Unit III: Quantitative Unit III: Quantitative Composition of CompoundsComposition of Compounds

A.A. Atoms and IsotopesAtoms and IsotopesB.B. Atomic Composition of Chemical Atomic Composition of Chemical

CompoundsCompoundsC.C. Formula and Molecular MassFormula and Molecular MassD.D. Calculations using Moles of AtomsCalculations using Moles of AtomsE.E. Calculations using Moles of CompoundsCalculations using Moles of CompoundsF.F. Mass Percent Composition of CompoundsMass Percent Composition of CompoundsG.G. Empirical and Molecular FormulasEmpirical and Molecular Formulas

The Structure of an AtomThe Structure of an Atom

nn

npp

pp

n

e

e

e

e

3-A

What is an atom?What is an atom?

An atom is a fundamental piece of matter, An atom is a fundamental piece of matter, composed of subatomic particles.composed of subatomic particles.

An single atom is the smallest amount of an An single atom is the smallest amount of an element that maintains properties of that element that maintains properties of that element.element.

Atoms are not created, changed, or Atoms are not created, changed, or destroyed during a chemical reactiondestroyed during a chemical reaction

3-A

n

n

npp p

p

n

ee

e

e

1.673 x 10-24 g

1.674 x 10-24 g

9.109 x 10-28 g

Electron (-1)

Neutron (0)

Proton (+1)

The Structure of an AtomThe Structure of an Atom

3-A

Atomic MassAtomic Mass

Protons and neutrons are regarded as Protons and neutrons are regarded as having one atomic mass unit (1 AMU)having one atomic mass unit (1 AMU)

Electrons have a mass of zero AMUElectrons have a mass of zero AMU

The total number of protons and neutrons in The total number of protons and neutrons in an atom is called the an atom is called the atomic massatomic mass

3-A

IsotopesIsotopes

While all atoms of the same element behave While all atoms of the same element behave similarly in chemical reactions, not all similarly in chemical reactions, not all atoms are identicalatoms are identical

Atoms always have the same number of Atoms always have the same number of protons, but may have different masses….protons, but may have different masses….

3-A

N

Isotopes of hydrogenIsotopes of hydrogen

P

e

H1

1

Atomic mass = 1

H2

1

Atomic mass = 2

H3

1

Atomic mass = 3

HydrogenDeuterium Tritium

N

3-A

IsotopesIsotopes

ZnZnzinc

65.38

30

ZnZn30

65

EEZ

AMass number

Atomic number

Element symbol

The isotope zinc-65

3-A

Practice with isotopesPractice with isotopes

CC6

13

Name the following isotopes

NN7

15

MgMg12

25

How many protons are in each of the following isotopes?

carbon-13 nitrogen-15

PP15

30

3-A

Fill in the gaps in the following table Fill in the gaps in the following table assuming each assuming each columncolumn represents a neutral represents a neutral

atom:atom:

Isotopic Symbol

Number of Protons 34

Number of Neutrons 28 42

Number of Electrons 95 23

Mass Number 241

3-A

Kinds of isotopesKinds of isotopes

Stable and radio isotopes Stable and radio isotopes

Oxygen for example has isotopes with Oxygen for example has isotopes with between 4 and 16 neutrons.between 4 and 16 neutrons.

Only those atoms with 8, 9, or 10 neutrons Only those atoms with 8, 9, or 10 neutrons are stable.are stable.

3-A

Atomic mass vs. Atomic weightAtomic mass vs. Atomic weight

Atomic MassAtomic Mass refers to the mass of a refers to the mass of a particular isotope of a particular elementparticular isotope of a particular element

Atomic weightAtomic weight refers to the weighted refers to the weighted average mass of all the isotopes of an average mass of all the isotopes of an element.element.

(you might also see this as relative atomic (you might also see this as relative atomic mass)mass)

Atomic Composition of Chemical Atomic Composition of Chemical CompoundsCompounds

To calculate formula weights we need to To calculate formula weights we need to know the number of atoms of each know the number of atoms of each element in a compound.element in a compound.

