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Stoichiometry

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Valerie Henderson

on 24 February 2016

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Transcript of Stoichiometry

Stoichiometry
Stoichiometry
The best definition for stoichiometry is this:
It’s a way of figuring out how much stuff you’re going to make in a chemical reaction,
or
how much stuff you’ll need
to make a chemical reaction do what you want.
Draw the diagram
When you're working on your stoichiometry...
Have this diagram out.
Check to make sure you're following it.
Once you're confident...ditch it.
Warm-up_Stoichiometry 1/23
Think of it in terms of peanut butter and jelly sandwiches...not exactly a chemical reaction but it will illustrate my purpose
If I'm hungry for a Pb&J...I'm going to figure out how much bread I have.
I have 10 slices of bread.
How many sandwiches can I make?
5 sandwiches
Notice that the only 2 places you can use the mole ratio to convert between different chemicals occurs between which two quantities?
moles
Liters
(volume)
where does the mole ratio come from?
The mole ratio comes
directly
from the BALANCED CHEMICAL EQUATION
atoms of
reactant
atoms of
product
6.02 x 10
23
atoms
1 mole
6.02 x 10
23
atoms
1 mole
The Great Balloon Stoichiometry Competition
Exercise vs Problem
An exercise is simpler.
You follow a prescribed method.
the stoichiometry we have been practicing for the last 2 units have been exercises.
For the most part, you know what you are doing and the method you will use to complete the exercise.
A problem is more difficult.
You aren't given a prescribed method.
You shouldn't know exactly what to do.
You will struggle a bit at first.
You may be initially given more OR less info than you need to solve the problem.
Problems often mimic real-life situations or problems.
DA: Dimensional Analysis
With your group, please
make 1 dollar
out of each set of coins.
Discuss
with your group if the dimes and the the nickels measure the same thing? (are they the same amount of money?)
Now, please write a mathematical relationship to show this.
1.
10 dimes
20 nickels
What's a Chemist's Favorite Tree?
1 mole
6.02 x 10 particles
23
12 g C
1 mol C
Stoichiome-TREE!
20 nickels
10 dimes
Do these 2 fractions equal the same thing?
They equal the same thing because each fraction equals the amount of 1 (dollar).
Did you write?
10 dimes = 20 nickels
Conversion factors change something to different version of form.
What is a factor?
Something that brings results or a cause.
What's a conversion?
The action of changing the "version" of a thing.
Let's think about this in math...(think: algebra)
A conversion factor is used to convert a measured quantity.
10 dimes
to a different unit of measure
pennies
without changing the relative amount.
What's the amount?
10 cents
How do we do this?
A fraction!
(that equals 1)
ratio
What's a ratio?
part
whole
part
part
ways to write ratios?
part: part
or!
a ratio is just...
From above...our conversion factors for dimes to nickels in 1 dollar is really just a set of proportions...
check it out...
Warm –up 5.1 “Mole Problems”

What is the conceptual idea (not the number value) of a mole? (why do we have moles?)

How many atoms are in 1 mole?

The masses on the periodic table tell you amount of _____ in 1 mole of that element. (hint: unit of measure)

Air Canada Powerpoint
Here’s one penny to get you started…
Warm-up 5.2 “Pennies from my high school to yours”
How many pennies would it take to stretch from here (JCHS) to the high school I went to.
Start with?
End with?
3942.73 Km
pennies!
How do I get there?
convert, convert, convert!
x
1,000 m
1 Km
x
100 cm
1 m
x
1 penny
? cm
How many m in 1 km?
1000 m = 1 km
conversion factors?
1,000 m
1 Km
1 Km
1,000 m
Which to use? (Which will cancel out Km?)
How many cm in 1 m?
100 cm = 1 m
What is the diameter of your penny?
~1.91 cm
Did I set it up right?
(unit cancellation dance)
Warm- up 5.2 "Pennies from my high school to yours"
"What is Percent Yield?"
What is a percent (think: type of ratio)
How do you figure out a percent?
part: whole
whole = 100
can you express the ratio another way?
part
whole
Important dimensional analysis note:
on top = multiply
bottom = divide
Just like fractions
5
3
3
3 x 5
What is this?
Can't you just cancel out the 3s?
3
3 x 5
1
5
They're just division...remember fractions by pizza?
Quick Note on Calculator Use
always use EE!
Then you don't have to worry about order of operations
on most calculators it's 2nd EE for scientific notation.
2 x 10
5
Press 2, 2nd, EE, 5
2 5
calculator will look like:
E
means 2 x 10
5
Sig Figs - dem zeros
decimal, to right and at the end = significant,
to the left of the decimal = not

