Transcript moles

Stoichiometry
The quantitative study of reactants
and products in a chemical reaction
Stoichiometry

Whether the units given for reactants or
products are moles , grams , liters (for
gases), or some other units, we use moles to
calculate the amount of product formed in a
reaction
Stoichiometry
Mass
Mass
Moles
Liters
Particles
Known
Moles
Liters
Particles
Unknown
Review before starting
 The Mole
 Molar Conversions
 Balancing Chemical Equations
Stoichiometry
Problem Types





Mole to Mole
Mole to Mass & Mass to Mole
Mass to Mass
Volume to Moles or Mass
Limiting Reactants & Per Cent Yield
The Balanced Equation
2 CO(g) +
O2 -->
2 moles
1 mole
2CO2(g)
2 moles
Coefficients show relative amounts
Mole Ratio
A conversion factor that relates the amounts in moles of any
two substances involved in a chemical reaction
2 CO(g) + O2 --> 2 CO2(g)
2 moles of CO is equivalent to 2 moles of CO2
The mole ratio between CO and
CO2 is 2:2 or 1:1
2mol CO
2 mol CO2
2 mol CO2
and 2 mol CO
Mass to Mass
140. grams of carbon monoxide reacts with an
excess of oxygen gas to theoretically produce
how many grams of carbon dioxide?
CO(g) + O2
140. g CO
2 CO2(g)
1 mole CO
28.0 g CO
2 moles CO2 44.0 g CO 2 = 220. g CO 2
2 moles CO 1 mole CO2
Problem Solving Strategy
(mass to mass)
Known: 140 g CO
Unknown: ? grams CO2
mass
mass
140. g CO
moles
5 moles CO
220. g CO2
moles
5 Moles CO2
mole bridge
20
Burlingame High School Chemistry
Example Problem

If a furnace burns an amount of coal containing 6.0
moles of FeS2, how many moles of SO 2 (an air
pollutant) is theoretically produced?
4FeS2 + 11O2
2 Fe2O3 + 8 SO2
What is the mole ratio of FeS 2 & SO 2 ?
4 moles FeS2 : 8 moles SO 2
4:8 or 1:2
Example Problem
If a furnace burns an amount of coal containing
6.0 moles of FeS 2 , how many moles of SO 2
(an air pollutant) is produced?
4FeS2 + 11O2
2 Fe2O3 + 8 SO2
Use the mole ratio - 4 mol FeS2 to 8 SO2
use the given
6 mols FeS2
8 mols SO2
4 mols FeS2
= 12 mols SO 2
Another
Example
If a furnace burns an amount of coal containing
100.0g of FeS 2, how many grams of SO 2 (an
air pollutant) is theoretically produced?
Remember the balanced equation
4FeS2 + 11O2
2 Fe2O3 + 8 SO2
Use dimensional analysis
100.0 g FeS2
Convert mass of reactant to
moles of reactant.
100.0 g FeS2 1 mole FeS2
120.0 g FeS2
Convert moles of reactant to
moles of product.
100.0 g FeS2 1 mole FeS2
120.0 g FeS2
8 mole SO2
4 mole FeS2
Convert moles of product to
grams of product.
100.0 g FeS2 1 mole FeS2
120.0 g FeS2
8 mole SO2
64.0 g SO2
4 mole FeS2
1 mole SO2
Multiply across the top and
bottom
100.0 g FeS2 1 mole FeS2
8 mole SO2
64.0 g SO2
=
120.0 g FeS2
4 mole FeS2
1 mole SO2
51200 g SO2
= 106.67 g SO 2 = 107 g SO 2
480
Moles to Number of
Particles
Another Conversion Factor
1 mole = 6.02 x 1023 particles
Moles of
substance
Number of
atoms or
molecules
6.02 x
x
1023
1mol
x
1mol
6.02 x 1023
Number of
=
atoms or
molecules
=
Moles of
substance
31
Number of Particles to
Moles
How many atoms are present in
0.35 mol of Na?

Number of Particles to
Moles
How many atoms are present in
0.35 mol of Na?

.35 mol Na
6.02 x 1023 = 2.1 x 1023 atoms
1 mol
Moles to Number of
Particles
How many moles are present in
3.00 x 10 21 molecules of C 2 H 6 ?

