 # Ch. 11 The Mole.

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Ch. 11 The Mole

11.1 Measuring Matter Mole- SI base unit used to measure the amount of a substance Equal to the number of representative particles (carbon atoms) in exactly 12 grams of carbon-12

Representative particle :
Elements –atom Covalent –molecules Ionic - formula units

Representative particle
Elements Atoms Molecules Compounds Ionic Compound Formula Unit Covalent Compound

Is a very large number because it is used to count extremely small particles.

Conversion of a Mole to a Particle
1 mole = 6.02 x 1023 # of moles x 6.02 x 1023 representative particles = # of representative particles

Ex: How many molecules are in 3.5 moles of sucrose?
3.5 moles sucrose x 6.02 x 1023 molecules = mole sucrose = 2.11 x 1024 molecules of sucrose

Conversion of a Particle to a Mole
Reverse conversion factor to solve for # of moles Ex: How many moles are in 4.50 x 1024 atoms of zinc? 4.50 x 1024 atoms Zn x 1 mol Zn 6.02 x 1023 atoms Zn = 7.48 mol Zn

STOP! YOUR TURN! Practice Problems 11.1

11.2 Mass and the Mole Just as a dozen bricks and a dozen feathers don’t have the same mass, moles of different substances also have different masses.

Molar mass- mass in grams of one mole of any pure substance
The molar mass of any element is numerically equal to its atomic mass and has the units g/mol

Using Molar Mass Ex: What is the mass in grams of moles of chromium? Moles Cr x grams Cr = grams Cr 1 mole Cr mol Cr x g Cr = 2.34 g Cr 1 mol Cr

Mols Mass Particles (atoms, molecules, formula unit)

Converting Mass to Atoms and Atoms to Mass
Ex: How many atoms of gold are in a pure nugget having a mass of 25.0 g? Known: Mass = 25.0 g Au Molar mass Au = g/mol Au Unknown: Number of atoms = ? Atoms Au

Mass Au x 1 mole Au = moles Au
# g Au 25.0 g Au x mol Au = ? mol Au g Au = mol Au Half way there!!

Moles Au x 6.02 x 1023 atoms Au = 1 mole Au 0.127 mol Au x 6.02 x 1023 atoms Au 1 mol Au = 7.65 x 1022 atoms Au Mass must always be converted to moles before being converted to atoms, and atoms must be converted moles before calculating their mass.

11.3 Moles of Compounds Chemical formula indicates types of atoms and number of each in one unit of the compound

Ex: CCl2F2 Carbon = one atom Chlorine = 2 atoms Fluorine = 2 atoms Ratio of carbon to chlorine to fluorine is 1 : 2 : 2

Conversions with Chemical Formulas
How many moles of fluorine atoms are in 5.50 moles of freon (CCl2F2)? 5.50 mol CCl2F2 x 2 mol F atoms = 11.0 mol F atoms mol CCl2F2

Molar Mass of Compounds
Mass of a mole of a compound equals the sum of the masses of every particle that makes up the compound. Suppose you want to determine the molar mass of potassium chromate (K2CrO4)

# moles x molar mass = # grams
2.000 mol K x g K = g K 1 mol K 1.000 mol Cr x g Cr = g Cr 1 mol Cr 4.000 mol O x g O = g O 1 mol O

Molar mass = g K 52.00 g Cr g O g K2CrO4

Converting Moles of a Compound to Mass
Step 1: Calculate molar mass of the compound. Step 2: Convert moles to grams using the molar mass as a conversion factor.

Converting Moles to Mass
Convert 2.50 mol CHCl3 to mass in grams. Step one: Calculate the number of grams in one mole of CHCl3 = grams Step up problem as before What’s given What you want What you want to get rid of 2.5 mol CHCl grams of CHCl3 1 mol of CHCl3 = grams of CHCl3

Converting Mass of a Compound to Moles
Use inverse of mole to mass conversion factor

Converting Mass of a Compound to Number of Particles
Step 1: Convert given mass to moles by using the molar mass as a conversion factor. Step 2: Convert moles to number of representative particles by multiplying by Avogadro’s number.

11.4 Empirical and Molecular Formulas
Percent Composition- percent by mass of each element in a compound % composition = mass of element x 100 mass of compound

Percent Composition from the Chemical Formula
Percent composition is always the same, regardless of the size of the sample To determine percent composition, assume a sample size of one mole

What is the percent composition of water?
Percent Hydrogen: 2.02 g H x 100 = 11.2% H 18.02 g H2O Percent Oxygen: 16.00 g O x 100 = 88.80% O Think about it as a cookie with parts, calculate its percentage

STOP! YOUR TURN! Page 331, #45

Empirical Formula Empirical Formula- formula with the smallest whole number mole ratio of the elements Which of these do you think is the empirical formula? HO or H2O2 ?

Empirical formula may be different from molecular formula
Ex: hydrogen peroxide Empirical formula: HO Molecular formula: H2O2

Finding the Empirical Formula
If percent composition is given, assume a 100 g sample, so change % to grams

Ex: percent composition is 40.05% S and 59.95% O
100.0 g of the sample is g S and g O

Convert mass of each element to number of moles
40.05 g S x 1 mol S = mol S 32.07 g S 59.95 g O x 1 mol O = mol O 16.00 g O

Ratio of S atoms to O atoms is
1.249 : 3.747 Must convert to whole numbers Since is smallest, divide both numbers by that value

Continued… 1.249 mol S = 1 mol S 1.249 3.747 mol O = 3 mol O
Ratio of S atoms to O atoms is 1 : 3 Empirical Formula = SO3

STOP! YOUR TURN! # 46 and 47 page 333

Molecular Formula Molecular Formula- specifies the actual number of atoms of each element in one molecule or formula unit of the substance

Molecular formula = (empirical formula) n
n is the factor by which the subscripts in the empirical formula must be multiplied to obtain molecular formula

Determining Molecular Formula
Empirical formula: C2H3O2 Molar Mass C2H3O2 = g C2H3O2 (calculated value) Molar Mass succinic acid = g (given value) I have one dozen eggs and they weigh 1 lb, I have 10 pounds of egg, how much dozen do I have? I have one mol of oxygen, it weighs 16g, but I did this experiment I now I have 32g of oxygen, how much mols of oxygen do I have?

n = molar mass succinic acid
molar mass C2H3O2 (empirical formula) n = g = 2.00 59.04 g (C2H3O2) 2 = C4H6O4

STOP! YOUR TURN! # 51 and 52

Empirical Formula = HO Given the molar mass is g/mol. What is the MOLECULAR FORMULA?

Class Discussion Why do one mol of sulfur not the same as one mol of hydrogen? CROSS OFF 11.5

11.5 The Formula for a Hydrate
Hydrate- a compound with a specific number of water molecules bound to its atoms

Analyzing a Hydrate In order to analyze a hydrate, water must be removed (usually from heating) Substance remaining after heating is “anhydrous” (meaning without water)