Chapter 09 - Stoichiometry

Foundations of College Chemistry, 14th Ed.

Morris Hein and Susan Arena

Accurate measurement and dosage calculations are critical in dispensing medicine to patients all over the world.

Calculations from Chemical Equations

Stoichiometry



_{Calculations involving chemical equations}



_{Chemical equations tell us exactly how}

Equations must always be balanced before calculation of any mass, moles, or volume of a reactant or product!

a A + b B c C + d D Example

Using X.X g of A, how much C will be formed?

Solving stoichiometry problems always requires the use of:

1. A balanced chemical equation (coefficients must be known!) 2. Conversion factors in units of moles (i.e. mole ratios)

Introduction to Stoichiometry

4

• Mole ratio: ratio (conversion factor) between any two

species in a chemical reaction. The coefficients of a

balanced chemical equation are used to generate mole ratios.

2Fe(s) + 3S(s) Fe₂S₃(s)

Fe and S 2 mol Fe and 3 mol S 3 mol S 2 mol Fe

Fe and Fe₂S₃ 2 mol Fe and 1 mol Fe₂S₃ 1 mol Fe₂S₃ 2 mol Fe

S and Fe₂S₃ 3 mol S and 1 mol Fe₂S₃ 1 mol Fe₂S₃ 3 mol S

Given the following balanced chemical equation, write the mole ratio need to calculate:

a. The moles of H₂O produced from 3 moles of CO₂

b. The moles of H₂ needed to produce 3 moles of H₂O.

CO₂ (g) + 4 H₂ (g) CH₄ (g) + 2 H₂O (l)

= 6.0 mol H₂O 3.0 mol CO₂ × 2 mol H_{1 mol CO}2O

2 a.

Mole ratio

Desired quantity in the numerator of the mole ratio:

known quantity in the denominator

Mole Ratios Practice

3.0 mol H₂O × _{2 mol H}4 mol H2

2O

Mole ratio

= 6.0 mol H₂

Given the following balanced chemical equation, write the mole ratio need to calculate:

a. The moles of H₂O produced from 3 moles of CO₂

b. The moles of H₂ needed to produce 3 moles of H₂O.

CO₂ (g) + 4 H₂ (g) CH₄ (g) + 2 H₂O (l)

b.

Desired quantity in the numerator of the mole ratio:

known quantity in the denominator

Mole Ratios Practice

Flow Chart for Stoichiometry Problems

Grams of Known

Atoms/Molecules of Known

Moles of Known Moles of Desired

Step 2 Step 3 Using the Mole Ratio Atoms/Molecules of Desired

Grams of Desired

Step 2

Step 4 Step 4

Problem Solving for

Stoichiometry Problems

2 Al + 6 HCl



2 AlCl

₃

+ 3 H

₂

•

_{How many molecules of HCl will react with 4}

atoms of Al?

•

_{How many atoms of Al are required to make 1}

molecule of AlCl

?

•

_{How many moles of H}

2

are made from 3 mole of

HCl?

•

_{If 4 moles of AlCl}

3

are produced, how much H

is

produced?

•

_{How much HCl is required to react with 1 mole of}

CH

₄

+ 2 O

₂



CO

₂

+ 2H

₂

O

•

_{How many moles of oxygen are required to react}

completely with 50.0 g CH

?

•

What mass of CH

, in grams, is required to react

with 96.0 g of O

?

•

Calculate the mass of CO

that can be produced

by burning 6.0 moles of CH

in excess O

?

•

What mass of CH

produces 3.01 x 10

_water

•

_{Smelling salts contain ammonium carbonate,}

which can decompose to form ammonia, which

acts as a mild heart stimulant. Ammonium

carbonate decomposes by the reaction

•

_(NH

4

)

CO

(s)



2 NH

(g) + CO

(g) + H

O(l)

•

_{How many g of NH}

3

will be formed from 0.500 g of

•

_{Calculate the number of moles of calcium}

chloride needed to react with excess silver

nitrate to produce 6.60 g of silver chloride.

•

₁

- write the equation ( a double

displacement reaction)

Reactions and Energy

•

Reactions will often generate energy (heat) or

produce (give off) energy. We can use stoichiometry

to calculate energy consumption or production.

•

Consider the combustion of methane (used in our

bunsen burners)

•

CH

(g) + 2 O

(g)



CO

(g) + 2 H

O(l) + 890 kJ

•

_{Calculate the amount of heat produced when 1.00 g}

13

Limiting Reactant

A limiting reactant in a chemical reaction is the substance that

• _{is used up first}

• _{stops the reaction}

14

Example of Everyday Limiting

Reactant

How many peanut butter sandwiches can be made from 8 slices of bread and 1 jar of peanut butter?

Problem Solving Strategy for Limiting Reactant Problems

1. Calculate the amount of product formed from each reactant present.

2. The reactant that gives the least amount of product is limiting; the other reactant is in excess.

3. The amount of product is determined by the

calculation from Step 1 with the limiting reactant.

4. If the amount of excess reactant is desired, determine the amount of excess reactant needed to consume the limiting reactant and subtract from the initial quantity present.

Limiting Reactants

16

Limiting Reactants

When 4.00 mol of H₂ is mixed with 2.00 mol of Cl₂, how many moles of HCl can form?

H₂(g) + Cl₂(g)  2HCl (g)

17

Checking Calculations

Change _{2.00 mol 2.00 mol} +4.00 mol

Excess 2.00 mol Excess

0 mol Limiting

The amount of products formed calculated by

stoichiometry are the maximum yields possible (100%). Yields are often lower due to side reactions, loss of product while isolating/transferring the material, etc. Theoretical yield: maximum possible yield for a reaction,

calculated based on the balanced chemical equation.

Actual yield: actual yield obtained from the reaction. Percent yield: ratio of the actual and theoretical yield

Actual yield Theoretical yield

% yield = × 100

Reaction Yield

Percent Yield

•

_{% Yield = (mass product/mass expected)*100(%)}

•

_{What could cause the yield to be < 100%?}

– _{impurities in the sample} – _{incomplete reaction}

•

_{What could cause the yield to be > 100%?}

– _{unexpected reactions} – _{Impure product}

•

_{Copper(I)sulfide reacts upon heating in}

oxygen gas to produce copper metal and

sulfur dioxide. How many grams of copper

can be obtained from 500.0 g of cuprous

sulfide using this process?

•

_{If 382.6 g of copper were obtained from}

the reaction above, what would be the

percent yield?

•

_{How much potassium chloride is produced}

from the reaction of 2.00 g potassium and

3.00 g chlorine gas? Which is the limiting

reagent?

Mg

N

+6H

O



3Mg(OH)

+2NH

•

_{How many moles of Mg(OH)}

2

would be produced

from the reaction of 0.10 mole of Mg

N

?

•

_{How many moles of NH}

3

would be produced from

the reaction of 500 g of Mg

N

?

•

_{How many molecules of water would be required}

to react with 3.64 g of Mg

N

?

•

_{What is the maximum number of grams of}

Mg(OH)

that can be produced by the reaction of

10.0 g of Mg

N

and 14.4 g of H

O?

•

_{What is the percent yield if 9.4 g of Mg(OH)}

2

are