1. Chemical Quantities & Stoichiometry
The Mathematics of Chemistry

2. Percent Composition of a Compound
% Composition = Percentage of Mass of each element present in the compound
Law of Constant Composition = The percent composition of a compound is always the same. (H20 is water, H2O2 is not)
Are these the same compound?
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3. Percent Composition of a Compound
If mass data from lab is known:
% Composition=
mass of element
mass of compound
× 100%
Example: When a g sample of a compound containing only magnesium and oxygen is decomposed, 5.40 g of oxygen is obtained. What is the percent composition of this compound?
% O =
mass of Oxygen
mass of compound
× 100%
= 39.7.% O =
5.40 g
13.60 g

4. Percent Composition of a Compound
If no mass data, but formula is known: - Pretend you have one mole of the compound
% Composition
mass of element in 1 mol compound
molar mass of compound
× 100% =
Example: Propane (C3H8), is obtained from petroleum. Calculate the percent composition of propane.
mass of C in 1 mol C3H8 = 3 mol × 12.0 g/mol = 36.0 g mass of H in 1 mol C3H8 = 8 mol × 1.0 g/mol = 8.0 g molar mass of C3H8 = 36.0 g/mol g/mol = 44.0 g/mol
36.0 g
% C =
× 100%
= 81.8% C
44.0 g

5. Empirical Formula ratio of moles of each element in a compound
A substance with 1 mole of N for every 2 moles of O =
Empirical Formula is the lowest whole number ratio
A substance with 1 mole of N for every 2.5 moles of O =
If ratio is not in whole numbers, multiply each element by a factor until you get all whole number
NO2
N1.0O2.5
= N2O5

6. Empirical Formulas For carbon dioxide, the empirical and molecular
are the
same—
CO2.
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7. Empirical Formulas
Two compounds can have the same empirical formula, yet be different compounds
Ethyne (C2H2), also called acetylene, is a gas used in welders’ torches.
Styrene (C8H8) is used in making polystyrene.
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8. Molecular Formulas
Methanal (formaldehyde), ethanoic acid (acetic acid), and glucose have the same empirical formula—CH2O.
This means they will also have the same % composition.
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9. Comparison of Empirical and Molecular Formulas
Change this chart so it shows molecular formula, empirical formula, % composition, & molar mass
Interpret Data
Notice that the molar masses of the compounds in these two groups are simple whole-number multiples of the molar masses of the empirical formulas, CH and CH2O.
Comparison of Empirical and Molecular Formulas
Formula (name)
Classification of formula
Molar mass (g/mol) CH
Empirical 13
C2H2 (ethyne)
Molecular
26 (2 × 13)
C6H6 (benzene)
78 (6 × 13)
CH2O (methanal)
Empirical and molecular 30
C2H4O2 (ethanoic acid)
60 (2 × 30)
C6H12O6 (glucose)
180 (6 × 30)
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10. Determining a Molecular Formula
Need example problem here…
To flow chart

11. Formula Determination Flow Chart
Divide each mole amount by smallest # of moles
Moles of
Each Element
Mole
Ratio
1 mole = molar mass of element
multiply ratio to get all integers
Mass of
Each Element
Empirical
Formula
Mass of element Mass of Cmpd
% =
x 100
Use a sample size of 100 grams
multiply emp. formula by factor
Factor =
Molar Mass of Compound
Molar Mass of Empirical Formula
% Mass
Composition
Molecular
Formula

12. Warm-Up: All 3 balloons are the same volume and at STP
CH4 N2
How do the number of moles in each balloon compare?
How do the number of particles in each balloon compare?
How does the mass of the gas in each balloon compare?
How do the speed of the molecules compare?
Which would be the most fun to light with a match?

13. Determining Volume of a Gas
If at STP: 1 mole of any gas = ______ L
If not at STP, the volume of a gas depends on what three things?
# of particles (moles)
Temperature
Pressure
Ideal Gas Law
22.4
PV = nRT

14. Volume of a Gas: Ideal Gas Law
PV = nRT
Kelvin
Temperature must be in ________
n = number of moles
R is called the Gas Constant
= atm•L / mol•K
= kPa•L / mol•K
= mmHg•L / mol•K
Notice all Volume is in Liters with these constants

15. Volume of a Gas: Ideal Gas Law
PV = nRT
Example A: moles of hydrogen gas will occupy what volume at 18ºC and 772 mmHg?

16. Volume of a Gas: Ideal Gas Law
PV = nRT
Example B: What is the mass of carbon dioxide in a 892 mL balloon at 15.0 ºC and a pressure of kPa?

17. The “Wholly Moley” Wheel
Molarity
Volume of Gas
PV=nRT
M = mol / Liter
Non- STP
Moles
At STP
1 mole = 22.4 L
Mass
# of Items

18. Synonyms: (what do they mean
Synonyms: (what do they mean?) Molar Mass (MM) Gram-Molecular Mass Molecular Weight Formula Weight
Molar Mass = mass
mole

19. Molar Mass Example 1 If 0. 34 moles of a compound has a mass of 23
Molar Mass Example 1 If 0.34 moles of a compound has a mass of grams, what is its molecular weight?

20. Molar Mass Example #2: A 30.6 g sample of gas occupies L at STP. What is the molar mass of this gas?

21. Molar Mass Example #3: A 2. 5 g sample of gas occupies 16. 42 L at 0
Molar Mass Example #3: A 2.5 g sample of gas occupies L at 0.43 atm and 18.2 °C What is the molar mass of this gas?

22. Non-Ideal Gas Behavior

23. Molar Mass Example #4: An unknown 6 carbon hydrocarbon gas is measured to have a density of 3.13 g/L at 27ºC and 750mmHg. Would the gas most likely be benzene or hexane?

24. The following items are placed inside of an airtight 0
The following items are placed inside of an airtight 0.53 m3 box on a day when the temp was 25˚C and the air pressure was inHg.
A 4.4L basketball at 2.4 atm
a 0.98L bicycle tire at 45psi
and a 1.7L football at 1280 mmHg
The box was placed in direct sunlight where it heated up to an inside temp of 85˚C and all 3 items exploded. What is the pressure inside the box after the explosion?

25. The Stoichiometry Road Map
Molarity
-or- Volume of Solution A
Molarity
-or- Volume of Solution B
Volume of Gas A
Volume of Gas B
Non- STP
Non- STP
Coefficients
Mole Ratio from Equation
Moles A
Moles B
At STP
At STP
Mass A
# of Items A
# of Items B
Mass B