MOLES!
ATOMIC MASS
 The of protons + the # of neutrons in an atom
(they weigh about the same)
 Example: Carbon’s atomic number is 6 and its atomic mass is 12amu
(atomic mass units)
 Atomic masses on the periodic table represent the average mass of
all of the isotopes of that element.
Problems with atomic mass:
1. Atoms are too small to be weighed
2. Atomic masses do not easily convert to grams
Why use atomic mass at all?
o Atomic mass gives information about the number of protons,
neutrons, and electrons an atom has
o It is useful to know relative mass
o Example: What ratio is needed to make H2O?
By atoms: 2:1
By mass: 2:16 (H mass = 2(1.008) and O mass = 16.00)
 It is useful to associate atomic mass with a mass in grams
THE MOLE
 Samples of matter contain many atoms
 The mole (abbreviated mol) is used in counting atoms.
- Just like a dozen = 12
- One mole = 6.022 x 1023 (HUGE #:
602,200,000,000,000,000,000,000)
(Since atoms are so tiny, a mole of atoms or molecules is a perfectly
manageable quantity!)
USING MOLES
 A sample of an element with a mass equal to that element’s average
atomic mass expressed in grams contains one mole of atoms.
 Example: 1 mol of Carbon has a mass of 12.01 g
SAMPLE PROBLEMS
1. Determining the mass of a sample of atoms
Compute the mass in grams of a sample of carbon containing 10
atoms:
2. Determining moles of atoms
Compute the number of moles of atoms and the number of atoms in a
10.0g sample of Aluminum.
3. Calculating Numbers of atoms
A silicon chip has a mass of 5.68 mg. How many silicon atoms are
present?
4. Calculating the # of moles and mass
a) Calculate the number of moles in a sample of cobalt containing
5.00 x 1020 atoms
b) What is the mass of the sample?
MOLAR MASS
 The mass of one mole is called molar mass
The mass of 1 mole of Carbon is 12.01g,
Expressed as 12.01g/mol
Ex: What are the molar masses for:
a) S
b) SO2
c) Cu3(BO3) 2
CONVERTING BETWEEN GRAMS & MOLES
 If we are given the # of grams of a compound, we can determine the
# of moles and vise-versa
 In order to convert from one to the other, you must calculate molar
mass.
g = mol x g/mol
mol = g ÷ g/mol
Chemical
Formula
HCl
H2SO4
g/mol
g
0.25
53.15
NaCl
Cu
Mol
3.55
1.27
Mathematical
Formula
MOLAR MASS AND # OF MOLECULES
Bees release 1µg (1.0 x 10-6 g) of Isopentyl acetate (C7H14O2) when they
sting.
a) How many molecules of isopentyl acetate are released in a typical
bee sting?
b) How many Carbon atoms are present?
Name:_________________________________________ __ Date: ________________________
Mole Practice Problems
1. What is the mass of 500 atoms of iron (Fe)?
2. How many atoms are there in 0.200g of Carbon?
3. What is the molar mass of Al2O3?
4. What is the molar mass of NH3?
5. What amount (moles) are present in 1.00 g of NH3?
6. What is the mass of 5.00 mol of NH3?
7. What mass of Nitrogen is present in 5.00 mol of NH3?
8. What number of molecules are present in 1.00g of NH3?
9. What number of Nitrogen atoms are present in 1.00 g of NH3?
10. How many moles are present in each sample?
a) 150.0 g of Fe3O2
b) 10.0 mg of NO2
c) 1.5 x 1016 molecules of BF3