Transcript The Mole - Downtown Magnets High School

The Mole
Chapter 11
Counting large numbers…
• Easier when you use counting units:
Dozen
Ream
Pair
Case
Gross
• Regardless of the object, the number that
the units represents is always the same
• Atoms and molecules are extremely small
• Impossible to actually count them (even in
the smallest of samples)
• Chemists need a unit for counting
accurately the # of atoms, molecules, or
formula units
• They use the mole
• Mole (mol) = the SI base unit used to
measure the amount of a substance
• 1 mole of anything contains 6.02 x 1023
representative particles
1 mole of apples =
6.02 x 1023 apples
1 mole of diamonds =
6.02 x 1023 diamonds
1 mole of baseballs =
6.02 x 1023 baseballs
1 mole of carbon atoms =
6.02 x 1023 carbon atoms
The same applies to moles of
compounds
• How many molecules are in 1 mol NH3?
6.02 x 1023 NH3 molecules
• How many molecules are in 1 mol
C 9H 8O 4?
6.02 x 1023 C9H8O4 molecules
6.02 x 1023
• Known as Avogadro’s number
• Where did it come from?
Amedeo Avogadro determined the volume of
one mole of gas in 1811
Converting Moles to Particles and
Particles to Moles
• How many roses are in 3 ½ dozen?
USE CONVERSION FACTORS!!!
3.5 dozen x 12 roses =
1 dozen
42 roses
• How many pencils are in 6 gross?
6 gross x 144 pencils =
1 gross
864 pencils
Converting Moles to Particles and
Particles to Moles
• How many particles of sucrose are in 3.50
moles of sucrose?
3.50 mol sucrose x 6.02 x 1023 rep. part.
1mol
2.11 x 1024 particles of sucrose
Converting Moles to Particles and
Particles to Moles
• How many moles contain 4.50 x 1024
atoms of zinc?
4.50 x 1024 atoms Zn x 1 mol zinc
6.02 x 1023 atoms Zn
7.48 mol Zn
Try some on your own…
1. How many Cu atoms are in 1.20mol Cu?
7.22 x 1023 Cu atoms
2. How many moles of Xe contain 1.54 x 1026
atoms of Xe?
2.56 x 102 mol Xe
Mass and the Mole
Mass of dozen limes = Mass of dozen eggs
• Moles of substances have different
masses, due to the differing sizes and
compositions
– 1 mol of C vs. 1 mol of Cu
• Molar mass= the mass in grams of one
mole of any pure substance
• The molar mass of any element is
numerically equal to its atomic mass and
has the units g/mol
• Ex:
– one atom of Manganese = 54.94amu
– molar mass of Manganese = 54.94g/mol
– So, when you measure out 54.94 g of
manganese on a balance you are actually
measuring out 6.02 x 1023 atoms of
manganese
– Look at figure 11-4 in your textbook
Using Molar Mass
• Jellybean example
– Jelly beans sold by the dozen
– 1 doz jelly beans = 35 g
– If a customer wants 8 dozen jelly beans,
should you count out 96 individual jelly beans
or could you USE A CONVERSION FACTOR?
Yep…you guessed it…use a
conversion factor!!
• You could figure out the mass of 8 dozen
jelly beans and measure the mass
8 doz jelly beans x 35 g jelly beans
1doz jelly beans
= 280 g of jelly beans
Try one on your own…
A customer comes into your candy store and
wants 16 doz jelly beans, use a
conversion factor to figure out how many
grams you should measure out.
16 doz x 35 g
1 doz
= 560 g jelly beans
Apply it to working in a chemistry lab…
You are a lab assistant and want 0.0450mol of
chromium. Using a conversion factor, calculate
the mass in grams of 0.0450mol of chromium.
Molar mass of chromium = 52.00g/mol
0.0450mol x 52.00g
1 mol
= 2.34 g Cr
You would need to measure out 2.34 g of
Chromium
Try one on your own…
You need 3.0 moles of Mn for a chemical
reaction. Calculate how many grams of
Mn you would need to measure out.
3.0mol x 54.9g
1mol
= 165g Mn
You know how to convert moles to
mass, let’s try converting mass to
moles!!
• Use the inverse of the conversion factor!!
Let’s say you have 200g of Mn. How many moles
do you have?
200g x 1mol
54.9g
= 3.64mol Mn
Let’s do some
more
converting!!
Mass to atoms
You are back at the candy store and at the end of
the day you have 910g of jelly beans left. Your
boss wants to know how many jellybeans this is.
Rather that count out the individual candies, you
decide to use your superior brain power and
calculate it mathematically!!
910g x 1 doz
35g
= 26 doz jelly beans left
Your boss is so impressed with your skills that he gives you
a raise!!
Now try it with atoms…
How many atoms of gold are in a pure gold
nugget with a mass of 25.0g?
• You can’t go directly from mass to
atoms, so you have to convert to moles
first
25.0g x 1mol
196.7g
= 0.127 mol Au
0.127mol x 6.02 x 1023 atoms
1mol
= 7.65 x 1022 atoms of Au
Chemical Equations
• Coefficients in a chemical equation
represent the numbers of individual
particles of substances in the equation
• …they also represent the numbers of
moles of particles
C3H8 + 5O2  3CO2 + 4H2O
Mole Ratios
• Mole ratio = ratio between the numbers of
moles of any two substances in a
balanced chemical equation
C3H8 + 5O2  3CO2 + 4H2O
2Al + 3Br2  2AlBr3
What mole ratios can be written for this
reaction?
1. 2mol Al
4. 3mol Br2
3molBr2
2mol AlBr2
2. 2mol Al
5. 2molAlBr3
2molAlBr3
2Al
3. 3mol Br2
6. 2molAlBr3
2mol Al
3molBr2
Courtesy of
Chemistry Matter and Change
Glencoe McGraw-Hill
Chemistry for Dummies
John T. Moore
www.karentimberlake.com