Things you must KNOW and what to expect Things you must KNOW

You must KNOW your polyatomics

You must KNOW how to write a balanced formula

You have to be able to balance a chemical equation

You need to know how to convert from grams to moles and moles to grams.

You need to be able to calculate molar mass

What to expect in this chapter You will be converting grams to moles of one

compound, then using the mole ratio (balanced equation) go to moles of another compound and then from moles to grams in the new compound.

You will learn how to identify a limiting reactant, and how to calculate percent yield.

What is Stoichiometry?

Stoichiometry: the study of quantitative relationships between amounts of reactants used and products formed by a chemical reaction.

Based on the Law of Conservation of Mass

(mass reactants = mass products)

To prove Law of Conservation of Mass

1. Start with a balanced equation

2. Coefficient indicates the #molecules/formula units and # of moles

3. Change moles to mass

4. Add up mass reactants and mass of products

They should equal

C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g)

What is Stoichiometry? : continued

Can use balanced chemical equation to get mole ratios

2 Al + 3 Br2 2 AlBr3

2 mol Al 2 mol Al 3 mol Br2

3 mol Br2 2 mol AlBr3 2 mol AlBr3

Find all the possible mole ratios for:

KClO3(s) KCl(s) + O2(g)

Stoichiometric Calculations

Use relationships to determine the amount of product formed or reactant needed. Moles to moles

Moles known x mol unknown

mol known

from problem

ratio from balanced

equation

=mol unknown

Moles MolesHow many moles of carbon

dioxide are produced when 10.0 moles propane (C3H8) are burned in excess oxygen?

1. Write & balance the equationC3H8 + 5 O2 3 CO2 + 4 H2O

2. Find the mole ratio between propane and carbon dioxide: 1 mol C3H8 : 3 mol CO2

3. Multiply ratio by known amount from problem

10 mol C3H8 x 3 mol CO2 = 30 mol O2

1 mol C3H8

Moles Mass

Moles to massDetermine the mass of sodium needed when sodium reacts with 1.25 moles of chlorine gas to form sodium chloride

1. Write & balance the equation2 Na + Cl2 2 NaCl

2. Find the mole ratio between propane and carbon dioxide: 2 mol Na: 1 mol Cl2

3. Multiply ratio by known amount from problem

1.25 mol Cl2 x 2 mol Na = 2.50 mol Na 1 mol Cl2

4. Change moles to grams

2.50 mol Na x 22.99 g Na = 57.48 g Na 1 mol Na

Mass Mass

Mass to mass

25.0 g NH4NO3 produces N2O and H2O when it decomposes. Find the mass of water produced.

1. Start with a balanced equationNH4NO3 N2O + 2 H2O

2. Change grams to moles25.0g NH4NO3 x 1 mol NH4NO3 = 0.312 mol NH4NO3

80.04g NH4NO3

3. Find the mole ratio : 1 mol NH4NO3 : 2 mol H2O

4. Multiply ratio by known amount from problem

0.312 mol NH4NO3 x 2 mol H2O = 0.624 mol H2O

1 mol NH4NO3

5. Change moles to grams

0.624 mol H2O x 18.02g H2O = 11.2 g H2O 1 mol H2O

Limiting Reactants

Limiting reactant: the reactant that runs out first

Excess reactant: the reactant that is left over

To solve limiting reactant problems:

1. Work the problem using both reactants and solve the number of moles produced of the same product The one that produces the least amount of moles is the

limiting reactant

Percent Yield

Theoretical yield is the maximum amount of product that can be produced (determined through stoichiometric calculations)

Actual yield is the amount produced in lab/from experiment Percent yield is the ratio of actual to theoretical expressed

as a percent.% yield = actual yield x 100

theoretical yield

Reasons for a decrease in percent yield Loss of products or reactants (spill, stick to side of containers) Human error Not all reactants react

Percent Yield continued

CH4 + 2 O2 CO2 + 2 H2O

Determine the % yield if 10.0 grams methane burned to produce 19.5 g water.

1. Use stoichiometric calculations to determine theoretical yield of water produced from 10.0 g methane.

2. Using 19.5 g H2O as the actual yield, solve for % yield.