Transcript Writing Empirical and Molecular Formulas PowerPoint

A: 16 Nov. 2010
Objective: SWBAT calculate percent
composition for a compound and
determine the percentage of water in a
hydrate using laboratory techniques
 Do now: Calculate the molar mass of
calcium chloride. (Hint: Write the correct
formula first!)

Agenda
Do now
II. Percent Composition by Mass: examples
and practice problems
III. Empirical vs. Molecular Formulas
IV. Pre-lab and using a Bunsen burner
Homework: Week 11 Homework #1-8
Read lab handout carefully!
I.
Percent Composition by Mass
What is the percent composition of each
element in dinitrogen trioxide?
 What is the percent composition of each
element in magnesium nitrate?
 What is the percent composition of each
element in glucose (C6H12O6)?

a tougher problem…
Calculate the number of grams of nitrogen
in 50.0 grams of dinitrogen trioxide.
 Calculate the number of grams of
magnesium in 10.0 grams of magnesium
nitrate
 Calculate the mass of carbon in 82.0 g of
glucose (C6H12O6)

Steps
1.
2.
3.
Calculate the molar mass of the
compound.
Calculate the percent composition by
mass of the element in the problem, but
keep it as a decimal, not a percent!
Multiply the percent composition of that
element x the mass given in the problem.
C: 17 Nov. 2010
Take Out Homework: Week 11 #1-5
 Objective: SWBAT differentiate between
empirical and molecular formulas and
calculate a molecular formula.
 Do now: (in your notebook) Calculate the
percent composition by mass of each
element in silver sulfide. (Hint: Write the
correct formula first!)

Agenda
Do now
II. Homework answers
III. Empirical vs. Molecular Formulas
IV. Calculating a Molecular Formula
V. Pre-lab and using a Bunsen burner
Homework: Week 11 Homework #6-8
I.
A: 18 Nov. 2010
Take out homework: Week 11 #1-5
 Objective: SWBAT to write empirical and
molecular formulas for a compounds, given
laboratory data
 Do now: What is the empirical formula of
the compound C8H4O12?

Agenda
Do now
II. Homework solutions
III. Determining molecular formulas
IV. Determining formulas given empirical
data
Homework: Week 11 Homework #6-12: Fri.
I.
Empirical and Molecular Formulas
Molecular Formula

Gives a complete list of all
the atoms in a molecule
Ex: H2O
 2 atoms of H, 1 of O

Ex: C6H12O6
 6 atoms of c, 12 of H, 6 of O

Empirical Formula



Just gives the lowest, whole number RATIO of
elements in the compound
 empirical: “by experiment”
Ex: Glucose (Molecular formula: C6H12O6)
 Empirical Formula: CH2O
 (what is the multiplier?)
Ex: Water (Molecular formula: H2O)
 Empirical Formula: H2O
What is the empirical formula for
each of the following?
1.
H2O2
2.
C6H6
3.
CH4
4.
C2H10
5.
C12H22O11
Finding Molecular formula
Ex 1. What is the molecular formula for a
compound with the empirical formula HO and a
molar mass of 36 g/mol?
 STEP 1 – find the total mass of the empirical
formula given to you
 STEP 2 – divide the molar mass from the
problem by the mass of the empirical formula
 STEP 3 – multiply each subscript in the emp.
form by the number you just found

Examples
Ex 2: CH with a molar mass of 39 g/mol.
 Ex 3: CH2O with a molar mass of 240
g/mol

Determine the molecular formula
1.
2.
3.
CH4N with molar mass 60 g/mol
CH2O with molar mass 150 g/mol
HgCl with molar mass 472 g/mol
Which have the same empirical and
molecular formulas?
C2H4O2 and C6H12O6
 NaCrO4 and NaCr2O7

Which is both an empirical and a
molecular formula?
C5H10O5 (ribose)
 C6H12O2 (ethyl butyrate)
 C55H72MgN4O5 (chlorophyll)
 C12H17ON (DEET)

C: 19 Nov. 2010
Take Out: % Water in a Hydrate Lab
Handout
 Objective: SWBAT calculate the percent
water in a hydrate and determine the
hydrate’s molecular formula.
 Do now: Which pair of formulas have the
same empirical formula? What is it?
a. C2H4O2 and C6H12O6
b. NaCrO4 and NaCr2O7

Agenda
Do now
II. Pre-lab discussion, safety and using a
Bunsen burner
III. Lab!
IV. Lab calculations and conclusions
Homework: Complete lab packet (#5 is
optional): Mon.
Advance notice: Test Weds.
I.
Lab – water of hydration
Hydrate – An ionic compound that contains
water molecules as part of its crystal structure
 Naming/writing formulas
 CaCl2 •2H2O – Calcium Chloride dihydrate
 This is a compound where each molecule of
calcium chloride is combined with 2
molecules of water in the crystal structure
 The prefixes are the same as for naming
compounds