extraneous zeros are eliminated with scientific notation
make me a sandwich (of zeros) = zeros between other numbers = significant
How to remember if they're significant or not...
d =
e =
m=
Rules of Figs...Sig Figs, That is!
1. any # not zero = a significant figure
2. zeros between any non-zero #
3. zeros at the end of a number and to the right of a decimal point are always significant
Who's significant?
43.00 1.010 9.000
4 sig figs
Struggling? change the # to scientific notation
Significant Figures
62/41 = 1.512195122
ugly!
How many #s do I round to?
answer already answered by #s you divided in the first place.
62 = 2 sig figs
41 = 2 sig figs
round answer to 2 figures
1.5
How did I know 62 and 41 had 2 significant figures?
1
5
DA: Dimensional Analysis
5. How many seconds are there in 2.5 hours?
(see whiteboard )
Do numbers 6 and 7 on your own.
How many sig figs do I get?
conversion factors within systems exact numbers (meaning they have infinite sig figs)
You're limited by your measurement of the diameter of the penny. (Remember the pogil with the rulers on it?)
You record the number based on the tick marks of the ruler and estimate one digit further...(in between the millimeter ticks)
1.91 cm
How many sig figs?
3
actual amount of product expressed as a percent of the theoretical amount of that substance.
% yield
=
actual amount of product from the lab
calculated amount from stoichiometry
Percent yield:
answer in a decimal...
Remember place value to convert it to a percent!
vocabulary time:
Zinc:
1 mole of atoms
salt:
1 mole of a formula unit
water:
1 mole of water molecules
Na and Cl is a formula unit
for molecular compounds, it's a molecule
These are all referred to as representative particles or simply particles
Lab - Stoichiometry and Limiting Reactant
Calculating percent composition from mass data...
13.60 g sample of a compound containing only magnesium and oxygen is decomposed, 5.40 g of oxygen is obtained. What is the percent composition of this compound?
What do you know?
mass compound = 13.60g
mass of oxygen = 5.40 g O
why just O?
remember that oxygen wants to be bonded to itself OR someone else...
mass of magnesium =
13.60 g -5.40 g = 8.20 g
How can we figure out % by mass of an element?
it's the mass of that element divided by the mass of the compound multiplied by 100%
So that would be...
% Mg?
% O ?
= mass of Mg
mass of compound
=
8.20 g
x
100%
13.60 g
=
60.3%
x 100%
=
mass of O
mass of compound
x
100%
=
5.40 g
13.60 g
x 100%
=
39.7%
% composition:
take a chemical formula (which is what type of ratio?) and convert it to a part to whole ratio.
So today, we're going to do it backwards...
If you're given % composition, you need to give a formula.
When we do this backwards, all we'll be able to tell is the simplest whole number ratio.
What type of formula is this called?
empirical formula
glucose formula?
What is the ratio of C to H to O?
It's 1:2:1
what kind of a compound is glucose?
molecular
ionic compounds tend not to be a problem. why?
they only have one formula.
It's a problem with molecular compounds.
what does it mean when something is derived empirically?
from data
data we're going to use is % composition
% by mass
mass to mole
divide by small
multiply til all
assume 100 g - to make your math easier
clearing fractions to make whole numbers
remember, what you do has to be done to ALL!!!!
Clearing Fractions...
To make whole numbers...
What do you multiply the entire thing by when you have .5?
.5 =
1
2
So you just multiply by the reciprocal
Which is 2
This will clear your fraction.
.3
1
3
=
What would you multiply by to make it a whole #?
(or to make it 1)
3
.25
Well, that's a quarter...
So you would multiply by 4
.2
Well that's a fifth
1
5
So multiply by...
5
unit ratio
Empirical Formula
What is an empirical formula?
it's just the lowest whole-number ratio of the elements in a compound.
For example, a person has 10 fingers and 2 hands. What would be the formula for this?
H F
2
10
Now what would the lowest whole number ratio of hands to fingers?
HF
5
What if we have a 3 handed alien with 5 fingers per hand?
That would be:
H F
3
15
What would the empirical formula be?
HF
5
chemical ratio = part: part
Outline part III #15
Outline part III #16
Find the simplest (empirical) formula of a compound found to consist of 26.56% K, 35.41% Cr, and 38.03% O.

