Download Stoichiometry – AP - Waukee Community School District Blogs

Check for Understanding-Whiteboard
 Name the following:
Cr2O3
FeO
NiCl2
S3Cl6
HI
HNO3
Write the chemical formula for the following:
Iron (III) Oxide
Calcium Nitride
Diphosphorus hexaiodide
Carbonic acid
Hydrochloric acid
Stoichiometry
Ch. 3
Lab A Partners
 Group 1:
 Group 2:
 Group 3:
 Group 4:
 Group 5:
Chapter 3 - Stoichiometry
 Atomic Mass
 The Mole
 Molar Mass
 Percent Composition
 Empirical and Molecular Formulas
 Balancing Chemical Equations
 Stoichiometry—Limiting Reactants
 Percent Yield
Learning Objectives – Part 1
 I can calculate average atomic mass of an atom from mass
spectrometry and isotopic data.
 I can calculate the molar mass of a compound and the percent
composition of an element in a compound.
 I can convert between mass, moles, and numbers of
representative particles (atoms, molecules, formula units) of a
substance.
Warm-up!
 Balance the following equations.
(a.)
(NH4)2Cr2O7(s) 
(b.)
NH3(g) +
Cr2O3(s) +
O2(g) 
NO(g) +
N2(g) +
H2O(g)
H2O(g)
The Mole
Definition:
The number equal to the number of carbon atoms in
exactly 12 grams of pure C-12. This number has been
determined (via Mass Spectrometry) to be 6.022 *10 23
(Avogadro’s Number).
One mole of something consists of 6.022 *10
23 units or particles of that substance. (Like a
dozen or ream)
Video
The mass of 1 mole of an element is equal to
its atomic mass in grams.
Example – The Mole
 Example Problem 1: Americium is an
element that does not occur naturally. It
can be made in very small amounts in a
device known as a particle accelerator.
Compute the mass in grams of a sample of
americium containing six atoms.
 Example Problem 2: A silicon chip used in
an integrated circuit of a microcomputer
has a mass of 5.68 mg. How many silicon
atoms are present in the chip?
Molar Mass
The mass in grams of one mole of the
compound.
Obtained by summing the masses of the
component atoms.



Example Problem 3: Juglone, a dye known for centuries, is
produced from the husks of black walnuts. It is also a natural
herbicide that kills off competitive plants around the black walnut
tree but does not affect grass and other noncompetitive plants. The
formula for juglone is C10H6O3.
(a.) Calculate the molar mass of juglone.
(b.) A sample of 1.56 x 10-2 g of pure juglone was extracted
from black walnut husks. How many moles of juglone does this
sample represent?
Molar Mass Practice Time - 10 minutes
Percent Composition
Mass percent of the elements can be
determined from the formula of the
compound by comparing the mass of
each element present in 1 mole of the
compound to the total mass of 1 mole of
compound.
 Hint: Always check that percentages add up to 100.

