Kinda like a recipe
4 Eggs + 3 Bacon ® 2 Omelets8 Eggs + ? Bacon ® ? Omlets4Al + 3O2 ® 2 Al2O3
8 Al+? O2 ® ? Al2O3
+ ®+ ®
3
Stoichiometry (more working with ratios)
Ratios are found within a chemical equation.
4Al + 3O2 ® 2 Al2O3
4 moles of aluminum react with 3 moles of oxygen to form 2 moles of aluminum oxide
coefficients give MOLAR RATIOS
Mole to mole conversions4Al + 3O2 ® 2 Al2O3 every time we react 3 moles of O2 we make 2
moles of Al2O3
2 moles Al2O3
3 mole O2
or2 moles Al2O3
3 mole O2
Mole to Mole conversionsHow many moles of O2 are produced when 3.34 moles of
Al2O3 decompose?
4Al + 3O2 ® 2 Al2O3
3.34 moles Al2O3
2 moles Al2O3
3 mole O2 = 5.01 moles O2
6
When N2O5 is heated, it decomposes:
2N2O5(g) 4NO2(g) + O2(g)
a. How many moles of NO2 can be produced from 4.3 moles of N2O5?
= moles NO2
4.3 mol N2O5
52
2
ON mol2
N O mol48.6
b. How many moles of O2 can be produced from 4.3 moles of N2O5?
= mole O2
4.3 mol N2O5
52
2
ON 2mol
O mol12.2
2N2O5(g) 4NO2(g) + O2(g)4.3 mol ? mol
2N2O5(g) 4NO2(g) + O2(g)4.3 mol ? mol
Mole – Mole Conversions
Units match
36.04 g reactants36.04 g reactants
In terms of mass – Law of Conservation2H2 + O2 ® 2H2O
2 moles H22.02 g H2
1 moles H2
= 4.04 g H2
1 moles O232.00 g O2
1 moles O2
= 32.00 g O2
What should be the mass of the products?
36.04 g reactants36.04 g products
In terms of mass – Law of Conservation2H2 + O2 ® 2H2O
2 moles H2O 18.02 g H2O1 mole H2O
= 36.04 g H2O
2H2 + O2 ® 2H2O
Try using stoichiometry to get the products mass.
Practice!Show your work. Have your periodic table out.
A. What is the mole ratio of N2O5 to O2B. How many moles of oxygen will be produced from 3 moles
of N2O5?
2N2O5(g) 4NO2(g) + O2(g)
Practice!Show your work. Have your periodic table
out.
A. What is the mole ratio of N2O5 to O22:1
B. How many moles of oxygen will be produced from 3 moles of N2O5?
2N2O5(g) 4NO2(g) + O2(g)
= 1. 5 Mol O2= 2 mol O2
3 mol N2O5
2 mol N2O5
1 mol O2
Periodic Table
MolesA
MolesB
Massg B
Periodic Table
Balanced Equation
Massg A
•Decide where to start based on the units you are given
•and stop based on what unit you are asked for
THE STEPS for StoichiometryREAD the problem and UNDERLINE what you
know and need to find outWRITE known and what you want to know
under equationSET UP house with known and what you want
to knowFIND pathway on green sheet to get from
known to what you want to knowMultiply/Divide and CANCEL units – What you
want goes on top of the table!!!!RECORD answer (sig figs and units)
14
When N2O5 is heated, it decomposes:2N2O5(g) 4NO2(g) + O2(g)
a. How many moles of N2O5 were used if 210g of NO2 were produced?
= moles N2O5
210 g NO2
2
52
N O mol4
ON mol22.28= 2.3
b. How many grams of N2O5 are needed to produce 75.0 grams of O2?
= grams N2O5
75.0 g O2
2
52
O 1mol
ON mol2506
2
2
N O g0.46
N O mol
2
2
O g 32.0
O mol
52
52
ON mol
ON g108
gram ↔ mole and gram ↔ gram conversions
2N2O5(g) 4NO2(g) + O2(g)210g? moles
2N2O5(g) 4NO2(g) + O2(g)75.0 g? grams
Units match
15
Aluminum is an active metal that when placed in hydrochloric acid produces hydrogen gas and aluminum chloride. How many grams of aluminum chloride can be produced when 3.45 grams of aluminum are reacted with an excess of hydrochloric acid?
First write a balanced equation.
Al(s) + HCl(aq) AlCl3(aq) + H2(g)2 6 2 3
Gram to Gram Conversions
16
Aluminum is an active metal that when placed in hydrochloric acid produces hydrogen gas and aluminum chloride. How many grams of aluminum chloride can be produced when 3.45 grams of aluminum are reacted with an excess of hydrochloric acid?
Al(s) + HCl(aq) AlCl3(aq) + H2(g) 2 6 2 3
Now let’s get organized. Write the information below the substances.
3.45 g ? grams
Gram to Gram Conversions
17
Aluminum is an active metal that when placed in hydrochloric acid produces hydrogen gas and aluminum chloride. How many grams of aluminum chloride can be produced when 3.45 grams of aluminum are reacted with an excess of hydrochloric acid?
Al(s) + HCl(aq) AlCl3(aq) + H2(g)2 6 2 33.45 g ? grams
Let’s work the problem.
= g AlCl33.45 g Al
Alg 27.0
Almol
We must always convert to moles.Now use the molar ratio.
Almol 2
AlClmol 2 3
Now use the molar mass to convert to grams.
3
3
AlClmol
AlClg 133.317.0
Units match
gram to gram conversions
More ExamplesTo make silicon for computer chips they use this reaction
SiCl4 + 2Mg ® 2MgCl2 + Si
How many moles of Mg are needed to make 9.3 g of Si?3.74 mol of Mg would make how many moles of Si?
How many grams of MgCl2 are produced along with 9.3 g of
silicon?