What is a percent?
part: whole
whole = ?
100
So, if your whole is 100 grams, what does that mean each percent is telling you?
Each percent tells you how many grams!
K = 26.56 g

Cr = 35.41 g

O = 38.03 g
How many moles of each?
x 1 mol K
39.10 g K
1 mol Cr
x
52.00 g Cr
x
1 mol O
16.00 g O
Mole Island is a helpful tool (meaning: You should be using this to solve all dimensional analysis problems!)
Let's do some thinking:
How many grams are there in 1 mole of an element or substance?
*where do I look this up?
*
How many particles (atoms or molecules) are there in 1 mole?
Can you write this as a conversion factor? (outline part I)
What is the mass of 1 mole of dihydrogen monoxide (in grams)?
for molecules (or ionic compounds):
The mass of 1 mole is the mass of each element present added together (make sure to account for when is more than 1 atom present - x molar mass by 2)
2(1.01) + 16.00 =
18 grams H O
2
1 mole H O
2
How many seconds are there in 3 days?
days
seconds
How do we get from days to seconds?
3 days
minutes
hours
x
24 hours
1 day
How many hours in 1 day?
24 hours = 1 day
What are the two conversion factors for this conversion?
24 hours
1 day
1 day
24 hours
Which one do I use?
x
60 minutes
1 hour
1 minute
60 seconds
x
=
259, 200 seconds
1 mole of gas at STP has a volume of 22.4 L
STP = Standard Temperature and Pressure
Temperature = 20 degrees celsius
Pressure = 1 atm (101.325 KPa)
What is the volume of 1 mole of gas @ STP?
1 mole gas at STP = 22.4 L
Can you write two conversion factors for this?
1 mole gas
22.4 L
22.4 L
1 mole gas
Which do I use when?
moles gas
liters gas
mole gas x
= liters gas
1 mole gas
22.4 L
moles gas
liters gas
liters (L) gas
x
= moles gas
22.4 L
1 mole gas
(L)
"Island Hopping"
Outline part II #1 a.
If 8.00 moles of water react with carbon dioxide, how many moles of glucose are produced?
What eqn. is this?
What type of reaction is this?
(reactions of our lives)
double replacement
First, I need to know the mole ratio...
(The ratio of moles of water to moles of glucose.)
in greek = "counting atoms"
6 moles H O
2
1 mole C H O
6
12
6
Where does the mole ratio come from?
balanced equation!
What do I start with?
8 mol H O
2
x
moles H O
2
moles C H O
6
12
6
How do I convert between the two?
mole ratio!
6 moles H O
2
1 mole C H O
6
12
6
=
1.33 mol C H O
6
12
6
How do I know what to round to?
How many significant figures in my measurement of 8.00 moles?
3 (D.E.M. zeros...)
=
=
=
.679 mol K
.68 mol Cr
2.376 mol O
.679 mol K
.68 mol Cr
2.376 mol O
K Cr O
.68
.68
2.376
.68
.68
.68
Divide by small (divide by smallest number of moles)
3.49
KCrO
Round up to 3.5
How do you clear the 3.5?
.5 is what as a fraction?
1
2
So, to get rid of that, you multiply by 2.
2(KCrO )
3.5
K Cr O
2
2
7
1 mole = 6.023 x 10
23
6.023 x 10
23
particles
1 mole
All roads lead through mole island.
Always converting to or from moles or to and then through moles.
1 mole
x grams
x grams
1 mole
Quick Dimensional Analysis Re-cap:
Conversion:
Conversion Factors:
Criss Cross Applesauce:
The equals sign one.
1 mole = 6.02 x 10 particles
23
An example:
made from the conversion
2 are possible
any conversion factor can be flipped (because the fraction equals 1)
An example from the above conversion:
1 mole
6.02 x 10 particles
23
6.02 x 10 particles
1 mole
23
Write quantity given in problem
Insert appropriate conversion factor (hint: What do you need on the bottom to cancel out the unit?)
Multiply the top, divide by the bottom
Need more help? DA: Dimensional Analysis from unit 1 (metric conversions)
