The Mole & Chemical Formulas

The Mole Mole, abbreviated mol, is the SI (metric) base unit to measure the amount of a substance It is called Avogadro’s number in honor of Italian physicist Amedeo Avogadro, who discovered one mole of gas in 1811 6.0221367 x 1023 representative particles 602,000,000,000,000,000,000,000

Use of the Mole One wouldn’t use this unit for everything A mole of marbles would cover the surface of the Earth down to approximately 6 kilometers Although a mole of $1.00 bills would be nice! It is used to measure the amount of molecules in a substance

Conversion Recall 5,280 feet = 1 mile How many miles would you have in 12,369 feet? It is the same system of proportions except now its… 1 mole = 6.02x1023 So, if you had 2.5 moles of H2O, how many atoms of water do you have?

Recall Determine the number of atoms in 2.75 moles of Zn? If you had 9 moles of Cu(OH), how many molecules are present? Lets change it up! How many moles of H2SO4 if you had 2.11x1024 molecules?

Mass & The Moles 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)

Periodic Table Recall The atomic masses on the periodic table are not whole numbers They are weighted averages of the masses of all naturally occurring isotopes of each element Knowing this and using this new idea… 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)!

So What Does This Mean? Periodic Table Transitions** You Try! Carbon has a mass of 12 Thus 12 g of carbon = 1 mole of Carbon Manganese (Mn) has an atomic mass of 54.94 amu 55 g of Mn = 1 mol Mn You Try! 3 mol Mn = ? g Mn 136 g Pb = ? Mol Pb Challenge* How many grams of gold are found in 7.65x1022 Au atoms? This works for compounds as well as single elements 1.00 g H2O = ? Mol H2O 1.00 g H2O = ? Molecules H2O

Percent Composition Percent Composition: The percentage by mass of each element in a compound 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%

Percent Composition 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 * 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 Try 8.00 g of Cu(OH)

Empirical Formula After you have identified the compound and its percent composition, you can find the formula of the compound 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 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!

Empirical Formula 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

Practice Problems Ni: 34.4%, S: 37.5%, O: 28.1% C: 69.7%, H: 14.0%, N: 16.3% C: 60.6%, H: 8.14%, P: 31.3%

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 It is possible because they have different molecular formulas This specifies the actual number of atoms of each element in one molecule or formula unit of the substance

Molecular Formula 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 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