/11/2020
1
Unit 8
Mathematical Relationships
Lesson 1:
Moles and Molar Mass
Objective
Determine the number of atoms in a
molecule
Calculate molar mass
Wednesday, March 11
• LT: Moles
• List ways that we group items
– Example: a dozen cookies = 12 cookies
Review
• Subscripts: Number of atoms in a formula
• Coefficients: Total number of molecules or
compounds
Ex: 3Ca3(PO4)2
Ca = 9 P = 6 O = 24
Examples: How many atoms
are in each substance?
• NH4NO3
• Al2(SO4)3
• 3Al2(SO4)3
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First Something Familiar
What do the following items have in common?
The Mole
1 dozen =
1 gross =
1 score =
1 mole =
12
144
20
6.02 x 1023
There are exactly 12 grams of carbon-12 in one mole of carbon-12.
• A mole is a number—it is a constant—a mole will always
equal 6.02 x 1023
• It relates the mass of a single atom in amu to the mass in
grams (It is the same number of particle as there are in
exactly 12 grams of carbon-12.)
1 mol = 6.02 x 1023 particles (atoms, molecules, things)
1 mol = gram formula mass
Mole
Molar Mass(aka Formula Mass or Molecular Mass)
• Mass of the formula expressed in grams
• Equal to 1 mole
• Units (g/mol)*** Elements molar mass are reported on the periodic table
(remember avg. atomic mass)
Calculating Molar Mass
1. Count the atoms in the compound
2. Look up the mass of each element on the
periodic table
3. Multiply mass by the number of atoms of each
element
4. Add up the total mass
* Always add g/mol unit
* Round to 2 decimal places
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Molar Mass Example: Elements
1. What it the molar mass of iron? 55.85 g/mol
2. What is the molar mass of copper?
Molar Mass Examples: Compounds
1. What is the molar mass of water?H2O = 2(1.01) + 16.0 = 18.01 g/mol
2. What is the molar mass of calcium
chloride?
Lesson 2:
Mole Conversions
Objective
Calculate moles given the number of
grams
Calculate grams given the number of
moles
Moles/Gram Relationship
• Molar Mass (g/mol)
• Equations
Calculations with Moles:Converting moles to grams
How many grams of lithium are in 3.50 moles of lithium?
3.50 mol Li= g Li
1 mol Li
6.94 g Li
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Calculations with Moles:Converting grams to moles
How many moles of lithium are in 18.2 grams of lithium?
18.2 g Li= mol Li
6.94 g Li
1 mol Li
Practice
• Pages 5-6
• Add Vocabulary
Objective
Complete 1 step mole conversions using
g, L, or particles.
Mole/Particle Relationship
• Avogadro's number
Calculations with Moles:Using Avogadro’s Number
How many atoms of lithium are in 3.50 moles of lithium?
3.50 mol Li= atoms Li
1 mol Li
6.022 x 1023 atoms Li2.11 x 1024
Mole/Liter Relationship
• At STP (Standard Temperature and Pressure), 1 mol of 6.02 x 1023
representative particles of any gas occupies a volume of 22.4 L
• Equations
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Calculations with Moles:Converting Liters to Moles
How many moles of Chlorine gas are in 33.6L Cl2?
33.6 L Cl2=
22.4 L Cl2
1 mol Cl21.50 mol Cl2
Calculations with Moles:Converting Moles to Liters
How many L of water are in 2.4 mol of H2O?
2.4 mol H2O=
22.4 L H2O
1 mol H2O54 L H2O
Practice
• Turn in Chalk lab
• Pages 7-8
• Add vocabulary
• Homework due tomorrow
• Quiz Friday
Lesson 3:
Percent Composition and Empirical Formulas
Objective
Determine the empirical formula from
molecular formula
Calculate the molecular formula from the
empirical formula
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Molecular to Empirical Formula
• The empirical formula of compound shows the
smallest whole-number ratio of the atoms in the
compound. (Cannot be reduced further.)
Determining Empirical Formula
• Divide subscripts by the greatest common
factor
• Example: molecular formula = C4H10
Divide by 2 (greatest common factor)
C2H5
Examples
Molecular Empirical
P4O10 P2O5
C10H22
C6H18O3
C5H12O
N2O4
Molecular Formula
• “Actual Formula” – the actual number of
atoms in a compound
– Whole # multiple of empirical
– If the empirical formula is CH4 a molecular
formula could be CH4, C2H8, C3H12, etc.
Calculating Molecular Formula
from Empirical Formula1. Calculate Molar mass (gram formula
mass) of the Empirical Formula
2. Divide molar mass given for molecular
formula by the empirical formula mass.
3. Multiply all of the subscripts in the
empirical formula by the number
(multiple) you calculated in step 2.
Example
1.The empirical formula for ethylene is CH2. Find
the molecular formula if the molecular mass is 28.1
g/mol.
empirical mass CH2 =
(1 C X 12.011 g/mol) + (2 H X 1.008 g/mol) = 14.03g/mol
28.1 g/mol
14.03 g/mol
(CH2) x 2 ⇒ C2H4
= 2.00
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Example
2. What is the molecular formula of a compound
that has an empirical formula of NO2 and
molecular mass of 92.0 g?
