1. Chapter 3 OWL Deadline 27-September
All material through Chapter 3 is fair game for Exam #1 on 24-September.
Lab the week of 10-September: Synthesis of Alum
Stoichiometry and Equations
The Mole
Stoichiometry and Compounds
Chemical Reactions
Limiting Reactants
% and Theoretical Yield

2. What is a mole? A standard unit of quantity.
The number of atoms present in exactly 12 grams of 12C.
Avogadro’s number is the constant for 1 mole
NA = 6.02 x 1023

3. Stoichiometry & Compounds
In terms of compounds, stoichiometry refers to the ratio of elements within a molecule or substance (including polyatomic ions and ionic compounds)
Any unique compound will have its formula
The formula will have a specific ratio of atoms
These ratios can be expressed in weight (grams for example) or quantity (moles usually)
H2O
C3H8
PO4-3
H2C2O42 H2O
Na2CO3

4. Empirical Formula Mass
Formula Mass (weight)
The sum of the atomic masses of all atoms in a formula unit of the compound, whether molecular or not.
Empirical Formula Mass
Molecular Formula Mass (another term for molecular mass)
The mass of some other unit containing multiple atoms
Molar Mass (weight)
The sum of the masses of all of the atoms or molecules in a mole of a compound, ordinarily expressed in grams/mole.
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Ca(OH)2
1 Ca 1(40.08) = amu
2 O 2(16.00) = amu
2 H 2(1.008) = amu
3 significant figures
74.1 amu
Total
Calculate the formula weight of the following compound from its formula. Report your answers to three significant figures.
Calcium hydroxide, Ca(OH)2
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What is the mass in amu of the nitrous acid molecule, HNO2?
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7. A sample of nitric acid, HNO3, contains 0. 253 mol HNO3
A sample of nitric acid, HNO3, contains mol HNO3. How many grams is this?
First, find the molar mass of HNO3:
1 H 1(1.008) = 1.008
1 N 1(14.01) = 14.01
3 O 3(16.00) = 48.00
63.018
(2 decimal places)
63.02 g/mol
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Next, using the molar mass, find the mass of mole:
= g
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9. Remember, moles (mol) are really used for counting and deriving data from those counts.
How many grams does 0.45 mol of sulfur trioxide weigh?

10. Percent Composition
Often used in chemical analysis and identifying empirical or molecular formulas
A weight ratio of the elements in a compound
Write the name of, identify the stoichiometry involved in, and determine the % composition of each element in
MgCO3

11. Empirical/Molecular Formulas and % Composition
Chemists may discover a compound and, if it is new to them, have no idea what the chemical formulas are
Empirical
Molecular
You can use a step-by-step approach to determine the Empirical and Molecular formulas even if you have limited data.
Determine % composition (wt %) of each element.
Calculate the mole ratio for each element.
Round up to whole numbers
Use the molecular weight and the empirical formula weight to calculate the molecular formula
We estimate, pretty closely, the molecular weight from techniques such as mass spectrometry

12. You’ve discovered what you think is a new compound:
2.00 grams of it were found
Chemical analysis shows that it has Carbon and Hydrogen
What kind of compound is it?
Chemical analysis shows that of the sample that you have, grams of Hydrogen.
Mass spectrometry shows that the molecular weight is grams/mole
What compound is it?

13. Figure 3.6: Combustion method for determining the percentages of carbon and hydrogen in a compound

14. Calculate % Composition Calculate Stoichiometry (Mole Ratios)
Empirical Formula
Calculate Ratio of Molecular Weight to Empirical Formula Weight
Use ratio from #3 to decipher molecular formula from empirical formula

16. Stoichiometry and Reactions
Chemical reactions take place with defined ratios of reactants and products
These defined ratios are based on the stoichiometry of the compounds or atoms in the reaction, and the stoichiometry related to the reaction itself.
Stoichiometric coefficients

17. Let’s identify
Products (and name them)
Reactants (and name them)
Stoichiometry of compounds in this reaction
Stoichiometric coefficients
Phases
Reaction Direction (arrow)
What important law is adhered to by this reaction (and all the reactions we will study in CHEM 111)

18. BaCl2 (aq) + Na2SO4(aq)  BaSO4(s) + 2NaCl(aq)
Let’s identify
Products (and name them)
Reactants (and name them)
Stoichiometry of compounds in this reaction
Stoichiometric coefficients
Phases
Reaction Direction (arrow)
What important law is adhered to by this reaction (and all the reactions we will study in CHEM 111)

19. Balancing Equations Remember, mass MUST be conserved!
All of the atoms of reactants must appear, in the same mole quantities, in the products
This can be extrapolated to weights later on with reaction stoichiometry

20. Practice Balancing Equations
Methane gas reacts with oxygen gas to produce carbon dioxide gas and liquid water.
Write the reaction
Balance elements that appear in only one compound first
Remember that compound stoichiometry (subscripts) can not change
Balance elements that appear in other compounds second
Round coefficients up to the lowest whole numbers
Check answer

21. Aqueous acetic acid reacts with aqueous sodium hydroxide to produce aqueous sodium acetate and water. This reaction is in “equilibrium”.

22. Reaction Stoichiometry
Once we have a balanced reaction, we can evaluate how much of one substance can form (or cause to be formed) another substance.
Reaction stoichiometry is almost ALWAYS studied using moles (stoichiometric coefficients and subscripts)

23. Propane is combusted in a cooking grill using atmospheric oxygen
Propane is combusted in a cooking grill using atmospheric oxygen. The products are carbon dioxide and water. How much water (mol and grams) will form if 10 grams of propane are consumed?

24. In the Haber Process, 2. 00 grams of hydrogen are consumed
In the Haber Process, 2.00 grams of hydrogen are consumed. How much nitrogen is consumed in with it?

25. Limiting Reactants

26. Often, we have reactants where:
There is an excess of one (compared to the others)
There is a limiting amount of another (compared to the others.
The Limiting reactant controls how much product can be produced
If in the reaction above, we have 4 moles of butane and 20 moles of oxygen (O2), which is limiting and which is excess?

27. Molecular view of 2H2 + O2  2H2O reaction with H2 as the limiting reactant
If we have one gram of hydrogen gas (H2), how many moles of O2 will be consumed and
how many grams of water will be formed?

28. Theoretical and % Yield
Rarely do reactions produce 100% of the product they should
Impurities
Some reactants/products produce other things
Spillage
The theoretical yield is what you would get, ideally.
Calculated from the balanced equation and limiting reactant
The % yield (percent yield) is what you actually produce, relative to the theoretical yield

29. If we have 1. 00 grams of hydrogen gas (H2), and 2
If we have 1.00 grams of hydrogen gas (H2), and 2.00 grams of O2 reacting to form hydrogen peroxide (H2O2 (g)), answer the following: -Write the balanced chemical reaction -Identify the limiting reactant -Calculate the theoretical yield -Calculate the percent yield if 1.5 grams of hydrogen peroxide is formed.