Chapter 3Mass Relationships in Chemical Reactions

Atomic Mass What is atomic

mass? Mass of an atom in

atomic mass units (AMU) 1 AMU is the mass of

1/12 of a carbon-12 atom

Also called atomic weight

Relative Atomic Mass What does the

relative atomic mass of an element represent?

Average mass of all known existing isotopes of that element

It is “unitless” Proportioned based

on abundance Ex. Carbon has a

mass of 12.011 which is an average of all of the carbon isotopes

Isotope ProportionsA certain sample of element Z contains 60% of 69Z and 40% of 71Z.

What is the relative atomic mass of element Z in this sample?

Isotope ProportionsChlorine exists as two isotopes, 35Cl and 37Cl. The relative atomic mass of chlorine is 35.45.

Calculate the percentage abundance of each isotope.

Molar Mass and NA

What unit is used to measure the number of atoms/molecules?

Moles (mol) – the number of atoms in 12 grams of a carbon-12 sample

1 mole = 6.02 x 1023

Avogadro's number (NA)

We use it like we would a pair (2), dozen (12), or a gross (144) to simplify our math because it is freaking huge

How Big Is A Mole?1. If you would distribute 1 mole of pennies among the

current population of the Earth, how much would everyone get?

2. How long would it take to spend this amount if you would spend $1,000/day?

3. How many people could you make from 1 mole of cells?

4. How long is 1 mole of seconds? 5. If we took a mole of regular 8.5 x 11 inch sheets of

paper and started laying them out side by side on the planet’s surface. What percentage of the Earth would be covered?

How Big Is A Mole? Answers1. $900 million dollars2. 2,500,000 years3. 10 billion4. 950,000 x age of the universe (14 billion

yrs)5. 100 %

…and it would be 17 million sheets thick

Molar Mass How is molar mass

different from relative atomic mass?

It is the mass of one mole of a substance

Abbreviated MM Same number as

atomic mass, but with different units Units: IB Units: g mol-1

Molar Mass What is the molar

mass (MM) of the following elements? Lithium Plutonium Bromine

6.94 239.05 79.91

Molar Mass How do we

determine the molar mass of a molecule?

What is the molar mass of water?

Add up the molar masses of each element within the molecule

H2O MM = 2(H) + 1(O) MM = 2(1.01) + 1(16.00 ) MM = 18.02

Molecular Vs. Molar What is the

difference between atomic, molecular and molar mass?

Different units for different purposes

Atomic/Molecular Mass AMU’s

Molar Mass

Can be used interchangeably

Molar Mass What is the molar

mass of each of the following species: KBr FeCl3 Sodium Hydroxide Lead (II) Oxide

119.01

162.20

40.00

223.19

Navigating How can we

navigate between mass (m), moles (n) and the number of atoms, molecules or particles?

We use molar mass (MM) and Avogadro's number (NA)

Double Circles

Double Triangles

NAAvogadro's number

mmass

nmoles

MMmolar mass

nmoles

Nnumber

Mass-Mole Conversions1. How many moles in 28 grams of CO2 ?

2. What is the mass of 5 moles of Fe2O3 ?

3. Find the number of moles of argon in 452 g of argon.

4. Find the grams in 1.26 x 10-4 mol of HC2H3O2.

5. Find the mass in 2.6 mol of lithium bromide.

Mole-Number Conversions1. How many moles of magnesium is 3.01 x 1022

atoms of magnesium?

2. How many molecules are there in 4.00 moles of glucose, C6H12O6?

3. How many moles are 1.20 x 1025 atoms of phosphorous?

4. How many atoms are in 0.750 moles of zinc?

5. How many oxygen atoms are in 0.400 moles of N2O5?

To the right, To the right How many atoms

are in 20.0 grams of gold?

Mass 20.0 g Divide by molar mass

to get to moles 0.102 moles

Multiply by Avogadro's number to get to atoms 6.14E22 Au atoms

To the left, To the left What is the mass of

1.00E24 carbon atoms?

Number 1.00E24 atoms

Divide by Avogadro's number to get to moles 1.66 moles

Multiply by molar mass to get mass 19.9 grams

Combined1. Find the mass in grams of 2.00 x 1022

molecules of diatomic fluorine.