Purpose: In order to compare one chemical in a chemical reaction to another, we must compare the amount of moles of each chemical.to convert from what you have to what you want. In chemistry lab, we don't have a mole-o-meter so we can only take chemical measurements in grams (mass) or liters (volume).
An Example:
You need to know how many particles are in 10 moles of a substance.
10 moles
x
6.02 x 10 particles
1 mole
23
=
6.02 x 10 particles
24
6.02 x 10 particles
1 mole
23
1 mole
6.02 x 10 particles
23
22.4 L
1 mole
1 mole
22.4 L
1 mole of any gas @ STP has a volume of 22.4 Liters
x is the mass of 1 mole of the chemical (BFF 1.0)
What is the conversion for particles (atoms or molecules) to moles?
1 mole = 6.02 x 10 particles
23
What are the 2 conversion factors that can be written from this conversion?
1 mole
6.02 x 10 particles
23
6.02 x 10 particles
1 mole
23
What is the conversion for grams of potassium to moles of potassium?
Which conversion factors are the same no matter what chemical you have?
particles (atoms/molecules) and liters
(atom/molecule island and liter island)
What is the conversion for liters?
At STP, 1 mole of any gas has a volume of 22.4 L.
22.4 L
1 mole
1 mole
22.4 L
What are the 2 conversion factors that can be written from this conversion?
1 mole = 22.4 L
STP = standard temperature and pressure
Each element has its own molar mass.
What is molar mass?
The mass of 1 mole of the substance.
Molar Mass:
How do you know molar mass of a substance?
The masses on the periodic table (BFF 1.0) = the mass of 1 mole of that chemical.
What is the mass of 1 mole of potassium?
The mass of 1 mole of potassium = 39.10 grams.
1 mole K = 39.10 g K
*On your BFF, it is rounded to 39.10 so you can write exactly the number you see on your BFF. Round at the end.
What are the 2 conversion factors that can be written from this conversion?
1 mole K
39.10 g K
39.10 g K
1 mole K
What is an example for converting from grams to moles?
In lab, you measured 47 g of silver using an analytical balance. How many moles of silver did you have?
47 g Ag
x
107.862 g Ag
1 mole Ag
=
Remember, if it's on top, multiply, if on bottom, divide!
0.4357 mole Ag
How do I know how what to round my answer to?
Rules of Figs!
(Sig figs that is!)
In our data, we had 47. How many significant figures are in 47?
2
So, we can report 2 significant figures in our answer.
0.44 mole Ag
Island Hopping
Stoichiometry TLC
5C + 2SO CS + 4CO
How many grams of carbon monoxide are produced from the reaction of 16.0 g of sulfur dioxide with carbon?
2
2
start
end
grams
sulfur dioxide
grams
carbon monoxide
mol
sulfur dioxide
mol
carbon monoxide
How do you convert between grams of a chemical and moles of the same chemical?
molar mass
mole
grams
= mass (in grams) of 1 mole of the chemical
grams SO
2
moles SO
2
molar mass:
S:
O:
2
32.07 (1) = 32.07
16.00 (2) = 32.00
+_______
64.07 g SO
1 mol SO
2
2
Two conversion factors for moles and grams of sulfur dioxide:
64.07 g SO
1 mol SO
2
2
1 mol SO
64.07 g SO
2
2
or
16.0 g SO
2
x
molar mass
1 mol SO
64.07 g SO
2
2
x
mole ratio
What is the mole ratio?
It's a way to convert between moles of one chemical and moles of a different chemical.
Where do you get the numbers of moles for the mole ratio?
The coefficients of the balanced equation.
(the numbers in front of the letters)
Which moles do you use?
The coefficients for the chemicals you're converting between.
5C + 2SO CS + 4CO
2
2
What moles are you converting from?
SO
2
What moles are you converting to?
CO
So you need the mole ratio for moles of SO to moles of CO
2
2 mol SO
4 mol CO
2
4 mol CO
2 mol SO
2
or
2 mol SO
2
4 mol CO
molar mass
How do you convert from moles of one chemical to grams of that chemical?
molar mass
(the mass of 1 mole of that chemical)
moles of CO
g of CO
molar mass CO
C: 12.01 (1) = 12.01
O: 16.00 (1) = 16.00

