Chapter 7
The Mole
What is a mole?
I have some examples on the front table
Our text says: a mole (mol) of a substance represents 6.02 X 1023
representative particles of that substance.
Wikipedia says: The mole is a unit of measurement for the amount of substance or
chemical amount. It is one of the base units in the International System of Units, and
has the unit symbol mol
What this means is that the mol is a word that stands for a number, like the word dozen stands for
the number 12. A mol of something has 6.02 X 1023 representative particles of that substance.
1 mol of X = 6.02 X 1023 representative particles (atoms, molecules, formula units, ions) of X
Looks like we could make a conversion factor out of that !!!!! (Yipee, Ch. 4 again)
1) One billion Pb atoms is how many mols of Pb?
2) How many Hydrogen atoms are in 0.5 moles of sucrose (C12H22O11)
What is different about a dozen cars and a dozen eggs?
What is the same?
What is different about a mol of water molecules and a mol of Pb atoms?
What is the same?
But why 6.02 X 1023?????
To answer that we need another term(s):
gram atomic mass (if its and atom)
gram molecular mass (if it’s a molecule)
gram formula mass (if its ionic)
molar mass (generic term for all of the above)
EX: What is the molar mass of the following:
1) Pb
2) NaCl
3) Water
4) Calcium chloride
5) C12H22O11
6) Tin (IV) phosphate
But wait…don’t those numbers look familiar?
ADD ‘EM UP!
Chapter 7
The Mole
That’s right, now we know something much more valuable:
The molar mass of something is the mass of 1.0 mol of that stuff in grams; just like adding
up the atomic masses gives us the mass of just one of them in amu’s.
Pb’s molar mass is 207.2
1 atom of Pb has a mass of 207.2 amus
1mol of Pb (6.02 X 1023 Pb atoms) has
a mass of 207.2 grams
Review and try molar masses:
1)
2)
H2O’s molar mass is 18, therefore:
3)
1 molecule of H2O has a mass of 18 amus (1+1+16)
1 mol of H2O (6.02 X 1023 molecules of H2O) has a mass of 18 grams
or 1 mol H2O = 18 g H2O (I feel more math coming on)
4)
EX:
How many mols of water are in 47.9 g of water?
5)
6)
7)
How many molecules of water is that?
8)
9)
How many molecules of water are in 47.9 g of water?
(the above in one step)
One mole of X is:
 6.02 X 1023 of X (in number)
 Has a mass in grams = the molar mass of X
 If X is a gas at STP has a
volume = 22.4 L
Convert the following to mols:
1) 12. 8 g of sodium acetate
2) 8.0 L of dinitogen pentoxide gas at
STP
3) 1.7 X1019 water molecules
How many molecules and what is the mass
of 150.0 mL of carbon dioxide at STP?
10)
MOLE MAP
Chapter 7
The Mole
Percent Composition
-
Percent composition tells us the percent by mass of each element within an element
-
We calculate this by: grams of the element X 100 %
grams of the compound
if we’re not given grams we use 1 mol of the stuff so we can use the molar mass
EX: What is the % composition of nitric acid?
1) What is nitric acid?
HNO3
2) What is its molar mass? 1 + 14 + 3(16)= 63 g/mol
3) Calculate:
% H = 1/63
= .0159 = 1.59 % H
% N = 14/63
= .222 = 22.2 % N
% O = [3(16)] / 63 = .762 = 76.2 % O
we could check by adding up and getting close to 100%
What is the % composition of calcium acetate?
-
Another way we find % composition is by experiment
In an experiment 0.500 g of an acid was analyzed. It was found to be 0.010 g H, 0.163 g
S, and 0.327 g O. What is the acid’s % composition?
Determining Empirical and Molecular formulas
-
Before we go into the math of this we have to define these two things:
Empirical formula: the lowest whole number ratio of the atoms in a compound
Molecular formula: the actual number of atoms in the compound
Chapter 7
-
The Mole
We can use % composition data to find empirical formulas
EX: What is the empirical formula of a compound that is 79.8 % C, and 20.2 % H
1) assume you have 100 g of the compound… that means you would have 79.8 g of C and
20.2 g of H
2) convert these to moles
79.8 g of C
1 mole of C = 6.65 mol
X
1
12 g of C
of C
20.2 g of H
1 mole of H = 20.2 mol
X
1
1 g of H
of H
3) divide by the smallest of the mole numbers
6.65 mol of C = 1 mol of C
6.65
20.2 mol of H = 3 mol of H
6.65
therefore the EF is: CH3
what if we don’t get whole numbers when we divide?
a) if it’s close to a whole number it probably is that number (4.06 = 4)
b) all the numbers may need multiplied by a whole number to become whole.
EX : C = 2.3, H = 3, N = 1
X 3 to get C7H9N3
Try finding the empirical formula for the acid in the previous part
Finding molecular formulas:
To find molecular formulas(MF) from empirical formulas(EF) we need another piece of
information. The compounds molecular mass(MM)
If we found a compounds EF to be CH3 and we also know its MM to be 30
Add up the weight of the EF
CH3 = 15
Find how many times that goes into the compounds MM
30/15 = 2
Multiply the EF through by that number to get the MF
CH3 X 2 = C2H6
Summary Problem:
1,6 diaminohexane is used to make nylon. Analysis of this compound finds its percent
composition to be 62.1% C, 13.8% H, and 24.1% N. Its molecular mass is also found to be 116
g/mol. What are the empirical and molecular formulas of this compound?