Lab – water of hydration


Anhydrous – The form of a hydrateforming compound with NO water in its
structure
Salt – Any ionic compound
Lab – water of hydration



Objective: To determine the percentage of
water in a hydrate, and to calculate the ratio of
the number of moles of water to the number of
moles of anhydrous salt molecules.
You will be removing the water from a hydrate
and calculating how much water was in it and
then using that value to calculate the formula
for the compound
  CuSO (s) + H O(g)
CuSO4·XH2O(s)  
4
2
heat
Using a Bunsen burner
1.
2.
3.
4.
Be sure the gas valves at your station are
turned off.
Attach the black tubing to the gas outlet at
your station and attach the other end to your
Bunsen burner. Be sure the tubing is
attached securely.
Open the gas valve at your station. You
should hear a soft hissing sound.
Immediately, hold the flint lighter above, and
at an angle to, the top of your Bunsen burner.
Squeeze to produce a spark.
Using a Bunsen burner
5.
6.
7.
8.
If the flame is not immediately ignited, turn off the
gas at your station and wait several minutes
until the gas has dissipated before trying again.
Once lit, turn down the gas at the station, and then
use the airflow adjustments on the Bunsen burner
to produce a quiet flame with smooth edges and
two distinct cones: a darker blue cone inside a
lighter blue cone.
To extinguish the flame, turn off the gas at your
station.
NEVER leave the Bunsen burner lit unless
you are actively using it!
Lab – % Water in a Hydrate





Never set a hot object directly on the lab bench:
place it on a wire mesh.
Never place a hot object on a balance. Let it
cool to the touch first.
Heat with the cover OFF, cool with the cover
ON.
Be careful not to burn your substance. Stir and
move the crucible in and out of the flame to
evaporate evenly.
A hot crucible and a cool crucible look the
same!!
Goggles stay on for the entire lab, until
your area is completely clean.
 When you finish the lab, clean up your
area completely.
 Return clean, dry equipment to the front
table.
 Then, sit down with your group and begin
calculations.

Homework
Lab worksheet due Monday with all
completed calculations. Show all work!
(#5 optional)
 Test Weds.
 You will be writing a formal lab report –
started in class next week.

A: 19 Nov. 2010
Take Out Homework: Week 11 #6-10
and Hydrates Lab worksheet!
 Objective: SWBAT calculate an empirical
formula from lab data, and write a lab
report for the hydrate lab.
 Do now: What is the molecular formula
for the compound with empirical formula
PCl2 and molar mass 306 g/mol?

Agenda
Do now
II. Homework solutions
III. Empirical formula practice problems
IV. Lab report writing overview
V. Lab report work time
Homework: Finish Week 11 Homework,
Lab report 1st draft due Friday, Dec. 3
I.
Finding Empirical Formula in the
Lab
When we do Analytical Chemistry in the
lab, we can find values for the amount of a
certain element in a compound
 Like when we found the amount of water
in CuSO4 • XH2O
 We have a system for taking the values we
get in the lab and figuring out the
empirical formula

Determine the empirical formula for a compound that
is 74.83% carbon and 25.17% hydrogen by mass
STEP 1 - Pretend you have 100 grams of the
substance, make the % values into numbers of
grams
STEP 2 - Convert each mass value into moles
STEP 3 - Divide all mole values by smallest # of
moles
STEP 4 - Use the resulting numbers as your
subscripts
Determine the empirical formula for a compound
that is 25.9% nitrogen and 74.1% oxygen.
STEP 1 - Pretend you have 100 grams of the
substance, make the % values into numbers
of grams
STEP 2 - Convert each mass value into moles
STEP 3 - Divide all mole values by smallest #
of moles
STEP 4 - Use the resulting numbers as your
subscripts
Finding Empirical Formula
Sometimes you won’t be given percentages,
but values for each element out of a total value
 Ex: What is the empirical formula for a
compound where a 10.0 g sample contains
9.41 g O and 0.59 g H?


