Transcript The Mole - Chemistry202013

Lets talk about it!
NOT THIS KIND OF MOLE!
The mole is used mostly in chemistry as a SI unit for
amount of substance.
In simplest terms a mole represents a number of
objects.
Suppose you’re engaged in a building project:
laying a floor. You’re going to put down 25
boards with 10 nails a piece: that’s at least 250
nails you’ll need. When you go to the hardware
store and ask for 250 nails, the clerk doesn’t
count out the nails for you. What do they do?
You weigh them!!
When we need a large number of a small item, it
doesn’t make sense to count them out; weighing
is more efficient. We probably won’t get exactly
the number we want, but if we’re dealing in large
numbers it won’t make much difference if there
are a few more or less.
In chemistry, we’re stuck with using just this
method to count atoms and molecules,
because it’s the only way we can do it.
Q: Can we count out atoms one-by-one?
A: No, they are too small, but we do need to
know how many we have in a bunch so we
can study their behavior, which is what
chemistry is all about.
Q: In chemistry labs or when cooking or
baking at home, how do we measure out
chemicals/reactants/ingredients?
A: We either measure it out per a certain
volume amount, such as in a graduated
cylinder or in a measuring cup, or we weigh
it out, like on a balance.
Why is this important?
It is important to know how much of a particular
substance we have in order to study its
behaviour.
Weighing atoms in order to count atoms is not an
exact science. If we were to weigh out 250 nails it
would be less accurate than actually counting
each nail. However, it is much more practical to
weigh the nails.
Because atoms and molecules are so small,
chemists created a quantity called a mole that
represents a large number of atoms. The unit of
a mole is like a dozen in that it represents a
certain number of atoms or molecules.
What is a dozen?
What do we call 2 things?
When can we use the terms a dozen or a pair?
We use terms such as dozen or pair in order to
represent a particular number of objects.
So what is a mole???
It is a very, very, very large number but the same
idea as a pair, a dozen, a gross, and so on. A pair
is 2 items, a dozen is 12 items, a gross is 144
items, and a mole is 6.02 × 1023 items. That is
602,000,000,000,000,000,000,000 items in a
mole. This number is usually expressed in
scientific notation.
One mole of anything is equal to 6.02 x1023
of those things...
1 mol of elephants =
elephants
1 mole of water molecules =
• A long while a go a man by the last name of
Avogadro came up with an idea of how to group
atoms! The mole is also referred to as
Avogadro’s number!
• If two elements were at equal volume,
temperatures and pressure, he figured there
would be the same number of particles in each.
But how big is a mole?
How big is a mole of basketballs?
What is another name for the mole?
Why do we use the mole?
http://ed.ted.com/lessons/daniel-dulek-how-big-is-amole-not-the-animal-the-other-one
Q: can a dozen represent any number besides 12
or can a pair represent a number other than
two?
A: No, these numbers are constants, which mean
that they always represent the exact same
number regardless of the item.
The mole is most specifically used to count atoms.
Even though a mole is like a dozen, the actual
number is much larger because atoms are much
smaller than the eggs or oranges we usually
measure by the dozen.
Recall Dimensional Analysis
Suppose you buy three and a half dozen roses and
you want to know how many roses you have. You
can multiply the known quantity (3.5 dozen) by a
conversion factor to express the quantity in the
units you want (number of roses).
Conversion factor: 12 roses
1 dozen
Now lets deal with particles:
NOTE: 1 MOLE = 6.02 X 1023 PARTICLES
PARTICLES (ATOMS OR MOLECULES)
How many particles of sucrose are in 3.50 moles
of sucrose?
Calculate the number of moles that contain 4.50 x 1024
atoms of zinc.
Example 3: How many molecules are in 4.00
mols of NaCl?
4.00 mols NaCl
x
6.02 x 1023molecules
1 mol NaCl
= 2.41 x 1024 molecules NaCl
Questions:
1. How is a mole similar to a dozen?
2. Why do chemists use the mole?
3. Do the following conversions using dimensional
analysis:
a) How many atoms in 2.50 mol Zn?
b) How many moles contain 5.75 x 1024 atom Al?
c) How many moles contain 3.58 x 1023 formula units
ZnCl2 ?
d) How many formula units given 3.25 mol AgNO3 ?
e) How many molecules in 11.5 mol H2O?
f) How many moles contain 3.75 x 1024 molecules CO2 ?
g) How many moles contain 2.50 x 1020 atoms Fe?.
