chemstoichMOLES
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Transcript chemstoichMOLES
2.C – Conserving
Matter
When a car’s gas empties, where do the atoms in
the gasoline go?
Keeping Track of Atoms
Law of Conservation of Matter: Matter is
neither created nor destroyed.
Since chemical reactions cannot create or
destroy atoms, chemical equations
representing the reactions must always be
BALANCED.
Atomic Perspective:
C
1 Carbon atom
O2
1 oxygen molecule
CO2
1 carbon dioxide molecule
1. What are the reactants in this chemical equation?
2. What are the products in this chemical equation?
3. Are there the same number of atoms on both sides of the equation?
a. Where any atoms destroyed or created?
b. Was the Law of Conservation of Matter maintained?
2 Cu (s) + O2 (g) 2 CuO (s)
COEFFICIENTS - indicates
the number of units of each
substance involved.
1. Does the oxygen molecule have a coefficient?
2. What do the subscripts represent?
3. Can subscripts be removed from chemical equations?
How do we Balance
Equations?
Number of
compounds in
the reaction
Coefficients
2 H2 + O2 2 H2O
Subscripts
# of atoms in a
compound
Subscripts balance charges within a compound.
Coefficients balance atoms in an equation
What do Coefficients Really Mean?
CH4 + 2 O2 CO2 + 2 H2O
H
H
C
H
O
O
O
H
O
O
C
O
H
O
O
H
Total:
1C
4H
4O
The equation is balanced.
H
Total:
1C
4H
4O
H
How to Balance By
Inspection:
1
Make a table of elements
_____ CH4 + _____ O2 _____H2 O + _____ C O2
Reactants
Products
C
H
O
Taken from Kelly Deters
How to Balance By
Inspection:
2
Count the number of each element or ion on the
reactants and products side.
Don’t forget to add all the atoms of the same element together—
even if it appears in more than one compound!
_____ CH4 + _____ O2 _____H2 O + _____ C O2
Reactants
Products
C
1
1
H
4
2
O
2
3
Taken from Kelly Deters
How to Balance By
Inspection:
3
Add coefficients to balance the numbers
Each time you add a coefficient, update your table with the new
quantities of each atom.
_____ CH4 + _____
2 O2 _____
2 H2 O + _____ C O2
Reactants
Products
C
1
1
H
4
2
4
O
2
3
4
4
Taken from Kelly Deters
How to Balance By
Inspection:
4
Place a “1” in any empty coefficient location
Filling each coefficient location lets you and the grader know that you
finished the problem rather than you left some blank because you
weren’t done!
_____
1 CH4 + _____
2 O2 _____
2 H2 O + _____
1 C O2
Reactants
Products
C
1
1
H
4
2
4
O
2
3
4
4
Taken from Kelly Deters
Choosing the Order of
Balancing
How do you know what order to balance in?
Start
Elements that appear
only 1 time per side
Save for later
Elements that are
uncombined
Pb + PbO2 + H+ Pb2+ + H2O
Save for later
Elements that appear
more than 1 time per side
Start
Elements in most
complicated molecules
Taken from Kelly Deters
To balance this equation, use the order: O,
H, Pb
How is Balancing Affected by
Order?
What happens if we balance in the order
determined in the last slide?
O, H, Pb
_____ Pb + _____ Pb O2 + _____ H+
Reactants
_____ H2 O + _____ Pb2+
Products
O
H
Pb
Taken from Kelly Deters
Let’s Practice #1
Example:
Balance the
following
equation
__ HCl + __ Ca(OH)2 __ CaCl2 + __ H2O
Taken from Kelly Deters
Let’s Practice #2
Example:
Balance the
following
equation
__ H2 + __ O2 __ H2O
Taken from Kelly Deters
Let’s Practice #3
Example:
Balance the
following
equation
__ Fe + __ O2 ___ Fe2O3
Taken from Kelly Deters
C.5 The Mole Concept
Definition:
Mole – SI unit for counting
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
What is a counting unit?
You’re already familiar with one counting unit…a
“dozen”
A dozen = 12
“Dozen”
12
A dozen doughnuts
12 doughnuts
A dozen books
12 books
A dozen cars
12 cars
A dozen people
12 people
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
How big is a mole?
Enough soft drink cans to cover the
surface of the earth to a depth of over
200 miles.
If you had Avogadro's number of
unpopped popcorn kernels, and
spread them across the United States
of America, the country would be
covered in popcorn to a depth of over
9 miles.
If we were able to count atoms at the
rate of 10 million per second, it would
take about 2 billion years to count the
atoms in one mole.
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
A Mole of Particles
Contains 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 “molecules”
(technically, ionics are compounds not
molecules so they are called formula units)
6.02 x 1023 Na+ ions and
6.02 x 1023 Cl– ions
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
What does a “mole” count
in?
