St. Johns County School District
The Mole & Chemical Formulas
KNOW HOW TO DO METRIC TO METRIC CONVERSIONS, DIMENSIONAL ANALYSIS AND SCIENTIFIC NOTATION!
The Mole
• Mole, abbreviated mol, is the SI (metric) base unit to measure the amount of a substance
o Called Avogadro’s number in honor of Italian physicist Amedeo Avogadro, who discovered one mole of gas in 1811
o 6.0221367 x 1023 representative particles: 602,000,000,000,000,000,000,000
• One wouldn’t use this unit for everything
o A mole of marbles would cover the surface of the Earth down to approximately 6 kilometers
o It is used to measure the amount of molecules in a substance
o A dozen oranges won’t have the same mass a dozen watermelons
▪ But you do have exactly the same amount of each: 12
▪ Since they have different compositions, their mass will be different as well
▪ Thus, the mass of 1 mole of silver nitrate (AgNO3) will be different than 1 mole of oxygen gas (O2)
• It is the same system of proportions except now it’s…
o 1 mole = 6.02 x 1023
o So, if you had 2.5 moles of H2O, how many molecules of water do you have? (1.51∙1023 molecules)
Periodic Table Recall
• The atomic masses on the periodic table are not whole numbers but are weighted averages of the all naturally isotopes
o Knowing this and using this new idea…
o Carbon – 12 is the standard atom used to determine the number of representative particles per mole
▪ We can calculate the molar mass or mass in grams of one mole of any pure substance (g/mol)!
• Periodic Table Transitions**
o Carbon has a mass of 12
o Thus 12 g of carbon = 1 mole of Carbon
• Manganese (Mn) has an atomic mass of 54.94 amu
o 55 g of Mn = 1 mol Mn
o 3 mol Mn = ? g Mn
o 136 g Pb = ? Mol Pb
• This works for compounds as well as single elements
o 1.00 g H2O = ? Mol H2O
o 1.00 g H2O = ? Molecules H2O
• Freon – CCl2F2
o This is a molecule whose formula is known
o In each molecule, there is 1 atom of Carbon, 2 atoms of Chlorine and 2 atoms of Fluorine
o If one were asked, how many moles of F are there in 5.50 moles of Freon, what would you do?
5.50 mol Freon * 2 mol F atoms = 11 mol F atoms
1 1 mol Freon
Percent Composition
• Percent Composition: The percentage by mass of each element in a compound
o It is determined by gravimetric and volumetric analyses for solids and liquids, respectively
Example: 100 g of an unknown substance has two elements: X & Y. X has 55 g and y has 45 g of the total 100.
(55 g of element X / 100 g cmpd.)*100% = 55%
(45 g of element Y / 100 g cmpd.)*100% = 45%
• If you know the compound in question, you can calculate the percent composition for the known atoms
Example:
18.02 g/mol H2O: Hydrogen (H) – 1.00794 g Oxygen (O) – 15.9994 g
1.00794 g H * 2 Hydrogen in water = 2.02 g
(2.02 g H / 18.02 g H2O) * 100 = 11.2 % H
16.00 g O * 1 Oxygen in water = 16.00 g
(16.00 g O / 18.02 g H2O) * 100 = 88.8 % O
So, water is 11.2% Hydrogen and 88.8% Oxygen
Empirical Formula
• After you have identified the compound and its percent composition, you can find the formula of the compound
o First, you must find the smallest whole number ratio of the moles of the elements in the compound
▪ It provides the subscripts for the chemical formula
o Recall:
▪ Empirical formula is the lowest whole number ratio of the elements
• CH2O … C2H4O2 … C3H6O3 … C4H8O4 … C5H10O5 … etc.
MOST IMPORTANT FACT
* Empirical formula DOES NOT have to match actual molecular formula
Example: HO is the empirical formula for hydrogen peroxide, H2O2, but HO is not hydrogen peroxide!
Example:
You can assume the mass of the compound is 100 g if they give you percent composition.
So if 40.05 % is Sulfur (S) and 59.95% is Oxygen (O), then there is 40.05 g S and 59.95 g O
Then simply convert to moles:
40.05 g S * 1 mol S = 1.249 mol S
1 32.07 g S
59.95 g O * 1 mol O = 3.747 mol O
1 16 g O
So the molar ratio is 1.249:3.747 for S:O atoms
This will produce integers but we want whole numbers
How do we simplify?
Take the smaller number and divide it and everything else by that number.
1.249/1.249 = 1 3.747/1.249 = 3
1:3 Ratio so for every one S, there is three O.
SO3
Molecular Formula
• Knowing what you know about building compounds, percent composition and empirical formulas… realize that hundreds of different substance have the same percent composition and empirical formula
o It is possible because they have different molecular formulas
o This specifics the actual number of atoms of each element in one molecule
• To determine the molecular formula, the molar mass of the compound must be determined through experimentation and compared with mass represented by the empirical formula
Example: Acetone has a molar mass of 26.04 g/mol while its empirical formula is CH or 13.02 g/mol
• Since the mass of the molecule is 26.04 g/mol, divide it by the empirical mass of 13.02 g/mol
o 26.04 g/mol / 13.02 g/mol = 2.00
▪ Thus the molecular formula has to have twice as many carbon and hydrogen atoms than the empirical formula, or C2H2
(Empirical Formula)*n = Molecular Formula
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