Chemical Quantities:Stoichiometry

Mole Ratios

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

Stoichiometry Problems

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?

Summary

Mass of Given

Moles of Given

Moles of Unknown

Mass of Unknown

use molar mass

use mole ratio

use molar mass