Explain how you can convert from the number
of representative particles of a substance to
moles of that substance.
http://www.youtube.com/watch?v=1R7NiIum2TI
Mass of a Mole
Avogadro’s number is also used to represent how
many atomic mass units can be found in a gram.
This allows us to use the mass numbers on the
periodic table for both the mass of an atom
(atomic mass) and the mass of a mole of atoms
(molar mass), we only need to change the units.
What is atomic mass?
Atomic mass is the weighted average mass of the
isotopes of that element.
Molar mass – the mass in grams of one mole of
any pure substance.
Calculating Molar Mass
Atomic mass = molar mass
amu
= g/mol
What is the molar mass of copper?
What is the molar mass of carbon?
• How can the molar mass of copper and carbon
be different if a mole is always 6.02x 102 3
atoms?
• This is the same as asking how the weight of a
dozen elephants can be different than the weight
of a dozen ants. It is the number of items that is
always the same, not the mass of weight or size
of those items.
What is the molar mass of H2O?
What is the molar mass of Ca(OH)2 ?
Calculate the molar mass of the
following:
a) N2O5
b) H2CO3
c) NaC2H3O2
d) CuClO3
e) Ba(NO2)3
f) Pb(HSO4)2
Grams  Moles, Moles  Grams
Imagine that you are buying jellybeans. Instead of
counting out each jellybean they are measured by
mass. You find that 1 dozen has a mass of 35 g.
What mass of jellybeans should you measure if you
want 5 dozen?
35g jellybeans / 1 dozen
Example 1:
Suppose that while working in a chemistry lab you
need 3.00 moles of manganese for a chemical
reaction. How can you measure that amount?
Example 2:
How many moles of calcium are in 525 g calcium?
How many moles of NaCl are in 234.0g NaCl?
How many grams would 2 moles of NaCl be?
Practice
Determine the mass in grams of each:
a) 3.57 mol Al
b) 42.6 mol Si
c) 3.45 mol H2O
d) 2.45 mol ZnCl2
Determine the number of moles in each:
a) 25.5 g Ag
b) 125g Zn
c) 300.0g Na2S
d) 1.00 kg Fe
Mass to Atoms, Atoms to Mass Conversions
Just as we could not make a direct conversion
from cm  km we cannot make a direct
conversion from mass to atoms. Instead we use
two step dimensional analysis.
Mass  Moles  Atoms
Atoms  Moles  Mass
Examples:
How many atoms are in 25.0 g of gold?
A party balloon contains 5.50 x 1022 atoms of He
gas. What is the mass in grams of the helium?
How many atoms are in each of the following samples?
a) 0.230 g Pb
b) 11.5g Hg
c) 45.6 g Si
d) 0.120kg CaCl2
What is the mass in grams of each of the following:
a) 1.00 x 1024 atoms Mn
b) 3.40 x 1022 atoms He
c) 1.50 x 1015 atoms LiCl
d) 1.50 x 1015 atoms U
Hand – In Questions:
1. How many atoms are in 55.2 g Li?
2. What is the mass of 6.02 x 1024 atom Bi?
3. How many molecules are in 0.000349g of calcium
iodide?
4. What is the mass of 3.34 x 1022 molecules H2O?
5. How many sodium chloride atoms in 117g of NaCl?
Volume of Gases
Determining the Number of particles in a Gas
Although the size can differ greatly between different gases
Avogadro's principle states that all gases that have the same
number of particles will have the same volume. 1000 atoms of
Xenon will have the same volume as Helium. The molar volume
for gases is the volume that 1 mole of gas will take up at 0
Celsius at 1.00 atm. Since all gases have the same volume at
STP (standard temperature and pressure), this means that 1
mol of any gas will have a volume of 22.4L.
** Conversion Factor: 1 mole = 22.4L at STP
STP = Standard temperature and pressure (temp = 0oC,
pressure = 1 atm)
Moles to Volume and Volume to Moles
Example1: What is the volume of a balloon that holds 1.78
moles of oxygen gas at STP?