3-B

Example: Hexanoic acid CH3(CH2)3CH2COOH

6 carbon atoms

2 oxygen atoms

12 hydrogen atoms

UNIT 3-C: Formula and Molecular UNIT 3-C: Formula and Molecular weightsweights

Molecular weight refers to covalent Molecular weight refers to covalent compoundscompounds

Formula weight refers to the weight of an Formula weight refers to the weight of an ionic compound’s formula unit.ionic compound’s formula unit.

3-C

UNIT 3-C: Lecture Problem 1 (pg 95)UNIT 3-C: Lecture Problem 1 (pg 95)

A.A. Find the formula weight of barium nitrate.Find the formula weight of barium nitrate.

B.B. Find the molecular weight of diphosphorus Find the molecular weight of diphosphorus pentaoxidepentaoxide

3-C

Unit 3-D: Calculations Using Moles Unit 3-D: Calculations Using Moles of Atomsof Atoms

When we talk about practical amounts of When we talk about practical amounts of elements or compound we are talking elements or compound we are talking about about huge huge numbers. numbers.

We need a convenient term that describes a We need a convenient term that describes a large group of things (similar to a dozen large group of things (similar to a dozen or gross). We use the term: or gross). We use the term:

‘‘mole’mole’

3-D

MolesMolesA ‘mole’ of anything always contains the A ‘mole’ of anything always contains the

same number, 6.022 x 10same number, 6.022 x 102323

This special number is called Avogadro’s This special number is called Avogadro’s number.number.

It just happens that there are 6.022 x 10It just happens that there are 6.022 x 102323 atoms of carbon-12 in exactly 12 grams of atoms of carbon-12 in exactly 12 grams of carbon-12.carbon-12.

3-D

Moles and massesMoles and masses

The atomic mass of C-12 is 12 amuThe atomic mass of C-12 is 12 amu

If the atomic mass of He-4 (for example) is If the atomic mass of He-4 (for example) is 4 amu.. (1/3 the mass of C-12)4 amu.. (1/3 the mass of C-12)

…….and there are 12 g of C-12 in one mole of C-.and there are 12 g of C-12 in one mole of C-12…..12…..

Then there will be 4 g of He-4 in one mole of He-4Then there will be 4 g of He-4 in one mole of He-4

3-D

Lecture Problem 3-2Lecture Problem 3-2A.A. What is the atomic weight of zinc (in What is the atomic weight of zinc (in

amu)?amu)?

B.B. How many grams are in a mol of zinc?How many grams are in a mol of zinc?

C.C. How many atoms are present in a mol How many atoms are present in a mol of zinc?of zinc?

Page 97Page 97

3-D

Lecture Problem 3-3Lecture Problem 3-3

A.A. How many grams are in 2.20 mol of How many grams are in 2.20 mol of iron?iron?

B.B. Calculate the number of mol in 2.45 x Calculate the number of mol in 2.45 x 1010-5-5 g of calcium. g of calcium.

Pg. 97Pg. 97

3-D

Unit 3 Part E:Unit 3 Part E:Calculations using Moles of Calculations using Moles of

CompoundsCompounds

Calculating molecular weights and Calculating molecular weights and molar massesmolar masses

If we add up the atomic weights of all the If we add up the atomic weights of all the atoms in a compound we get the atoms in a compound we get the molecular weight. (units of amu)molecular weight. (units of amu)

If we want to talk about the mass of 1 mol If we want to talk about the mass of 1 mol of a compound we replace the units of of a compound we replace the units of amu with the units g/mol.amu with the units g/mol.

3-E

Lecture Problem III-4 (pg. 99)Lecture Problem III-4 (pg. 99)

What is the molar mass of ammonium What is the molar mass of ammonium sulfide?sulfide?

(NH4)

2S

Molar mass of (NH4)

2S is 68.17 g/mol

3-E

Unit factorsUnit factorsThe concept of the ‘mol’ is very useful in The concept of the ‘mol’ is very useful in

constructing unit factors..constructing unit factors..