2. Find the number of molecules in 60.0 g of dinitrogen monoxide.

3. How many molecules are in 5 mg of aspartame, C14H18N2O5?

4. Calculate the mass, in grams, of 6.69x1021 ethane (C2H6) molecules.

5. Urea [(NH2)2CO] is used for fertilizer and many other things. Calculate the number of N,C,O and H atoms in 1.68 x 104 grams of urea.

6. The density of water is 1.00 at 4°C. How many water molecules are present in 2.56 mL of water at this temperature?

Combined

PRELAB 1Gum

PRELAB 1Sugar in Chewing Gum How can we test

for the mass of sugar in chewing gum?

PRELAB 1Sugar in Chewing Gum

In your notebook, please calculate the following with proper uncertainties, sig figs and units.1. Experimental mass of sugar in the gum with

the correct uncertainties

2. Percent mass of sugar in your piece of gum

3. Percent error =

PRELAB 1Sugar in Chewing Gum Compare and contrast your measured

percentage of sugar, and the actual percentage from the nutrition label for the brand of gum that you used.

What are some possible reasons for any differences in these values, and how you might increase the accuracy of your data collection?

PRELAB 1Sugar in Chewing Gum

What mathematical relationship/formula did you use to determine percent by

mass? Why?

If you were given one molecule of sucrose, C6H12O6, what information do you think you would need to figure out the percent mass

of carbon? Why?

Percent Composition What is percent

composition?

What two pieces of info do we need?

Relative measure of the mass of each different element present in the compound

x 100

Mass of the element and the compound

Percent Composition How do we

calculate it?1. Calculate the mass

of the compound

2. Calculate the mass of each element

Percent Composition What is the

percent mass of hydrogen in water?

2 hydrogens 2(1.01) = 2.02 g/mol

Water is H2O 18.02 g/mol

2.02/18.02 x100 = 11.2% (tenths place)

Percent Composition Calculate the percent composition of

oxygen in sodium sulfate. Na2SO4 142.04 g/mol 4 oxygens 4(16.00) = 64.00 g/mol (64.00/142.04) x 100 = 45.1%

Percent Composition Tartrazine, or yellow dye # 5, is

commonly found in many foods. Its molecular formula is C16H9N4Na3O9S2 . What is the percent composition of Carbon and Sulfur? The molar mass of Tartrazine is 534.4 g/mol.

Working in Reverse If we start with

%’s, what can that possibly lead us to?

Working the formula in reverse and using mole ratios, we can get the empirical and possibly the molecular formula

Determination of the Empirical Formula

How do we determine it?

1. Assume that you have 100% of the element and set equal to mass

2. Divide each element by its molar mass

3. Divide each by the smallest amount of moles

4. If numbers aren’t close to whole, multiply all of them by a coefficient

Butane’s Empirical Formula Butane is made up of only carbon and

hydrogen. If hydrogen makes up 17.34% of butane’s mass and carbon the remaining 82.66%, what is the empirical formula of butane? C2H5

What is the molecular formula for butane if its molecular mass is 58.14 g/mol? Multiply each subscript by 2 C4H10

Determine the Empirical Formula Determine the empirical formula of a

compound having the following percent composition by mass… K: 24.75% Mn: 34.77% O: 40.51%Empirical Formula: KMnO4

What is the molecular formula if its molecular mass is 158.04 g/mol?

ANSWER: KMnO4 (potassium permanganate)

Chemical Reactions and Equations

What is the difference between a chemical reaction and equation?

A chemical reaction is the process of making a new substance(s) Ex. Iron oxidizing

A chemical equation is a symbolic representation of the reaction 4Fe + 3O2 → 2Fe2O3

Chemical Reactions and Equations

How do we read a chemical equation?

4Fe(s) + 3O2(g) → 2Fe2O3(s) Reactants (left) Products (right) → Goes in one direction ↔ Reversible reaction Coefficients represent

the mole ratios Notations (s, l, g, aq)

represent phases/states Conservation of mass Balanced

Chemical Reactions and Equations

What is a balanced equation?