Example Problem 7: Calculate the mass percent of
ethanol, C2H5OH.
Percent Composition – Practice 10 minutes
Empirical/Molecular Formulas
Empirical – Molecular Formula
Determination
1. Mass Percent to Grams (100 g sample)
2. Convert each element from grams to
moles
3. Divide all mole values by the smallest.
4. Subscripts obtained if whole number. =
Empirical Formula
5. Divide molar mass by empirical
formula mass to obtain integer to multiple
all subscripts in formula. “Method 1”
Calculating Empirical Formulas
Example Problem 10: Determine the empirical
and molecular formulas for a compound
that is given the following analysis (in mass
percents): 71.65% Cl; 24.27% C; 4.07% H. The
molar mass is known to be 98.96 g/mol.
Check for Understanding- Whiteboard
A sample compound weighing 83.5
g contains 33.4 g of sulfur. The rest is
oxygen. What is the empirical
formula and chemical name?
Empirical/Molecular Formulas
Empirical – Molecular Formula
Determination
1. Mass Percent to Grams (100 g sample)
2. Convert each element from grams to
moles
3. Divide all mole values by the smallest.
4. Subscripts obtained if whole number. =
Empirical Formula
5. Divide molar mass by empirical
formula mass to obtain integer to multiple
all subscripts in formula. “Method 1”
Example
A sample of 100g of acetic acid contains
39.9 g C, 6.7 g H, and 53.4 g O. The molar
mass of the compound was determined
by experiment to be 60.0 g/mol.
Determine the molecular and empirical
formula.
Learning Objectives –Part 2
 I can write a balanced chemical equation and predict the amount
of product formed from a given mass of reactant or the amount of
reactant required to produce a desired amount of product.
 I can identify limiting reactants, and calculate the amount of product
formed when given the amounts of all the reactants present.
 I can calculate the percent yield of a reaction.
Chemical Equations
Chemical Reactions
 Reorganization of the atoms in one or more substances.
 Bonds are broken and new ones form.
 Atoms are neither created nor destroyed.
 All atoms present in the reactants must be accounted for among the products. =
Balancing a chemical equation!
Reactants (initial substances) left of arrow
Products (substances produced) right of
arrow
The relative number of reactants and
products in a reaction are indicated by
the coefficients in the balanced equation.
Balancing Chemical Equations
ALWAYS check to make sure the equation
you are working with is balanced!!!
Use element tables to help.
Writing and Balancing the Equation for a
Chemical Reaction
 1. Determine what reaction is occurring. What are the reactants, the
products, and the physical states involved?
 2. Write the unbalanced equation that summarizes the reaction described in
step 1.
 3. Balance the equation by inspection, starting with the most complicated
molecule(s). Determine what coefficients are necessary so that the same
number of each type of atom appears on both reactant and product sides.
Do NOT change the identities (formulas/subscripts) of any of the reactants or
products.
Example
• Nitrogen and oxygen combine to form dinitrogen pentaoxide.
• Calcium nitrate reacts with lithium sulfide forming calcium sulfide
and lithium nitrate.
Partner Practice
 Solutions of lead (II) chloride and sodium chromate
react to produce a precipitate of
lead (II) chromate and a solution of sodium chloride.
 Solid calcium reacts with solid sulfur to produce solid
calcium sulfide.
 Hydrogen gas reacts with fluorine gas to produce
hydrogen fluoride gas.
Warm-up Question
Write the following balanced chemical reactions.
 Liquid carbon disulfide reacts with oxygen gas to produce
carbon dioxide gas and sulfur dioxide gas.
 Aqueous solutions of sodium chloride and silver nitrate react to
produce aqueous sodium nitrate and a precipitate of silver
chloride.
 Bubbling chlorine gas through a solution of potassium iodide
gives elemental iodine and a solution of potassium chloride.
Combustion Analysis - POGIL
 Analyze substances for carbon and hydrogen.
 A sample is burned in the presence of oxygen, which converts all its
carbon to carbon dioxide and all its hydrogen to water.
 The compounds are absorbed using appropriate materials and their
amounts determined.
 The results provide the mass of each type of element in the
compound, which can be used to determine mass percent of each
element and/or empirical formula.
Peer activity- Combustion Analysis
ALL GROUP Members participate in building
understanding! Below are side jobs to learning and
collaborating.
 Getter 1 (1)
 Group member that Collects and distributes materials for beginning of
activity/day.
 Group member that Collects and returns materials at the conclusion of
activity/day.
 Reporter (2)
 Group member that shares thoughts with class; only member that can ask
teacher question
 Starter (3)
 Group member that begins activities and keeps an eye on the time
Combustion Analysis Activity Groups
Getter
Starter
Reporter
Group 1:
Jenna-
Jacob-
Kaitlyn
Group 2:
Kirsten-
Tyler-
Atul
Group 3:
Caleb-
Tess-
Emily
Group 4:
Aravin-
Sura-
Alyssa
Group 5:
Natalie-
Rabab-
Tim
Check for Understanding -whiteboard with a partner!
Hydrates
 Example Problem 17: (Hydrates) Washing soda is a
hydrate. Its formula can be written as Na2CO3∙xH2O.
When a 2.558-g sample of washing soda is heated at
25°C, all the water of hydration is lost, leaving 0.948 g of
Na2CO3. What is the value of x? [10; Na2CO3∙10H2O]
Stoichiometry  Flowchart on page 111
1. Balance the equation for the reaction
2. Convert the known mass of the reactant or
product to moles of that substance.
3. Use the balanced equation to set up the
appropriate mole ratios.
4. Use the appropriate mole ratios to
calculate the number of moles of the desired
reactant or product.
5. Convert from moles back to grams if
required by the problem.
Example
Example Problem 18: (A template for solving
stoichiometry problems!!)

(a) What mass of oxygen will react
with 96.1 grams of propane?

(b) How many liters of carbon dioxide
gas at STP (1 atm, 273K) would be formed?

(c) How many water molecules are
produced?
Stoichiometry Model
 Example Problem 19: Solid lithium hydroxide is used in
space vehicles to remove exhaled carbon dioxide from
the living environment by forming solid lithium carbonate
and liquid water. What mass of gaseous carbon dioxide
can be absorbed by 1.00 kg of lithium hydroxide?
Practice

Calculate the mass of oxygen produced if 2.50 g of potassium chlorate are
completely decomposed by heating.

(0.976 g O2)
Practice

If 20.0 g of magnesium react with excess hydrochloric acid, how many
grams of magnesium chloride are produced?

(78.4 g MgCl2)
Practice

How many grams of chlorine gas must be reacted with excess sodium
iodide if 10.0 g of sodium chloride are needed?

(6.07 g Cl2)
Practice

What mass of copper is required to replace silver from 4.00 g of silver
nitrate? (Hint: Copper (II) product is formed!)

