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Intro AnalogyLet’s make some sandwiches.
2 pieces bread + 3 slices meat + 1 slice cheese 1 sandwich
What if we want to make 50 sandwiches? How many ingredients would we need?
50 x entire equation100 pieces bread + 150 slices meat + 50 slices
cheese 50 sandwiches
Application Chemical equations are like recipes for
reactions. Coefficients in a chemical equation indicate
the relative number of particles of each substance taking part in a reaction.
Application
CO(g) + 2H2(g) CH3OH(l)If we multiplied the equation by 12 it would
still be balanced.
12CO(g) + 24H2(g) 12CH3OH(l)
Because 12 = 1 dozen we can now read the coefficients as dozens.
1 dozen CO(g) + 2 dozen H2(g) 1 dozen CH3OH(l)
ApplicationWe could also multiply the original balanced
equation by 6.02 x 1023.
(6.02 x 1023)CO(g) + (1.204 x 1024)H2(g) (6.02 x 1023) CH3OH(l)
Because 6.02 x 1023 = 1 mole, we can now read the coefficients as moles.
1 mol CO(g) + 2 mol H2(g) 1 mol CH3OH(l)
Application
Coefficients in a balanced chemical equation give a ratio of the # of molecules, the # of moles, or even the # of dozens present in
the reaction.
We will focus on the mole relationships that balanced chemical equations provide.
Stoichiometry The study of the quantitative, or
measurable, relationships that exist in chemical formulas and chemical reactions.
In order to successfully complete stoichiometry problems you must understand mole relationships.
Stoichiometry: Relating Moles to Molecules
Propane (C3H8) is a fuel commonly used for cooking on gas grills and for heating in rural
areas where natural gas is unavailable. Propane reacts with oxygen gas to produce heat and the products carbon dioxide and water. Give the balanced equation for this
reaction and state the meaning of the equation in terms of numbers of molecules
and moles of molecules.
Stoichiometry: Relating Moles to Molecules
C3H8 + 5O2 3CO2 + 4H2O
Molecules:
1 molecule C3H8 + 5 molecules O2 3 molecules CO2 + 4 molecule H2O
Moles:
1 mole C3H8 + 5 moles O2 3 moles CO2 + 4 moles H2O
Stoichiometry: Using Mole Relationships
A mole ratio is the ratio of moles of one substance to moles of another substance in a balanced chemical equation.
Mole ratios can be used to convert the number of moles of one substance in a reaction to the number of moles of another substance.
Mole Analogy
For every 3 school buses that arrive at school, 2 will transport marching band
members and 1 will transport choir members.
3 buses 2 marching band + 1 choir
If 12 buses show up to school, how many will transport choir members?
Mole Analogy
3 buses 2 marching band + 1 choir
12 buses x
4 of the 12 buses will carry choir members
The ratio becomes your conversion factor.
3 buses 1 choir
= 4 choir
Mole Ratios
What is the mole ratio of water to oxygen when water decomposes? What is the mole ratio of water to hydrogen in the reaction?
2H2O(l) 2H2(g) + O2(g)
2 mol H2O : 1 mol O2
2 mol H2O : 2 mol H2 = 1:1
Stoichiometry: Using Mole Ratios
What number of moles of O2 will be produced by the decomposition of 5.8 moles of
water?
Given: 5.8 mol H2O
Unknown: moles of O2
5.8 mol H2O ? moles of O2
Stoichiometry: Using Mole Ratios
2H2O(l) 2H2(g) + O2(g)
5.8 mol H2O x
If 5.8 mol of water decompose, 2.9 mol of oxygen gas will be produced.
2 mol H2O1 mol O2 = 2.9 mol O2
Mole-Mole Practice Problem 1
Methane (CH4) burns in oxygen to form carbon dioxide and water. (a) How many moles of oxygen gas are required to react with 7.4 moles of methane? (b) How many
moles of carbon dioxide gas will be produced by reacting 2.6 moles of oxygen
with excess methane?
CH4(g) + 2O2 CO2(g) + 2H2O(g)
(a) How many moles of oxygen gas are required to react with 7.4 moles of
methane?
7.4 mol CH4 ? mol O2
7.4 mol CH4 x = 14.8 mol O21 mol CH4
2 mol O2
CH4(g) + 2O2 CO2(g) + 2H2O(g)
(b) How many moles of carbon dioxide gas will be produced by reacting 2.6 moles of
oxygen with excess methane?
2.6 mol O2 ? mol CO2
2.6 mol O2 x = 1.3 mol CO21 mol CO2
2 mol O2
Mole ratios (from the coefficients in a balanced chemical equation) allow us to convert moles of one substance in a chemical reaction to moles of another substance in the reaction.
We learned last semester that we can convert the mass of a substance to moles of that substance.
By combining the two processes, we can convert quantities of one substance to quantities of another.
Mass-Mass ProblemsMass of one substance is given and you are
asked to find the mass of another substance.
1. Convert mass of given to moles of given (using molar mass).
2. Convert moles of given to moles of unknown (using the mole ratio).
3. Convert moles of unknown to mass of unknown (using molar mass).
Mass-Mass Practice
Solid lithium hydroxide has been used in space vehicles to remove exhaled carbon dioxide from the living environment. This
produces solid lithium carbonate and liquid water. What mass of carbon dioxide can 1.00 x 103 g of lithium hydroxide absorb?
Mass-Mass PracticeHydrofluoric acid, an aqueous solution containing dissolved hydrogen fluoride, is
used to etch glass by reacting with the silica, SiO2, in the glass to produce
gaseous silicon tetrafluoride and liquid water. (a) Calculate the mass of hydrogen
fluoride needed to react with 5.68 g of silica. (b) Calculate the mass of water
produced by the reaction in part a.
Further Applications How would you convert a given number of
particles of a substance to the mass of another substance?
How would you convert the moles of a given substance to the mass of another substance?