The number of atoms on the left has to equal the number on the right

Use coefficients to balance them

Unbalanced: CH4 + O2 → CO2 + H2O

Balanced:

Chemical Reactions and Equations How do we balance the equations? KClO3 → KCl + O2 2KClO3 → 2KCl + 3O2

Al + O2 → Al2O3

4Al + 3O2 → 2Al2O3

C2H6 + O2 → CO2 + H2O 2C2H6 + 7O2 → 4CO2 + 6H2O

Chemical Reactions and EquationsBalance the following chemical equations: K2CO3 + HCl KCl + H2O + CO2

K2CO3 + 2HCl 2KCl + H2O + CO2

CaCO3 + HNO3 Ca(NO3)2 + H2O + CO2

CaCO3 + 2HNO3 Ca(NO3)2 + H2O + CO2

Pb(NO3)2 + NaI PbI2 + NaNO3

Pb(NO3)2 + 2NaI PbI2 + 2NaNO3

Al2(SO4)3 + NaOH Al(OH)3 + Na2SO4

Al2(SO4)3 + 6NaOH 2Al(OH)3 + 3Na2SO4

Amounts of Reactants and Products

What is stoichiometry? The study of any numerical observation within a chemical reaction

Using the mole method to determine how much reactant is needed (or product created) based on the number of moles

Amounts of Reactants and Products

What can be said about the reaction: H2 + O2 → H2O

Reactants: Hydrogen and Oxygen

Products: Water Unbalanced Balanced: 2H2 + O2→2H2O Proportion: 2 moles of

hydrogen and 1 mole of oxygen create 2 moles of water

Amounts of Reactants and Products

How many moles of water should be formed with 8 moles of oxygen?

What happens when our measurements are not in moles?

1:2 Ratio 8 moles of oxygen is

equivalent to 16 moles of water

Convert them into moles before starting using molar mass

Amounts of Reactants and Products

I have the formula: Mg + HCl → MgCl2 + H2

What is the mass of MgCl2 that is formed when 20.0 g of HCl reacts with Mg? 1. Balance it

Mg + 2HCl → MgCl2 + H2

2. Convert into moles (20.0 g)/(36.45 g/mol) = 0.549 mol HCl

3. Use the mole ratio method 2:1 ratio → 0.275 mol MgCl2

4. Convert back into mass 0.275 mol MgCl2 → 26.2 g

Amounts of Reactants and Products

What mass of silver nitrate as a solution in water would need to be added to 5.0 g of sodium chloride to ensure a complete precipitation of the chloride?

AgNO3(aq)+ NaCl (aq) AgCl(s) + NaNO3(aq)

Amounts of Reactants and Products

What mass of barium sulfate would be produced from 10.0 g of barium chloride in the following reaction?

BaCl2 + H2SO4 BaSO4 + HCl

Balanced: BaCl2 + H2SO4 BaSO4 + 2HCl

Amounts of Reactants and Products

What volume of ammonia gas would be needed to produce 40. g of ammonium nitrate in the following reaction.

NH3 (g) + HNO3 (aq) NH4NO3 (aq)

Amounts of Reactants and Products

In a fermentation reaction, glucose is converted to alcohol and carbon dioxide according to the following equation. What mass of alcohol and carbon dioxide would be produced from 10. g of glucose?

C6H12O6 C2H5OH + CO2

Balanced: C6H12O6 2C2H5OH + 2CO2

Amounts of Reactants and Products

What is the difference between theoretical and actual yield?

What is its purpose?

Theoretical is the predicted amount of product via stoichiometry

Actual is how much product you obtained experimentally

To determine the efficiency of the procedure and/or experiment

Reaction Yield

Reaction Yield What is the % yield

of CO2 if you started with 10.00 g of O2 obtained 22.53 g of CO2?

CO + O2 CO2

Balanced: 2CO + O2 → 2CO2

100% ltheoretica

actualyield

Limiting Reagent

What is a limiting reagent?

The reactant that is depleted first during a reaction is limiting

Prevents a reaction from achieving 100% yield

Any reactants remaining after the reaction completes are called excess reagents i.e. anything in excess

cannot be a limiting reagent

Limiting Reagent 124 g of aluminum react with 601 g of iron (III)

oxide. Calculate the mass of Al2O3 formed.