Example 2: How many moles of helium at STP will fit into a
39.6 L container?
Volume to Particles and Particles to Volume
Example 1: How many particles are in 3.72 L of gas at STP?
Example 2: What volume is occupied by 3.24 x 1024
molecules of H2O gas at STP?
Volume to Mass and Mass to Volume
Example 1: Determine the mass of 42.1L of NO2(g)
@ STP.
Example 2: Determine the volume of the 1.00g of
nitrogen gas at STP.
Welcome to Stoichiometry
Background
Comes from the Greek words stoicheion, meaning
element and metron, meaning measure.
In the real world, chemists perform reactions to make
useful chemical compounds such as medicines,
fibers, pigments, cosmetics and structural materials.
Because some reactants are expensive chemists don’t
just mix huge quantities of chemicals together and
hope for the best; rather they use stoichiometry to
calculate the mass relationships between products
and reactants.
Terminology:
Stoichiometry: The Study of relationships
between the amounts of reactants used and
products formed by a chemical reaction, based
on the Law of Conservation of Mass.
mole ratio: The ratio of the product and
reactant in a chemical reaction. The
numbers that fit into this ratio are pulled directly
out of the balanced equation for the reaction.
Mole ratios are the key to stoichiometric
calculations.
They give us the ratios between the amounts
of reactants and products in any given reaction.
We get mole ratios from the coefficients.
That’s why we need to start with a balanced
equation.
Chemical Equation:
2 Al(s) + 3Br2(l)  2AlBr3(s)
For every 2 moles of aluminum, 3 moles of
bromine are used to produce the reaction.
** The Law of Conservation is important when
dealing with stoichiometry. If the equation isn’t
properly balanced then you will use the incorrect
mole ratios in your calculations.
Mole Ratios
Example: Write all possible mole ratios
2 Al(s) + 3Br2(l)  2AlBr3(s)
Quizam Next Wednesday
What is a mole?
How big is a mole?
Who is Avogadro?
Molar/ Molecular Mass
Dimensional Analysis
- particles, volume, mass, moles
Law of Conservation of Mass
Steps to solve a stoichiometric equation. (Write a
balanced chemical equation, convert to moles,
use the mole ratio, and find what required.)
Determine All Possible Mole Ratios
Make sure to BALANCE!
a) Al (s) + O2 (g)  Al2O3(s)
b) Fe (s) + H2O (l)  Fe3O4 (s) + H2 (g)
c) HgO (s)  Hg(l) + O2 (g)
Mole to Mole Conversions
Moles of known x moles of unknown = moles of unknown
moles of known
Example 1:
How many moles of H2 are we going to need to
react with 12.0 moles of O2 to produce H2O2?
How much H2O2 will this produce?
Step 1: Write balanced equation
H2 + O2  H2O2
Step 2: Write down what you are given and convert
to moles.
Step 3: Use mole ratios to convert to desired
chemical.
Example2:
How many moles of hydrogen are produced
when 0.0400 mole of potassium is used?
2K(s) + 2H2O(l)  2KOH(aq) + H2(g)
Particles to Particles Conversions
Particles  moles  Particles
**Always convert to moles first!
2K(s) + 2H2O(l)  2KOH(aq) + H2(g)
How many atoms of H2 are produced if you start
with 3.02 x 1022 atoms of H2O?
Grams to Grams
a) Determine the mass of sodium chloride
produced when 1.25 grams of chlorine gas reacts
with sodium.
2 Na (s) + Cl2 (g) 2NaCl (s)
Volume to Volume
b) The volume of chlorine needed in order to
produce 26.0 L of NaCl.
Can you convert grams to volume?
c) Determine the volume of chlorine gas used
when 1.25 grams of sodium chloride is produced.
Practice:
2NaN3 (s)  2Na(s) + 3N2 (g)
a) Determine all mole ratios.
b) Determine the mass of N2 produced if 100.0 g of
NaN3 is decomposed.
c) Determine the volume of N2 gas produced if 2.4 x1023
atoms of NaN3 are used.
d) How many moles of sodium are produced if 7.2 mol
of NaN3 reacts?
e) What must you always convert to first?