Using the example of glucose (CUsing the example of glucose (C66HH1212OO66))

With a molar mass of 180.18g/mol With a molar mass of 180.18g/mol

Write as many unit factors as possible Write as many unit factors as possible utilizing the mol concept.utilizing the mol concept.

3-E

Lecture Problem 3-5 (pg. 101)Lecture Problem 3-5 (pg. 101)

A.A. How many mol of hydrogen atoms are How many mol of hydrogen atoms are in 83.5 g of Cin 83.5 g of C66HH1212OO66??

B.B. How many water molecules are in How many water molecules are in 0.946 mol of water?0.946 mol of water?

Extra practice:Extra practice: How many total atoms are in How many total atoms are in 55.3 g of sulfur dioxide?55.3 g of sulfur dioxide?

3-E

Using mols to describe Using mols to describe composition of compoundscomposition of compounds

Lecture problem III-6 (pg. 102)Lecture problem III-6 (pg. 102)A sample of ammonium dichromate contains 3.07 mol A sample of ammonium dichromate contains 3.07 mol

of nitrogen atoms. What is the mass of the sample of nitrogen atoms. What is the mass of the sample of ammonium dichromate?of ammonium dichromate?

(NH4)

2Cr

2O

7 has a molar mass of 252.10 g/mol

386.97 g of (NH4)

2Cr

2O

7 rounded to 387 g

3-E

Lecture problem III-7 (pg. 103)Lecture problem III-7 (pg. 103) How many grams of bromine are present in a sample How many grams of bromine are present in a sample

of Ca(BrOof Ca(BrO33))22 that is found to contain 10.0 g of that is found to contain 10.0 g of

calcium?calcium?

Ca has a molar mass of 40.08 g/mol and

Br has a molar mass of 79.90 g/mol

39.87 g of Br rounded to 39.9 g

3-E

Unit 3-F: Mass Percent Unit 3-F: Mass Percent CompositionComposition

1.1. Calculate the formula or molecular Calculate the formula or molecular weight of the compound.weight of the compound.

2.2. Determine the total weight of each Determine the total weight of each element in the compound.element in the compound.

% composition of an element = % composition of an element = Total weight of that elementTotal weight of that element x 100 x 100formula or molecular weightformula or molecular weight

Unit 3-F: Mass Percent Unit 3-F: Mass Percent CompositionComposition

Lecture problem III-8 (pg. 105).Lecture problem III-8 (pg. 105).Calculate the percent by mass of calcium in calcium Calculate the percent by mass of calcium in calcium

phosphate.phosphate.

Ca3(PO

4)

2 molar mass of 310.2 g/mol

38.76% Ca by mass in CaPO4

Unit 3-G: Empirical and Molecular Unit 3-G: Empirical and Molecular FormulasFormulas

The The empirical formulaempirical formula of a compound of a compound represents the smallest ration of atoms that are represents the smallest ration of atoms that are present in a compound.present in a compound.

The formula for an The formula for an ionic compoundionic compound is in essence an is in essence an empirical formulaempirical formula

The The molecular formulamolecular formula is the true formula, is the true formula, representing the number of atoms of each representing the number of atoms of each element that are present in a single molecule of element that are present in a single molecule of a compounda compound

Unit 3-G: Empirical and Molecular Unit 3-G: Empirical and Molecular FormulasFormulas

Consider dinitrogen tetraoxideConsider dinitrogen tetraoxide

Based on the compound name we can write its Based on the compound name we can write its molecular formula: Nmolecular formula: N22OO44

The empirical formula describes the smallest ratio The empirical formula describes the smallest ratio of atoms that are present in a compound.of atoms that are present in a compound.

