Chemical Measurements

1
Chemical Measurements

• Unit 4 Stoichiometry
• Chapter 10 The Mole

2
Overview

• Measurements are essential.
• Measuring the number of atoms involved in a
  chemical reaction is impractical.
• The mole establishes a relationship between the
  number of particles involved in a reaction and
  the mass of that reactant or product.
• Mole relationships are used to determine
  composition, empirical and molecular formulae.

3
Measurements

• We use measurements all of the time!
• Atomic Mass is a measurement that we have already
  used in class.
• units for atomic mass A.M.U.
• atomic
  mass unit
• Used to express masses of atoms on a relative
  scale. We compare everything to carbon-12.

4
Atomic Mass Formula Mass

• Atomic Mass
• The weighted average of the masses of the
  existing isotopes of an element
• Ex. Carbon C 12 a.m.u.
• Formula Mass
• The sum of the atomic masses of all atoms in a
  compound
• Ex. Carbon Dioxide CO2 44 a.m.u.

5
Calculating Formula Mass

• Methylene chloride (CH2Cl2) is used as a solvent
  in paint strippers. What is the formula mass of
  methylene chloride?
• C 1 atom x 12 amu
• H 2 atoms x 1 amu
• Cl 2 atoms x 35 amu
• Formula Mass 12(1) 1(2) 35(2) 84 AMU

6
Moving from AMU ? 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.
• We know that a ratio exists that describes one
  atom to another Mass Ratio
• Ex. The ratio between Hydrogen and Oxygen is
  always 116, no matter how many atoms are
  involved.

7
Moving from AMU ? grams

• We are missing something!
• We dont know how many atoms are necessary to
  make up a mass in grams that is equal to an
  elements atomic mass
• We need some sort of conversion factor

8
The Mole

• Definition
• The number of atoms of that element equal to the
  number of atoms in exactly 12.0 grams of
  carbon-12.
• The number of atoms in one mole of atoms is
  always the same!
• 6.02 x 1023 atoms

9
Same Number, Different Mass

• Element atoms/mole mass of 1 mole
• Copper 6.02x1023 atoms Cu 63.5 g
• Mercury 6.02x1023 atoms Hg 201 g
• Sulfur 6.02x1023 atoms S 32.1 g
• Iron 6.02x1023 atoms Fe 55.8 g

10
Molar Mass

• Definition
• The mass in grams of 1 mole of a substance
• We calculate molar mass the same way as formula
  mass.
• Atomic/Formula Molar
• Mass Mass
• Ex. Calcium 40 amu 40g
• Calcium Chloride 111 amu 111g

11
Calculating Molar Mass

• Methylene chloride (CH2Cl2) is used as a solvent
  in paint strippers. What is the molar mass of
  methylene chloride?
• C 1 atom x 12 amu
• H 2 atoms x 1 amu
• Cl 2 atoms x 35 amu
• Formula Mass 12(1) 1(2) 35(2) 84 AMU
• Molar Mass 84 g/mole