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WednesdayJanuary 23, 2013

(What is Stoichiometry?; Mole Ratios)

Bell RingerWednesday, 1-23-13

- Sodium metal reacts with chlorine gas to produce sodium chloride.
- Write the balanced chemical equation.

- Na + Cl2æNaCl

- 2

- 2

- What do the blue balance numbers really mean?

- The blue balance numbers represent either the number of particles of each substance or the number of moles of each substance.

Announcements

Happy National Pie Day!

Announcements

I will not be available after school today.

What is Stoichiometry?

Stoichiometry involves the quantity relationshipsbetween reactants and products in a chemical reaction.

Stoichiometry is used when you have a problem where you are given the quantity of one substance and you need to determine the quantity of another substance.

Stoichiometry can be used to determine how much of a substance to start with as a reactant, or how much of a substance will be produced in a reaction.

What is Stoichiometry?

All reaction stoichiometry calculations start with a balanced chemical equation.

The coefficients (balance numbers) of the equation represent the relative numbers of moles of reactants and of products.

The Dual Meanings of Coefficients

It is extremely important to recognize what the coefficients of a balanced chemical equation represent.

These coefficients can be interpreted to represent two different things:

- the number of particles of each substance in the reaction.

or

- the number of moles of each substance in the reaction.

For example, consider the balanced equation below:

H2 + O2→ H2O

This can be interpreted as 2molecules of hydrogen gas react with 1molecule of oxygen gas to produce 2molecules of water.

or

2moles of hydrogen gas react with 1mole of oxygen gas to produce 2moles of water.

2

2

Mole Ratios

A mole ratio is a conversion factor that relates the amounts in moles of any two substances involved in a chemical reaction.

Mole ratios use the coefficients from a balanced chemical equation.

Example: aluminum oxide decompose to produce aluminum and oxygen gas. Write and balance the equation.

Al2O3→ Al + O2

This means that 2 moles of aluminum oxide decompose to produce 4 moles of aluminum and 3 moles of oxygen gas.

The mole ratio of Al2O3 to Al is 2:4, or 1:2

The mole ratio of Al2O3 to O2 is 2:3

The mole ratio of Alto O2 is 4:3

3

2

2

4

Mole Ratios

Sample Problem

What are the mole ratios when cadmium (II) metal reacts with hydrobromic acid to produce cadmium (II) bromide and hydrogen gas.

First, Write and balance the equation.

Cd+ HBræ CdBr2+ H2

This means that 1 moleof cadmium reacts with 2 molesof hydrobromic acid to produce 1 moleof cadmium (II) bromide and 1 moleof hydrogen gas.

What is the mole ratio of Cdto H2?1:1

What is the mole ratio of HBrto CdBr2? 2:1

What is the mole ratio of CdBr2 to H2?1:1

2

Worksheet

Introduction to Stoichiometry and Mole Ratios

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