L4 - Compositional Stoich

Counting
Atoms
The Mole
Concept:
Mg burns in air (O2) to
produce white
magnesium oxide,
MgO.
Measuring
Matter
How can we figure out how
much oxide is produced
from a given mass of Mg?
The Mole
1 mole is the amount of
substance that contains as many
particles (atoms, molecules) as
there are in 12.0 g of 12C.
Avogadro’s Number:
Amedeo Avogadro
1776-1856
6.02214199 x 1023
There is Avogadro’s number of particles in
a mole of any substance.
One Mole of:
S
C
Hg
Cu
Fe
Mole Relationships
• One mole of atoms, ions, or molecules contains
Avogadro’s number of those particles
• One mole of molecules or formula units contains
Avogadro’s number times the number of atoms
or ions of each element in the compound
Representative particles
Refers to the species present in a substance
1. How many atoms of sodium are
in 0.5 mole of sodium metal?
•Usually atoms, molecules or formula units
Examples:
1 mole of iron atoms
1 mole of hydrogen molecules
1 mole of water molecules
1 mole of sodium chloride formula units
Reference
2. How many molecules of water are in
0.015 mole of water?
Using Moles
• Moles provide a bridge from the molecular
scale to the real-world scale
3. How many atoms of hydrogen
are in 0.015 mole of H2O?
Molar Mass
• The molar mass of an element is the average
atomic mass for the element that we find on the
periodic table in the units of grams.
• Because there are 6.022 x 1023 carbon-12 atoms
in one mole, and one mole of carbon-12 has a
mass of 12.01 grams, the atomic mass equals the
molar mass of a substance in grams
• Therefore:
1 mol Na = 22.99 g
1 mol Ar = 39.95 g
1 mol O2 = 31.98 g
Notice, not 15.99g
4. What is the mass of 7.98 mol
Aluminum metal?
6. What is the mass in grams of one
sodium atom?
5. What is the mass in milligrams of
3.45 moles of Boron?
Formula Weight (FW)
• Sum of the atomic weights for the atoms
in a chemical formula
• So, the formula weight of calcium
chloride, CaCl2, would be
Ca: 1(40.1 amu)
+ Cl: 2(35.5 amu)
111.1 amu
• These are generally reported for ionic
compounds
Molecular Weight (MW)
• Sum of the atomic weights of the atoms
in a molecule
• For the molecule ethane, C2H6, the
molecular weight would be
C: 2(12.0 amu)
+ H: 6(1.0 amu)
30.0 amu
7. What is the formula mass of
ammonium carbonate, (NH4)2CO3?
8. How many atoms (total) are in
2.50 g ammonium carbonate?
Equipment Used to Accurately
Measure Volume
% Composition
Given that the chemical formula describes the
number of atoms in a compound, the percent
mass of atoms in a compound can be
determined.
• Percent Composition = % by mass of each
element in a compound
(atoms of element)(AW)
X 100%
=
(FW compound)
9. How many molecules of ethanol
are in a 10.0 mL sample? (DCH3CH2OH
= 0.798 g/mL)
Percent Composition
So the percentage of carbon in ethane
is…
(2)(12.0 amu)
%C =
(30.0 amu)
24.0 amu
x 100
=
30.0 amu
= 80.0%
What is the percent mass of carbon in carbon
tetrachloride? Chlorine? (assume 1 mol)
10. Calculate the percent
composition (in grams) of
carbon in ethane, heptane
and decane.
12. An 8.129 g sample of
hydrated Magnesium Sulfate,
MgSO4• xH2O, is heated until all
of the water of hydration is
driven off. The mass of the
remaining anhydrous
magnesium sulfate, MgSO4, is
3.967 g. Determine the value of
x in the hydrated compound.
11. What is the mass
percent of water in
Nickel (II) Nitrate
undecahydrate?

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