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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