Download DP Chemistry 11 Part 3 Mass and the Mole

DP Chemistry 11
Part 3
Mass and the Mole
The Periodic Table shows the atomic masses of the elements. Atomic
masses are measured in amu’s (atomic mass units) because they are
too small to measure directly.
The atomic mass of an element, expressed in grams, is equal to the
mass of one mole of that element.
Therefore:
If mercury (Hg) has an atomic mass of 200.6 amu, 1 mole of
mercury atoms has a mass of 200.6 g
If carbon (C) has an atomic mass of 12.0 amu, 1 mole of carbon atoms
has a mass of 12.0 g
If sulfur (S) has an atomic mass of 32.1 amu, 1 mole of sulfur atoms
has a mass of 32.1 g
This is also true of molecules.
One mole of a compound = the molecular mass of the compound
To determine the molecular mass of a compound add up the individual
atomic masses:
1 mole of water (H2O) = 2 hydrogens + 1 oxygen
= 2 (1) + 16
= 18 g
1 mole of glucose (C6H12O6 ) = 6 carbon + 12 hydrogen + 6 oxygen
= 6 (12) + 12(1) + 6 (18)
= 192 g
The mass, in grams, of one mole of any pure substance is called
its molar mass. The units are given as g/mol.
Example:
As given on the Periodic Table (refer to www.ptable.com) , an
atom of iron has an atomic mass of 55.845 amu.
Therefore, the molar mass of iron is 55.845 g/mol
and 1 mole of iron (6.02 x 1023 atoms of iron) has a mass of
55.845 g
Why are atomic masses exact integers? Recall, from grade 10
Chemistry, that most elements have isotopes.
Definition: Isotopes are the atoms of the same element with
different numbers of neutrons.
There are 3 isotopes of hydrogen:
atom
protons
neutrons
mass
H-1
1
0
1 amu
H-2
1
1
2 amu
H-3
1
2
3 amu
Most hydrogen atoms have a mass of 1 amu. When all masses
are added up, the average mass is 1.008 amu. This is called the
relative mass.
Although atomic mass is measured in amu, relative atomic mass
has no units.
The symbol used to show relative atomic mass is Ar
Relative molecular masses (Mr )are calculated using the relative
atomic masses (Ar) of the elements making up a substance.
To review:
Avogadro’s Constant = 6.02 x 1023
= 1 mole
molar mass = the mass in grams of one mole of any pure
substance, derived from the Periodic Table
- measured as g mol-1 (read as g/mole)
Relative atomic mass = Ar = average atomic mass of a
substance
- no units
Relative molecular mass = Mr = sum of the Ar of all atoms
making up a molecule
- no units
Assignment #3
due date: Monday October 5, 2009
1. Review: Avogadro’s number: How much time, in years, would
it take to count the number of particles in one mole of any
substance? (assume that it takes 1 second to count 1 particle).
2. How many particles make up 0.05 moles of Sulfur?
3. How many moles of carbon are found in 200 grams of
carbon? (atomic mass of carbon = 12.0 amu)
4. Complete page 10, questions 2,3,4