 # The Mole: A measurement of Matter

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The Mole: A measurement of Matter
Chapter 10 Chemical Quantities Section 10.1 The Mole: A measurement of Matter

What is a Mole? 6.02 X 1023 is called Avogadro’s number
A mole (mol) of a substance is 6.02 X 1023 representative particles of that substance. Usually atoms or molecules. 6.02 X 1023 is called Avogadro’s number

Converting Number of Particles to Moles
Av. number Molecules or atoms e.g. How many moles of magnesium in 1.25 X atoms of magnesium = mol Mg Page 291: #3 & 4 Moles - Particles

Converting Moles to Number of Particles
e.g. How many atoms are in 2.12 mol of propane (C3H8)? Number of atoms = 2.12 x 6.02 x x 11 = 1.4 x atoms Page: 292 #5 & 6

The mass of a Mole of an Element
The mass of a mole of an element is its molar mass. How can you calculate the molar mass of a compound? To calculate the molar mass of a compound: 1. Find the number of grams of each element. 2. Add the masses of the elements in the compound. e.g. what is the molar mass of calcium oxide? Chemical formula = CaO, Ca = 40, O = 16 Molar mass = = 56g Page: 296, #7 & 8

Moles – Atoms Number of Moles Avogadro’s Number If atoms Atoms or
Molecules Particles Number of Moles Avogadro’s Number If atoms Multiplies by number of atoms

Section 10.2 Mole – Mass & Mole – Volume Relationships

Converting Mass to Moles
e.g. How many moles of iron(III) oxide are contained in 92.2g of pure Fe2O3? Mass = 92.2g Molar mass = 2x x16 = 159.6g/mol Number of moles = 92.2/159.6 = mol Fe2O3 Page: 299, # 18 & 19 Moles - Mass

Converting Moles to Mass
Mass = number of Moles x molar mass e.g. What is the mass of 9.45 mol of aluminum oxide? Chemical Formula = Al2O3 Number of moles = 9.45 mol Molar mass = 2x27 + 3x16 = 102g Al2O3 Mass = x 102 =964g Al2O3 Page: 298, # 16 & 17

The volume of a Mole of an Element
The volume of a mole of an element is called molar volume The molar volume of a gas at STP is 22.4 L Standard Temperature and Pressure To calculate the volume of a gas at STP: Volume of gas = Number of Moles x 22.4 e.g. Determine the volume of 0.6 mol sulfur dioxide gas at STP. Volume of SO2 = 0.6 x 22.4 = 13.4 L SO2 Page: 301, #20 & 21 Moles - Volume

Calculating Molar Mass from Density
Molar mass = density at STP x molar volume at STP e.g. The density of a gaseous compound carbon and oxygen is found to be 1.964g/L at STP. What is the molar mass of the compound? Molar mass = x 22.4 = 44g/mol Page: 302, #22 & 23 Moles - Density

Moles – Mass Mass Number of Moles Molar

Moles – Volume Volume Number of Moles 22.4

Moles – Density Molar mass Density 22.4

Section 10.3 Percent Composition and Chemical Formulas

The percent composition is the relative amounts of the elements in a compound.
The percent by mass of an element in a compound is the number of grams of the element divided by the mass in grams of the compound, multiplied by 100 Calculating Percent Composition from Mass Data e.g. When a 13.6 g sample of a compound containing only magnesium and oxygen. 5.4 g of oxygen is obtained. What is the percent composition of this compound? Mass of compound = 13.6 g Mass of oxygen = 5.4 g Mass of magnesium = 13.6 – 5.4 = 8.2 g Page 306, # 32 & 33

Calculating Percent Composition from the Chemical Formula
e.g. Propane (C3H8) is one of the compounds obtained from petroleum. Calculate the percent composition of propane. Mass of C in C3H8 = 36 g Mass of H in C3H8 = 8.08 g Molar mass of C3H8 = g/mol Page: 307, # 34 & 35

Empirical Formula The empirical formula of a compound shows the smallest whole-number ratio of the atoms in the compound. It shows the kinds and lowest relative count of atoms or moles of atoms in molecules or a compound. An empirical formula may or may not be the same as a molecular formula. For example, the lowest ratio of hydrogen to oxygen in hydrogen peroxide is 1:1 So the empirical formula of hydrogen peroxide is HO The actual molecular formula of hydrogen peroxide has twice the number of atoms as the empirical formula. The molecular formula is (HO) x 2 or H2O2 Notice that the ratio of hydrogen to oxygen is still the same. The molecular formula tells the actual number of each kind of atom present in a molecule of the compound. For carbon dioxide, the empirical and molecular formulas are the same – CO2

e.g. A compound is analyzed and found to contain 25.9% nitrogen and
74.1% oxygen. What is the empirical formula of the compound? 25.9% N % O Step 1. Assume that you have 100 g of the compound, 25.9 g g Step 2. Divide each by its atomic mass. 1.85 mol 4.63 mol Step 3. Divide each by the smallest number of moles. 1.85 4.63 1.85 1.85 1 2.5 Multiply all by 2 The empirical formula is N2O5 Page: 310, # 36 & 37

Molecular Formula is the same as the empirical formula or it is a simple whole-number multiple of its empirical Once you determined the empirical formula of a compound, you can determine its molecular formula, but you must know the compound’s molar mass and empirical formula mass. e.g. Calculate the molecular formula of a compound whose molar mass is 60 g/mol and empirical formula is CH4N Empirical formula = CH4N Molar mass = 60 g/mol Molecular formula = C?H?N? Step 1. Calculate the empirical formula mass Empirical formula mass = 1x12 + 4x x14 = g/mol Step 2. Divide the molar mass by the empirical formula mass 60 = 2 30.04 Step 3. Multiply the formula subscripts by this value (factor) Molecular formula = 2 x CH4N = C2H8N2 Page: 312, # 38 & 39 Table page: 311