(0.747 g Cu)
Limiting Reactants
 Think of this as just a multi-part stoichiometry problem.
 Example 21:
 The Haber process (the AP Exam likes to ask questions about this…) is
making ammonia for fertilizer production from the nitrogen in the air
reacted with hydrogen gas. The hydrogen gas is obtained from the
reaction of methane with water vapor. This process has saved millions
from starvation. Suppose 25.0 kg of nitrogen reacts with 5.00 kg of
hydrogen to form ammonia. What mass of ammonia can be
produced? Which reactant is the limiting reactant? What is the mass of
the reactant that is in excess?
Practice
 Zinc metal reacts with hydrochloric acid by the following reaction:
Zn(s) + 2HCl(aq)
ZnCl2(aq) + H2(g)
 If 0.30 moles of zinc is added to hydrochloric acid containing 0.52 mol
HCl, how many moles of H2 are produced?
Practice
 In a process for producing acetic acid, oxygen gas is bubbled into
acetaldehyde, CH3CHO, containing manganese (II) acetate under
pressure at 60C.
 2CH3CHO(l) +
O2(g)
2HC2H3O2(l)
 In a laboratory test of this reaction, 20.0 g of CH3CHO and 10.0 g of
O2 were put into a reaction vessel.
 How many grams of acetic acid can be produced by this reaction
from these amounts of reactants?
 How many grams of excess reactant remain after the reaction is
complete?
Percent Yield
Theoretical Yield
The amount of a product formed when
the limiting reactant is completely
consumed.
Percent Yield
The actual yield of product is often given
as a percentage of the theoretical yield.
(Actual yield ÷ Theoretical Yield) x 100% =
Percent Yield
Percent Yield Model
 Example Problem 24: Methanol (CH3OH), also called methyl
alcohol, is the simplest alcohol. It is used as a fuel in race
cars and is a potential replacement for gasoline. Methanol
can be manufactured by combination of gaseous carbon
monoxide and hydrogen. Suppose 68.5 kg CO(g) is reacted
with 8.60 kg H2(g). Calculate the theoretical yield of
methanol. If 3.57 x 104 g CH3OH is actually produced, what
is the percent yield of methanol?
HCl(aq) + NaOH(aq)  NaCl(s) + H2O(l)
Start with 5.0 grams of NaOH and after the reaction I have 6.3 grams of NaCl.
What is the percent yield of the reaction?
With A Partner
 A student runs a reaction to prepare 40.0g of aspirin and yet recovers only
15.5g. What is the percent yield (please show/be able to explain your work)?
CH4 + 2O2  CO2 + 2H2O
If you react 5.00 g of methane, CH4, with 12.00g of oxygen, O2, what is the
theoretical yield of H2O?
After completing the reaction there was 4.60g of H2O, what is the percent
yield?
Quiz Unit 1
 Complete Individually!
 Pre-lab for Lab B DUE TOMORROW!
Read through lab handout (in black binder)
Complete the 2 pre-lab questions
Write an abstract of Part 2 (omit Part 1 –
sample data provided)
Lab Safety
 Lab Safety Video
 Lab Safety Contract
 Lab Expectations
Labs over Ch. 3
 Lab A: Empirical Formula
 Complete Lab Report
 Due _______
 Lab B: ALUM Metal Hydrate Formula
 Complete Pre-lab
 Due _________
Quiz Analysis
 Locate a DIFFERENT colored pen/pencil.
 3 minutes reflect individually.
 3 minutes communicate with a peer.
 Questions?!
Lab Report Revision Process
 Different colored pen/pencil or separate sheet ATTACHED to draft 1.
 Review
 Individually
 With Peer
 With Mrs. Bechtum
 DUE: Tuesday, January 20th!
 Questions?
Lab B: Analysis of ALUM
 Pre-Lab Questions
 Safety
 Part 1: Sample Data
Trial #1
Trial #2
Measured melting point
92.1°C
92.8°C
Literature melting point
92.5°C
92.5°C
Alum Lab Pre-lab Questions
 1. When measuring a melting point, why is it necessary to raise the
temperature very slowly in the vicinity of the melting temperature?
 2. Washing soda is a hydrated compound whose formula can be written
Na2CO3x H2O, where x is the number of moles of H2O per mole of Na2CO3.
When a 2.123 g sample of washing soda was heated at 130 C, all of the
water of hydration was lost, leaving 0.787 g of anhydrous sodium
carbonate. Calculate the value of x.
Lab B: Analysis of ALUM
 Part 2:
 It is important to heat slowly at first so that the evolving water of
hydration does not carry the alum crystals with it!
 Multiple heatings (record number and duration of heatings recommend 5 minute increments)
 The remainder of the class period is yours to review/prepare for
tomorrow’s exam and/or begin working on Lab B report.
Let me know if you have questions!
Tomorrow…
 Exam 1!
 2 Parts
 Multiple Choice – 40 minutes (no calculator)
 Free Response – 40 minutes (calculator)
 NO ELECTRONIC DEVICES ALLOWED IN CLASSROOM!
 Begin working on Lab B Report (Due Friday!!)
UNIT 1 Exam!
 2 Parts
 Multiple Choice – 40 minutes (no calculator)
 Free Response – 40 minutes (calculator)
 NO ELECTRONIC DEVICES ALLOWED IN CLASSROOM!
 NO Talking!
 You CAN do this!!
 For tomorrow…
 Complete Lab B Report
 Read and take notes on Ch. 4 (Sections 1-6)
 Unit 2 Review Packet and Topic Check list available on blog and on front table