A mole = 6.02 10 (called Avogadro’s number)
23
6.02 1023 = 602,000,000,000,000,000,000,000
“mole”
6.02 1023
1 mole of doughnuts
6.02 1023 doughnuts
1 mole of atoms
6.02 1023 atoms
1 mole of molecules
6.02 1023 molecules
This means a 12 ounce bottle of water would have
19.7 “moles” of water…a much easier-to-work-with
Adopted from "Chemistry You
number!
Need to Know" by Kelly
Deters
Molar Mass
Adopted from
"Chemistry You Need
to Know" by Kelly
Definition
Molar Mass – The mass for one mole
of an atom or molecule.
Other terms commonly used for the same meaning:
Molecular Weight
Molecular Mass
Formula Weight
Formula Mass
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
Mass for 1 mole of atoms
The average atomic mass = grams for 1 mole
Average atomic mass is found on the periodic table
Element
Mass
1 mole of carbon atoms
12.01 g
1 mole of oxygen atoms
16.00 g
1 mole of hydrogen
atoms
1.01 g
Unit for molar mass: g/mole or g/mol
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
Molar mass for molecules
The molar mass for a molecule = the
sum of the molar masses of all the
atoms
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
Calculating a Molecule’s
Mass
To find the molar mass of a molecule:
1
Count the number of each type of atom
2
Find the molar mass of each atom on the periodic
table
3
Multiple the # of atoms molar mass for each atom
4
Find the sum of all the masses
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
Example: Molar Mass
Example:
Find the
molar
mass for
CaBr2
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
Example: Molar Mass
1
Example:
Find the
molar
mass for
CaBr2
Count the number of each type of atom
Ca
1
Br
2
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
Example: Molar Mass
2
Example:
Find the
molar
mass for
CaBr2
Find the molar mass of each atom on the periodic table
Ca
1
40.08 g/mole
Br
2
79.91 g/mole
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
Example: Molar Mass
3
Example:
Find the
molar
mass for
CaBr2
Multiple the # of atoms molar mass for each atom
Ca
1 40.08 g/mole =
40.08 g/mole
Br
2 79.91 g/mole =
159.82 g/mole
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
Example: Molar Mass
4
Example:
Find the
molar
mass for
CaBr2
Find the sum of all the masses
Ca
1 40.08 g/mole =
40.08 g/mole
Br
2 79.91 g/mole =
+ 159.82 g/mole
199.90 g/mole
1 mole of CaBr2 molecules would have a mass of
199.90 g
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
Example: Molar Mass &
Parenthesis
Be sure to distribute the subscript outside the
parenthesis to each element inside the parenthesis.
Example:
Find the
molar
mass for
Sr(NO3)2
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
Let’s Practice #2
Example:
Find the
molar
mass for
Al(OH)3
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
Using Molar Mass in
Conversions
Adopted from
"Chemistry You Need
to Know" by Kelly
Example: Moles to Grams
Example:
How many
grams are
in 1.25
moles of
water?
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
Example: Moles to Grams
When converting between grams and moles, the
molar mass is needed
Example:
How many
grams are
in 1.25
moles of
water?
1.25 mol H2O
H 2 1.01 g/mole =
2.02 g/mole
O 1 16.00 g/mole = + 16.00 g/mole
18.02 g/mole
1 mole H2O molecules = 18.02 g
18.02 g H2O
1
22.53 g H2O
= _______
mol H2O
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
Let’s Practice #3
Example:
How many
moles are
in 25.5 g
NaCl?
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
Let’s Practice #3
Na 1 22.99 g/mole = 22.99 g/mole
Cl 1 35.45 g/mole = + 35.45 g/mole
58.44 g/mole
Example:
How many
moles are
in 25.5 g
NaCl?
25.5 g NaCl
1 mole NaCl molecules = 58.44 g
1
mole NaCl
0.44
= _______
mole NaCl
58.44 g NaCl
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
Example: Grams to
Molecules
Example:
How many
molecules
are in
25.5 g
NaCl?
Adopted from "Chemistry You
Need to Know" by Kelly
Deters
Example: Grams to
Molecules
Example:
How many
molecules
are in
25.5 g
NaCl?
Na 1 22.99 g/mole = 22.99 g/mole
Cl 1 35.45 g/mole = + 35.45 g/mole
58.44 g/mole
1 moles NaCl molecules = 58.44 g
1 mol = 6.021023 molecules
25.5 g NaCl
1
mol NaCl
58.44 g NaCl
6.021023 molecules NaCl
1
mol NaCl
2.63 1023 Adopted
from "Chemistry You
= _________
molecules
NaCl
Need to Know" by Kelly
Deters
Let’s Practice #4
Example:
How many
grams is a
sample of
2.75 × 1024
molecules of
SrCl2?
Adopted from "Chemistry You
Need to Know" by Kelly
Deters