To solve: Just write down the number of
grams of each, then proceed as before.
Practice Problems
1.
2.
3.
4.
Calculate the empirical formula of a
compound composed of 79.8% C and 20.2% H
Calculate the empirical formula of a
compound composed of 67.6% Hg, 10.8% S
and 21.6% O.
Calculate the empirical formula of a
compound where a 50.0 gram sample
contains 29.4 g of C, 4.90 g H and 15.7 g O.
Calculate the molecular formula of the
compound in #3 if the molar mass is 204
g/mol.
For the rest of class…
Choose one section
 Spent the rest of this period (quietly!)
making a rough draft in your notebook
 I’ll pass back papers

Homework!
Week 11 HW, # 11 – 15: Fri.
 Lab report first draft: Friday. Dec. 3

C: 23 Nov. 2010



Take Out Homework: Week 11 #9-16
Objective: SWBAT solve all types of “determine
the formula” problems.
Do now: Put these steps in order to solve a
“calculate the empirical formula” problem:
a. Divide all numbers by the smallest number of
moles.
b. Change % to grams
c. Round to the nearest whole number and use
that number as the subscript
d. Divide by atomic mass to get moles
Agenda
Do now
II. Homework solutions
III. Lab report rubric
IV. “Determine the formula” self-assessment
V.
Practice Problems
VI. Unit Review Game!
Test Wednesday
Homework: Finish Practice Section III
(Section IV is optional but helpful!)
Lab report: Due next Friday
I.
C: This week
Today: Extra help after school!!! 3:30-4:30
Room 203
 Wednesday: Unit Test
 Atoms/molecules  moles  mass problems
 Determining empirical/molecular formulas
 Thursday/Friday: No class! 
 Next week Mon: Lab report work time (we
just ran out of time this week)

Hand out lab report rubric
Title
 Your lab report should be titled with a title of your
choosing.
 It should be relevant to the lab and should not just be the
same as the title of a handout your teacher gave you.
 Do not have it on a separate title page.
 Include your name, the date, class section and the names
of your lab partners
Background
 Explain any important concepts that are central to
Information
understanding the experiment.
Provide an overview  In paragraph form, define any scientific terms necessary
of necessary
for understanding the lab.
information
 Link these concepts and terms together into a coherent
paragraph
Objective
State the reasons
for the experiment
 Explain what the purpose of the experiment or what
goals you have in completing the lab. What do you hope
to accomplish?
Summary of
 Write a summary of what you did. Do not
Procedure
rewrite the given procedure word for word. The
Summarize what
summary should be in paragraph form and,
you did
ideally, in past tense.
 Include a labeled diagram of the experimental
set-up.
Data and
 Include a data table that shows what data you
Results
collected. Be sure it shows what each number
Present the
means!
results of your
 Data should be presented neatly!
experiment.
 Include all calculations you did. Show each step
and a short description of what you did for each
step.
 Include paragraph(s) of qualitative data and
descriptions.
Analysis
 Describe what your data mean. That is, what do the
Discuss patterns and
results of your calculations tell you?
trends in the data
 Describe what your qualitative observations mean.
Conclusions
Explain what you
can draw from the
data
 Was the experiment successful at completing the
objectives above? Explain why or why not.
 Calculate the actual percent water by mass of
CuSO4·5H2O.
 Compare your percent water to the actual percent water
by calculating percent error. Show your calculations.
 Describe all sources of error in the experiment. Explain
how they affected your data (for example, did they make
your mass of water increase or decrease? Did this make
your moles of water increase or decrease? Did this make
your number of molecules of water increase or decrease?
Be specific!)
 Explain how each of your errors could be minimized and
improved.
Self-Assessment
1.
2.
Silently, on your own, solve the first three
problems (Type A, B and C) and check
your answers. (12 min.)
Work on Section II: Use the results of
Section I to determine which types of
problems you need to practice most!
 For example, if you got “Type A” wrong
in Section I, do Section II, Type A.
After you take the self-assessment…
Parts of Section II are optional, but
recommended if you got any of Section I
wrong.
 You MUST complete Section III – Mixed
Practice, and check your answers.
 Section IV is optional (but recommended!) –
Practice Quiz, and check your answers.
 Anything you don’t finish by the end of 1st
period is homework

Homework
Test Wednesday
Finish Practice Problems Section III: due
tomorrow
Section IV is optional, but a great way to
study!
Study your mass-moles-atoms/molecules
quiz, and redo problems you got wrong
Lab report: Due next Friday
24 Nov. 2010


Objective: SWBAT show what you know
about moles conversions and determining
formulas on a test!!
Questions?
Periodic table is on the last page.
 You can rip it off!
 When you are finished, do one of the
following:
 Begin working on your lab report first
draft in your notebook.

 Lab
report work day Monday!
 Begin
working on extra credit problems.
(due Monday!)
Lab report work time
Save your work to a USB or to your email,
NOT to the desktop or laptop!!
 Stay in your seat and work quietly.
 Follow the checklist!!
 Raise your hand if you have questions.
 Lab report due Friday, December 3 in
class.