There is 1 nitrogen atom for every 2 oxygen atomsThere is 1 nitrogen atom for every 2 oxygen atoms

So the smallest ratio, or empirical formula) is…So the smallest ratio, or empirical formula) is…

NONO22

Unit 3-G: Empirical and Molecular Unit 3-G: Empirical and Molecular FormulasFormulas

Given the percent composition of an Given the percent composition of an unknown compound, how can we unknown compound, how can we determine its identity?determine its identity?

(sometimes actual masses for each element may be given)(sometimes actual masses for each element may be given)

The first step is to turn the mass percentage The first step is to turn the mass percentage into a ratio of atoms in the compound, into a ratio of atoms in the compound, called an empirical formula.called an empirical formula.

Solving Empirical Formula ProblemsSolving Empirical Formula Problems1.1.If starting with % composition assume exactly If starting with % composition assume exactly

100 grams of sample. Then percentages 100 grams of sample. Then percentages become mass quantities. become mass quantities. (If starting with mass, skip to (If starting with mass, skip to step 2)step 2)

2.2.Calculate the number of mol of each element Calculate the number of mol of each element present using our unit analysis method.present using our unit analysis method.

3.3.Find the simplest mol ratio by dividing all of the Find the simplest mol ratio by dividing all of the mol quantities by the smallest number of mol of mol quantities by the smallest number of mol of an element present.an element present.

Lecture problems III-9 (page 108-109)Lecture problems III-9 (page 108-109)

A.A. Calculate the empirical formula of a Calculate the empirical formula of a compound that contains 56.4% phosphorus and compound that contains 56.4% phosphorus and 43.6% oxygen43.6% oxygen

B.B. A compound contains 1.79g of phosphorus and A compound contains 1.79g of phosphorus and 2.29g of oxygen. What is the empirical formula 2.29g of oxygen. What is the empirical formula of the compound?of the compound?

C. It is found that 2.231 grams of selenium react C. It is found that 2.231 grams of selenium react with fluorine to form a selenium fluoride with a with fluorine to form a selenium fluoride with a mass of 5.452 grams. What is the empirical mass of 5.452 grams. What is the empirical formula of this compound?formula of this compound?

Finding the Molecular FormulaFinding the Molecular FormulaFind the empirical formula for the compound.Find the empirical formula for the compound.

– Calculate the empirical molar mass.Calculate the empirical molar mass.

Divide the molecular molar mass by the empirical Divide the molecular molar mass by the empirical molar mass. This is our multiplying factormolar mass. This is our multiplying factor

Multiply the quantity of each element in the Multiply the quantity of each element in the empirical formula by this factor.empirical formula by this factor.

The result is the molecular formula.The result is the molecular formula.

Molecular Formula DeterminationMolecular Formula DeterminationLecture problem (III-10 pg. 111):Lecture problem (III-10 pg. 111):A.A. What is the molecular formula for galactose if the What is the molecular formula for galactose if the

empirical formula is CHempirical formula is CH22O and the molar mass is O and the molar mass is

180.2 g/mol?180.2 g/mol?

B.B. A molecular compound contains 40.0% carbon, A molecular compound contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass. 6.7% hydrogen and 53.3% oxygen by mass.

A 0.100 mol sample of this compound weighs 6.00 A 0.100 mol sample of this compound weighs 6.00 grams. What is the molecular formula of the grams. What is the molecular formula of the compound?compound?

Extra practice with ‘mystery metal’ Extra practice with ‘mystery metal’ problemsproblems

0.125 mol of chlorine gas react with 0.0625 mol of 0.125 mol of chlorine gas react with 0.0625 mol of a metal to form 16.3 grams of a binary ionic a metal to form 16.3 grams of a binary ionic compound. What is the metal, and what is the compound. What is the metal, and what is the formula for the compound?formula for the compound?

5.11 grams of chlorine gas reacts with 0.0480 mol 5.11 grams of chlorine gas reacts with 0.0480 mol of a metal to form 14.56 grams of a new solid of a metal to form 14.56 grams of a new solid compound with the formula MClcompound with the formula MCl33. What is the . What is the metal (symbol)? metal (symbol)?