3.00 x 1021 molecules C2H6
1 mole C2H6
= 4.98 x 1 0-3 moles
6.02 x 1 023 molecules
The Balanced Equation
2 CO(g) +
2 moles
:
O2(g) -->
1 mole
2 CO2(g)
: 2 moles
2(6.02x1023) : (6.02x1 023): 2(6.02x1 023)
molecules
molecules
molecules
56g CO
:
gases
2 volumes :
32 g O2
1 volume
:88 g CO2
:2 volumes
Volume Conversions
Volume of a gas is dependent on the
temperature & pressure
In this unit we use
Standard Temperature & Pressure (STP)
for our problems
STP = 0 0 Celsius & 1 atm of pressure
Volume
Conversions
A new conversion factor!!
At SIP 1 mole of any gas occupies 22.4 Liters.
1 mole gas
22.4 L
and
22.4 L
1 mole gas
Volume Conversions
If 5 g of magnesium is added to a solution of
hydrochloric acid, what volume of hydrogen
gas is produced at STP?
Volume
Conversions
If 5.0 g of magnesium is added to a solution of
hydrochloric acid, what volume of hydrogen gas is
produced at STP?
1. Start with a balanced equation
Mg(s) + 2HCl(aq)
MgCl 2(aq) + H 2(g)
Volume
Conversions
If 5.0 g of magnesium is added to a solution of
hydrochloric acid, what volume of hydrogen gas is
produced at STP?
1. Balanced equation
Mg(s) + 2HCl(aq)
MgCl2(aq) + H2(g)
1. Dimensional Analysis
5.0g Mg
mass Mg
1 mol Mg
24.3 g
mols Mg
1 mol H2
1 mol Mg
mols H2
22.4 L = 4.6L
1 mol H2
volume H2
Limiting Reagents
To make a dozen brownies the recipe calls for
2 cups flour, 112 grams chocolate, 25O ml water .
You have 2 cups flour 50 grams chocolate, & 250 ml water
If you want to make quality brownies you will make
less than a dozen and have flour & water left over!
What is the limiting reagent ?
Limiting
Reagents
Zinc & Sulfur react to form zinc (II) sulÞde
according to the following equation
8 Zn(s) + S8
8ZnS(s)
If 2.00 mol of Zn are heated with 1.00 mole S 8,
identify the limiting reactant.
How many moles of excess reactant will there
Limiting
Reagents
Zinc & Sulfur react to form zinc (II) sulfide according to
the following equation
8 Zn(s) + S8
8ZnS(s)
If 2.00 mol of Zn are heated with 1 .00 mole S8, identify
the limiting reactant?
2 mol Zn 1 mol S 8 = .25 mole S 8
8 mol Zn
How many moles of excess reactant?
Zn is limiting
(there isn’t enough to
react with all the S8)
Limiting
Reagents
Zinc & Sulfur react to form zinc (II) sulfide according to
the following equation
8 Zn(s) + S8
8ZnS(s)
If 2.00 mol of Zn are heated with 1 .00 mole S8, identify
the limiting reactant?
2 mol Zn 1 mol S8 = .25 mole S 8
8 mol Zn
How many moles of excess reactant?
.75 moles of S8
Zn is limiting
(there isn’t enough to
react with all the S8)
Percent Yield
 So far we have been doing stoichiometry
problems that represent theoretical yields
 Actual Yield - the measured amount of
product that you really get in the reaction.
Percent
Yield
 Percent Yield is the ratio of the actual yield
to the theoretical yield multiplied by 100
 Percent yield =
actual yield
theoretical yield
x 100
Percent Yield

Quicklime, CaO, can be prepared by roasting
limestone, CaCO3, according to the following reaction.
CaCO3(s)
CaO(s) + CO2(g)
 When 2.00 x 1 03g of CaCO3(s) is heated the actual
yield of CaO is 1.05 x 1 03g. What is the percent yield?
Percent Yield
CaCO3(s)
CaO(s) + CO2(g)
Given: 2.00 x 1 03g of CaCO 3(s)
actual yield of CaO is 1.05 x 1 03g
To solve
1. Find theoretical yield (mass
mass problem)
2. Find percent yield (actual/theoretical x 1 00)
2.00 x 1 0 3g CaCO 3(s)
1 mol CaCO3
100.g
1 mol CaO
56.0 g
1 mol CaCO 3 1 mol CaO
= 11 20g CaO
Percent Yield = 1.05 x 10 3g x 100 = 93.8%
1.12 x 103g
Quest
1. Na2SIO3 (s) + 8 HF(aq) - H2SiF6 (aq) + 2 NaF(aq) + 3
H2O (l)
How many moles of HF are needed to react with 0.300
mol of Na2SiO3?
How many grams of NaF form when 0.500 mol of HF
reacts with excess Na2SiO3?
How many grams of Na2SiO3 can react with 0.800 g of
HF?
Quest
Quest
Quest
Quest
Quest