3. Measurements in Chemistry
• We use measurements all of the time!
• Atomic Mass is a measurement that we have
already used in class.
o units for atomic mass = atomic mass unit (amu)
o Used to express masses of atoms on a relative
scale. We compare everything to carbon-12.
4. Particle location Mass (g) Mass
(amu)
Proton
Inside
nucleus
1.673X10 -24
1.0073=1
Neutron
Inside
nucleus
1.675X10 -24 1.0087=1
Electron
Outside
nucleus 7.109X10 -28
0.0006=0
5. Atomic Mass & Formula Mass
• Atomic Mass
o The weighted average of the masses of the
existing isotopes of an element
o Ex. Carbon C 12.01 amu
• Formula Mass
o The sum of the atomic masses of all atoms in a
compound
o Ex. Carbon Dioxide CO2 44.01 amu
6. Calculating Formula Mass
• What is the formula mass for SO2?
o S: 1 atom x 32.1 amu
o O: 2 atoms x 16.0 amu
Formula Mass = 32.1 amu + 32.0 amu= 64.1 amu
7. Calculating Formula Mass
• What is the formula mass for H2O2?
o H: 2 atoms x 1.01 amu
o O: 2 atoms x 16.0 amu
Formula Mass = 2.02 amu + 32.0 amu = 34.02 amu
8. Calculating Formula Mass
• Methylene chloride (CH2Cl2) is used as a solvent in paint
strippers. What is the formula mass of methylene
chloride?
o C: 1 atom x 12.01 amu
o H: 2 atoms x 1.01 amu
o Cl: 2 atoms x 35.45 amu
Formula Mass = 12.01(1) + 1.01(2) + 35.45(2) = 85.0 amu
9. Moving from AMU to grams
• The use of atomic mass units (amu) are impractical
in the chemistry lab where the preferred unit of
measurement is grams.
• Scientists needed to establish a relationship between
# of atoms and masses of atoms.
10. The Mole
• Definition
o The number of atoms of that element equal to the
number of atoms in exactly 12.0 grams of carbon-
12.
o Abbreviated mol (without the ‘e’)
• The number of atoms in one mole of atoms is
always the same! Avogadro’s Number (N)
o 6.02 x 1023 atoms
o 602,000,000,000,000,000,000,000
11. Same Number, Different Mass
Element # atoms/mol mass of 1 mole
Carbon 6.02x1023 atoms
12.01 g C
Copper 6.02x1023 atoms
63.55 g Cu
Tin 6.02x1023 atoms
118.71 g Sn
Why do the amounts of each look different?
12. Calculating Molar Mass
• Methylene chloride (CH2Cl2)
o C: 1 atom x 12.01 amu
o H: 2 atoms x 1.01 amu
o Cl: 2 atoms x 35.45 amu
• Formula Mass = 12.0(1) + 1.0(2) + 35.5(2) = 85.0 amu
• Molar Mass = 84.95 g/mol
13. Molecules and Moles
• The number of molecules in 1mole of a
molecular compound is 6.02 x 1023 (same as
with atoms in a mole)
• 1 mol of water (H2O) contains 1 mol of water
molecules but 2 mol of hydrogen atoms and
1 mol oxygen.
• How many moles of Ca2+ and F- are in 1
mole of calcium fluoride?
o 1 mole of Ca2+ ions and 2 moles of F- ions
14. Molar Mass
• Definition
o The mass in grams of 1 mole of a substance
• We calculate molar mass the same way as
formula mass.
o Atomic/Formula Molar Mass
o Ex. Calcium 40.08 amu = 40.08 g/mol
15. Same Number, Different Mass
Practice
• What is the formula and molar mass for propane,
C3H8?
– C: 3 atoms x 12.01 amu = 36.03 amu
– H: 8 atoms x 1.01 amu = 8.08 amu
Formula Mass = 36.03 amu + 8.08 amu = 44.11 amu
Molar Mass = 44.11 g/mol
16. Same Number, Different Mass
Practice
• What is the formula and molar mass for glucose,
C6H12O6?
– C: 6 atoms x 12.01 amu = 72.06 amu
– H: 12 atoms x 1.01 amu = 12.12 amu
– O: 6 atoms x 16.00 amu = 96.00 amu
Formula Mass = 72.06 amu + 12.12amu + 96 amu =
180.18 amu
Molar Mass = 180.18 g/mol