+ _______
28.01 g CO
1 mol CO
28.01 g CO
1 mol CO
1 mol CO
28.01 g CO
or
x
1 mol CO
28.01 g CO
=
13.98969588 g CO
13.9 g CO
14.0 g CO
How to Calculate Percent Composition from a Chemical Formula:
What is a percent?
part
whole
whole = 100
How do you calculate a percent?
part
whole
x 100
=
_______ %
How to Calculate Percent Composition from a Chemical Formula:
What is the percent composition of KClO
3
Step 1: Calculate the molar mass of the compound. (This is your whole (total))
K
Cl
O
39.10 g K
1 mol K
x 1 mol K
35.45 g Cl
1 mol Cl
x 1 mol Cl
3
16.00 g O
1 mol O
x 3 mol O
= 39. 10 g K
= 35.45 g Cl
= 48.00 g O
+ _______________
122.55 g KClO
3
1 mol KClO
3
molar mass of KClO
3
= The mass in grams of 1 mol KClO
3
Step 2: Calculate what percent each atom is in the whole compound.
How to calculate the percent of each atom in the whole compound:
% K =
part of compound that is K
total mass of the compound
x 100
= mass of K in compound
total mass of whole compound
x 100
Basically, what percent of the mass of KClO is made up of K?
3
% K = 39.10
122.55
x 100
=
31.9053448 % K
The convention for number of sig figs using data from the periodic table is to report 3.
31.9 % K
Step 2 continued
% Cl:
35.45
122.55
x 100
=
28.9269686 % Cl
= 28.9 % Cl
% O
3
48.00
122.55
x 100
= 39.1676867 % O
= 39.2 % O
You analyze some iron ore for your mining company. You recover 35.0g of iron from 50.0g of an iron oxide. What is the empirical formula and name for this iron oxide?
mass Fe = 35 g
mass (Fe + O) = 50 g
What is the mass of O?
mass (Fe + O) - mass Fe = mass O
total mass of compound (whole mass of compound)
50g FeO - 35 g Fe = 15 g O
You need % composition to begin to calculate empirical formula.
% Fe =
part Fe
whole compound
x 100
% Fe =
mass Fe
mass of compound
x 100
% Fe =
35
50
x 100
=
70 % Fe
% O =
part O
whole compound
% O =
mass O
mass of compound
% O =
15
50
x 100
= 30 % O
Calculating the Empirical Formula Step 1: % mass
% grams
Fe 70 %
70 g Fe
O 30%
30 g O
mass moles
70 g Fe
x
1 mol Fe
55.85 g Fe
=
30 g O
x
1 mole O
16.00 g O
=
1.25335721 mol Fe
1.875 mol O
Step 3: Divide by small (divide the moles of each chemical by the smallest number of moles)
Calculating the Empirical Formula Step 2: mass moles
1.25335721
1.875
1.25335721
1.25335721
= 1
= 1.49598214
~ 1.5
Fe:
O:
3
2
Convert decimal to an improper fraction
To clear the fraction, multiply it by the denominator.
(Have to also multiply the 1 as well)
1 x 2 = 2
1.5 x 2 = 3
Fe O
2
3
Fe
O
?
2-
--
--
--
+++
+++
6-
=
6 +
2
3
3+
iron(III) oxide
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