Stoichiometry04/17

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

Formation of Ammonia…other ways to think about this

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?

For ExampleThe U. S. Space Shuttle boosters use this reaction

3 Al(s) + 3 NH4ClO4 ® Al2O3 +

AlCl3 + 3 NO + 6H2O

How much Al must be used to react with 652 g of NH4ClO4 ?

How much water is produced?How much AlCl3?