Stoichiometry

Calculations based on a balanced chemical equation

Chapter 9 (12)

Why do I need to know about stoichiometry?

Sample recipe for 1 loaf of bread:8 cups flour1 cup sugar2 cups milk1.5 cups eggs1/2 cup of butter1/8 cup yeast

How many loaves of bread can be made

from: • 3 cups eggs

2 loaves • 1 cup

milk 1/2 loaf

• 6 cup flour 3/4 loaf

Example using stoichiometry:

8 Fl + 1 S + 2 M + 1.5 E + 1/2 B + 1/8 Y => 1 Lf

“Chemical Equation” for bread recipe:

Sample recipe for 1 loaf of bread:8 cups flour1 cup sugar2 cups milk1.5 cups eggs1/2 cup of butter1/8 cup yeast

Review of Balancing Equations

KClO3 --> KCl + O2322

What do the coefficients mean?

a) Molecules

“2 molecules KClO3 produces 3 molecules of O2”

Review of Balancing Equations

KClO3 --> KCl + O2322

What do the coefficients mean?

a) Molecules

“2 molecules of KCl are formed when 3 molecules of O2 are formed”

Review of Balancing Equations

KClO3 => KCl + O2322

What do the coefficients mean?

b) Moles

“2 moles of KCl are formed when 3 moles of O2 are formed”

Review of Balancing Equations

KClO3 => KCl + O2322

What do the coefficients mean?

b) Moles

“2 moles of KCl are formed when 2 moles of KClO3 are decomposed”

In the following reaction how many moles of PbCl2 are formed if 5.000 moles of NaCl react? 2 NaCl + Pb(NO3)2 PbCl2 + 2 NaNO3

5.000 moles NaCl

2 moles NaCl

1 moles PbCl2 =

2.500 moles PbCl2

In the following reaction how many moles of NH3

are formed if 4.0 moles of H2 react? N2 + 3 H2 => 2 NH3

4.0 moles H2

3 moles H2

2 moles NH3 =

2.7 moles NH3

Complete Problems 1-5 on the practice page.

In the following reaction how many grams of NH3

are formed if 4.00 moles of H2 react? N2 + 3 H2 => 2 NH3

4.00 moles H2

3 moles H2

2 moles NH3 =

45.3 g NH3

1 moles NH3

17 g NH3

Grams A

Moles A Moles B

Grams B

1 1

coefficients

mw mw

In the following reaction how many moles of NH3

are formed if 10.0 grams of H2 react? N2 + 3 H2 => 2 NH3

10.0 grams H2

2.016 g H2

1 moles H2 =

3.31 mol NH3

3 moles H2

2 mole NH3

Complete Problems 6-10 on the practice page.

Grams A

Moles A Moles B

Grams B

1 1

coefficients

mw mw

In the following reaction how many grams of NH3

are formed if 25.0 grams of N2 react? N2 + 3 H2 => 2 NH3

25.0 g N2

28.02 g N2

1 moles N2 =

30.3 g NH3

1 moles N2

2 mole NH3

Grams A

Moles A Moles B

Grams B

1 1

coefficients

mw mw

1 mole NH3 17 g NH3

Complete Problems 10-15 on the practice page.

How many grams of NH3 are formed if 25.0 grams of N2 react with 10.0 g of H2? N2 + 3 H2 => 2 NH3

25.0 g N2

28.02 g N2

1 moles N2 =

30.3 g NH3

1 moles N2

2 mole NH3

1 mole NH3 17 g NH3

10.0 grams H2

2 g H2

1 moles H2

=

3 moles H2

2 mole NH3

1 mole NH3

17 g NH3

56.7 g NH3

(Solve the problem separately with each number)

(The smaller answer is the only correct one)56.7 g NH3

Complete problems 16-20.

How many grams of NH3 are formed if 25.0 grams of N2 react with 10.0 g of H2? N2 + 3 H2 => 2 NH3

25.0 g N2

28.02 g N2

1 moles N2 =

30.3 g NH3

1 moles N2

2 mole NH3

1 mole NH3 17 g NH3

10.0 grams H2

2 g H2

1 moles H2

=

3 moles H2

2 mole NH3

1 mole NH3

17 g NH3

56.7 g NH3 56.7 g NH3

How much of the excess reagent is left over?

How many grams of H2 (the excess reagent) are required to react with 25.0 g of N2 (the limiting reagent) ? N2 + 3 H2 => 2 NH3 25.0 g N2

28 g N2

1 moles N2 =

5.36 g H2

1 moles N2

3 mole H2

1 mole H2 2 g H2

REQUIRED

Left over = Given amount – Required amount = 10.0 g H2 - 5.36 g H2

= 4.64 g H2

How many grams of NH3 are formed if 10.0 grams of N2 react with 15.0 g of H2?

How much of the excess reagent is left over?

N2 + 3 H2 => 2 NH3

Percent Yield Calculations

Terms:

Theoretical Yield = the CALCULATED amount of product expected

Actual Yield = the EXPERIMENTAL amount that was actually obtained

% Yield =

Actual

TheoreticalX 100

What is the percent yield in a reaction where 1.50 mol of NH3 was obtained after reacting 10.0 g of H2 with excess nitrogen? N2 + 3 H2 => 2 NH3

10.0 grams H2

2.016 g H2

1 moles H2 =

3.31 mol NH3

3 moles H2

2 mole NH3

Theoretical Yield

% yield = 1.50

3.31X 100 = 45.3%