Chem_chap3b

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CHAPTER 3B
Stoichiometry
Counting Atoms
Amu – atomic mass unit
1 amu = 1.66 x 10-24 g
Ex.
1 carbon atom = 12 amu
1 Fe atom = 55.85 amu
1 Na atom = ??? Amu
Counting Atoms


Calculate the mass, in amu, of a sample of Fe that
contains 15 atoms.
Calculate the mass, in amu, of a sample of carbon
that contains 62 atoms.
Counting Atoms

Calculate the number of copper atoms present in a
sample that has a mass of 1779.4 amu.
1 Cu atom = ??? Amu

Calculate the number of argon atoms present in a
sample that has a mass of 3755.3 amu.
Moles
An amount of matter that contains as many
objects as the number of atoms in exactly 12
grams of pure carbon 12. This number is 6.02
x 1023.
Avogadro's number = 6.02 x 1023
Moles
1 dozen eggs = 12 eggs
1 mole eggs = 6.02 x 1023 eggs
1 mole of elephants = 6.02 x 1023 elephants
1 mole of Al = 6.02 x 1023 Al atoms
Moles
But they all have different masses.



1 mol of C and 1 mol of Mg have the same number of
particles
but C has a mass of 12 amu and Mg a mass of 24 amu
If C has a mass of 12 g by definition then Mg = 24
grams.
So…..
1 mole of the element = 6.02 x 1023 atoms of that
element and…
1 amu of an element = grams of 1 mole of element
Element
Al
Au
S
Fe
# of atoms
6.02 x 1023
6.02 x 1023
6.02 x 1023
6.02 x 1023
Mass (g)
26.98
196.97
32.07
55.85
Moles
1 mole
1 mole
1 mole
1 mole
Mass/Mole
1 mol of an atom= average atomic mass of the
element
Ex. 1mol H = 1.008 g H
1mol C = 12.01 g C
1mol Na = 22.99 g Na
Can then use this as a conversion factor
Mass/Mole
Compute the number of moles of atoms in a
25.0g sample of calcium.
1mol Ca = 40.08 g
Mass/Mole




Calculate the number of moles of atoms in 57.7 g
sample of sulfur.
Calculate the number of moles in 43.15 g of
carbon.
Calculate the mass in grams of 0.251 moles of
sodium.
Calculate the mass in grams of 5.58 moles of
copper.
Molar Mass
The total mass of a compound.
What is the mass of H20?
1 atom H2O = 1 atom O and 2 atoms H
So…
1 mol H20 = 1 mol O and 2 mols H
Mass of 1 mole of O = 1 x 16.00g
Mass of 2 moles of H = 2 x 1.008g
Mass of 1 mole of H2O
= 16.00g
= 2.016g
= 18.12g
Molar Mass

What is the molar mass of C3H8?

What is the molar mass of NH3?
Molar Mass




What is the mass in grams of 1.48 moles of
potassium oxide?
Calculate the mass of 4.85 mol of acetic acid,
HC2H3O2.
How many moles of formaldehyde (H2CO) does a
7.55 g sample represent?
How many moles of tetraphosphorus dioxide does a
250.0 gram sample represent?
Percent Mass Composition
Determining the percentage of how much of the total
weight each element weighs.
Ex. Determine the mass percent of each element in
sulfuric acid (H2SO4).
%H=
%S=
%O=
Percent Mass Composition
Ex. Determine the mass percent of each element in
C3H7OH
Formulas


Empirical formula – a formula of a compound that
has the smallest whole-number ration of atoms
present.
Molecular formula – the actual formula of a
compound.
Ex. Determine the empirical formula for
 C4H10
 H2 O 2
 CCl4
Calculating Empirical Formulas
1.
2.
3.
4.
Find mass of each element
Number of moles of each element
Divide each elements moles by the smallest
number of moles.
If the moles are not whole numbers, multiply each
element by the number that converts the fraction to
whole numbers
Calculating Empirical Formulas
Ex. A 1.500 gram sample of a compound containing
only carbon and hydrogen is found to contain 1.198
gram of carbon. Determine the empirical formula
for this compound.
Calculating Empirical Formulas
Ex. A sample of phosphoric acid contains 0.3086
grams of hydrogen, 3.161 grams of phosphorus,
and 6.531 grams of oxygen. Determine the
empirical formula for phosphoric acid.
Calculating empirical Formulas
Ex. The simplest amino acid, glycine, has the following
mass percents: 32.00% carbon, 6.714% hydrogen,
42.63% oxygen, and 18.66% nitrogen. Determine
the empirical formula for glycine.
Mole Ratios
A balanced equation of a reaction gives a ratio of
moles of each compound involved in the reaction.
Remember a mole has no dimension until you add one.
Ex. NO + H2 → N2 + H2O
Ex. SiH4 + NH3 → Si3N4 + H2
Mole Ratios
Methane burns in oxygen to form carbon dioxide
and water.
CH4 + 2O2 → CO2 + 2H2O


What number of moles of oxygen gas is required
to react with 7.4 moles of methane?
How many moles of carbon dioxide will be
produced by reacting 2.6 moles of oxygen with
excess methane?
Mole Ratios
Hydrogen sulfide gas reacts with oxygen to produce
sulfur dioxide gas and water.



How many moles of oxygen are required to react
with 5.6 moles of hydrogen sulfide?
How many moles of sulfur dioxide gas will be
produced by reacting 7.3 moles of hydrogen sulfide
with excess oxygen?
How many moles of sulfur dioxide gas will be
produced by reacting 7.3 moles of oxygen with
excess hydrogen sulfide?
Mole-Mass Conversion
Solutions of sodium hydroxide absorb carbon
dioxide from the air, forming sodium carbonate
and water.


Calculate the mass of carbon dioxide gas that is
required to react with a solution containing 10.0
grams of sodium hydroxide.
Calculate the mass of sodium carbonate that is
produced when 10.0 grams of sodium hydroxide
reacts with an excess of carbon dioxide.
Mass/Mole
Steps for calculating

Balance equation

Convert mass of known compound to moles.

Use mole ratios from balanced equation as
a conversion to moles of desired compound.

Convert moles of desired compound to mass
Limiting Reactants
A reactant that stops the reaction because it is consumed.
Steps to calculating limiting reactants
 Balance the equation.
 Convert to moles.
 Use the mole ratios to determine limiting reagent.
 Using limiting reactant compute the number moles of
desired product.
 Convert to moles.
Limiting reactants
Iron(III) oxide reacts with carbon monoxide to
form iron metal and carbon dioxide gas. In a
certain experiment 5.0 grams of Iron (III) oxide
is reacted with 5.0 grams of carbon dioxide.


What mass of iron will be produced, assuming a
complete reaction?
What mass of carbon dioxide will be produced,
assuming a complete reaction?
Limiting reactants
Iodine reacts with chlorine gas to form iodine
chloride. In a certain experiment, 10.0 grams of
iodine is reacted with 10.0 grams of chlorine gas.

What mass of aluminum chloride will be produced,
assuming a complete reaction?
Percent yield
actual yeild
x100%
Theoretical yeild


In a certain experiment, the expected yield is 1.325 grams.
The actual yield was 1.279 grams. Determine the percent
yield.
2NH3 + CO2 → CN2H4O + H2O
If 100.0 grams of ammonia is reacted with 100.0 grams of
carbon dioxide and 120.0 grams of urea are produced.
Determine the percent yield.

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