N= 14.007 x 1 = 14.007g
O= 15.999 x 2 = 32.00g
+ = 46.01g/mol
92.0 g/mol
46.01 g/mol
(NO2) x 2 ⇒ N2O4
= 1.9995 = 2
Practice
• Page 9
• Add Vocabulary
Objective
Calculate the percent composition of a
substance
Determine the empirical formula from
percent composition
Percent Composition
• The percentage by mass of each element
in a compound
• Formula
Steps for Calculating Percent
Composition1. Find the molar mass of each element and
the compound.
2. Divide element x 100
compound
Examples
1. What is the percent composition of
CaCl2?
2. What is the percent by mass of nitrogen in
NH4NO3?
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Empirical Formula from %
Composition
1. Assume a 100g sample so that your % =
g
2. Convert each sample to moles
3. Divide all values by the smallest
4. These results are your subscripts of the
empirical formula
Example: Empirical Formula
from Percent Composition
• A compound is analyzed and found to
contain 25.9% nitrogen and 74.1%
oxygen. What is the empirical formula of
the compound?
Practice
• Page 10-12
• Add Vocabulary
• HW#3 Due Wednesday
• Quiz Wednesday
Practice
• Page 10-12
• Add Vocabulary
• Quiz!!!
Lesson 4:
Mole Ratio/Stoichiometry
Objective
Calculate mole ratios for chemical
reactions.
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Mole Ratio
• Mole ratio is the ratio between the
amounts in moles of any two compounds
involved in a chemical reaction. Mole
ratios are used as conversion factors
between products and reactants in many
chemistry problems.
• Simply put, the ratio of the coefficients in a
balanced chemical equation.
Mole Ratio Steps
1. Balance the Chemical Equation
2. Use the coefficients as ratios
Mole Ratios
__H2 + __O2 __H2O
1. What is the molar ratio between H2 and
O2?
2. What is the molar ratio between O2 and
H2O?
Example: Mole Ratios Conversions
__H2 + __O2 __H2O
3. How many moles of oxygen are consumed
when 0.6 moles of hydrogen burns to produce
water?
4. How many moles of water are produced when 5
moles of hydrogen gas are consumed?
Example: Mole Ratios
__N2 + __H2 __NH3
5. What is the molar ratio between H2 and N2?
6. What is the molar ratio between H2 and NH3?
Example: Mole Ratios
__N2 + __H2 __NH3
7. How many moles of nitrogen gas would be
needed to produce 10 moles of ammonia in the
above reaction?
8. How many moles of ammonia are produced
when 7.5 moles of hydrogen are consumed?
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Practice
• Page 13
• Add vocabulary
Stoichiometry
• The quantitative relationship between two
or more substances in a chemical
reaction.
Mass to Mass Stoichiometry Full Stoichiometry #1
__CH4 + __O2 __H2O + __CO2
• If 50. g of CH4 are combusted, how many
g of H2O will be produced?
Full Stoichiometry #2
• Write and balance the following equation:
Hydrochloric acid reacts with Magnesium metal
to produce hydrogen gas and magnesium
chloride.
• If 5.45 g of magnesium react, how many L of
hydrogen gas will you produce?
Practice• Pg 14-16
• Pg 15 exclude # 3 and 6
• Add vocabulary
• HW #4 Due Monday
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Combined Stoichimetry Practice
• Pages 16
• Add Vocabulary
• HW #4 Due Monday
• Quiz Monday Lesson 5:
Limiting Reactants and Percent Yield
Limiting Reactant
• A limiting reactant is the substance that is
completely consumed during a chemical
reaction.
Limiting Reactant Steps
1. Balance Equation
2. Convert both reactants into product
3. Reactant that produces the least is the
limiting reactant. The other is the excess.
Limiting Reactant Example
2 Al + 6 HBr → 2 AlBr3 + 3 H2
a. When 3.22 moles of Al reacts with 4.96 moles
of HBr, how many moles of H2 are formed?
b. What is the limiting reactant?
c. For the reactant in excess, how many moles are
left over at the end of the reaction?
Percent Yield
• Percent yield is the ratio of the actual yield
to the theoretical or expected yield.
• Theoretical yield: amount of product that
should be produced by a chemical
reaction according to stoichiometric
calculations.
• Actual yield: amount of product actually
obtained from a chemical reaction.
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Percent yield = Actual yield x 100
Theoretical yield
Examples
• What is the percent yield for the
decomposition of 24.8 g of calcium
carbonate if the reaction produces 13.1 g
of solid product?
• CaCO3 CaO + CO2
• What is the percent yield if 1.87 g
aluminum is reacted with an excess of
copper (II) sulfate and 3.74 g of solid metal
(Cu) is produced?
• Al + CuSO4 Cu + Al2(SO4)3
Practice
• Pages 17-18
• Add Vocabulary
• Quiz
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