Al + Fe2O3 → Al2O3 + Fe

2Al + Fe2O3 → Al2O3 + 2Fe

1. Balance!2. Determine the number of moles of each reactant.3. Divide by the molar coefficient to determine the

limiting reagent4. Use that number of moles to determine the

amount produced using proper stoichiometry

Limiting Reagent Table

Reaction 2Al Fe2O3 Al2O3 2Fe

Molar Mass (g/mol)

Initial Mass (g)

Initial Moles (mol)

Limiting Reagent (mol)

Final Moles (mol)

Final Mass (g)

Limiting Reagent Table

Reaction 2Al Fe2O3 Al2O3 2Fe

Molar Mass (g/mol)

26.98 159.70 101.96 55.85

Initial Mass (g) 124 601 0 0

Initial Moles (mol)

Limiting Reagent (mol)

Final Moles (mol)

Final Mass (g)

Limiting Reagent TableReaction 2Al Fe2O3 Al2O3 2Fe

Molar Mass (g/mol)

26.98 159.70 101.96 55.85

Initial Mass (g) 124 601 0 0

Initial Moles (mol) 4.60 3.76 0 0

Limiting Reagent (mol)

Final Moles (mol)

Final Mass (g)

Limiting Reagent TableReaction 2Al Fe2O3 Al2O3 2Fe

Molar Mass (g/mol)

26.98 159.70 101.96 55.85

Initial Mass (g) 124 601 0 0

Initial Moles (mol) 4.60 3.76 0 0

Limiting Reagent (mol)

2.30 3.76 0 0

Final Moles (mol)

Final Mass (g)

Limiting Reagent Table

Reaction 2Al Fe2O3 Al2O3 2Fe

Molar Mass (g/mol) 26.98 159.70 101.96 55.85

Initial Mass (g) 124 601 0 0

Initial Moles (mol) 4.60 3.76 0 0

Limiting Reagent (mol)

2.30 3.76 0 0

Final Moles (mol) 4.60 2.30 2.30 4.60

Final Mass (g) 124 367 235 257

Note: The mass of reactants equals the approximate mass of the products

Limiting Reagent How much chlorine is produced in the following reaction

when 47.25 g of manganese (IV) oxide reacts with 48.2 g of hydrochloric acid?

MnO2 + 4HCl → MnCl2 + Cl2 + 2H2OReaction MnO2 4HCl MnCl2 Cl2 2H2O

Molar Mass (g/mol)

Initial Mass (g)

Initial Moles (mol)

Limiting Reagent (mol)

Final Moles (mol)

Final Mass (g)

Limiting Reagent How much chlorine is produced in the following reaction

when 47.25 g of manganese (IV) oxide reacts with 48.2 g of hydrochloric acid?

MnO2 + 4HCl → MnCl2 + Cl2 + 2H2OReaction MnO2 4HCl MnCl2 Cl2 2H2O

Molar Mass (g/mol) 86.93 36.46 125.83 70.90 18.02

Initial Mass (g) 47.25 48.2 0 0 0

Initial Moles (mol)

Limiting Reagent (mol)

Final Moles (mol)

Final Mass (g)

Limiting Reagent How much chlorine is produced in the following reaction

when 47.25 g of manganese (IV) oxide reacts with 48.2 g of hydrochloric acid?

MnO2 + 4HCl → MnCl2 + Cl2 + 2H2O

Reaction MnO2 4HCl MnCl2 Cl2 2H2O

Molar Mass (g/mol) 86.93 36.46 125.83 70.90 18.02

Initial Mass (g) 47.25 48.2 0 0 0

Initial Moles (mol) 0.5435 1.32 0 0 0

Limiting Reagent (mol)

Final Moles (mol)

Final Mass (g)

Limiting Reagent How much chlorine is produced in the following reaction

when 47.25 g of manganese (IV) oxide reacts with 48.2 g of hydrochloric acid?

MnO2 + 4HCl → MnCl2 + Cl2 + 2H2OReaction MnO2 4HCl MnCl2 Cl2 2H2O

Molar Mass (g/mol) 86.93 36.46 125.83 70.90 18.02

Initial Mass (g) 47.25 48.2 0 0 0

Initial Moles (mol) 0.5435 1.32 0 0 0

Limiting Reagent (mol)

0.5435 0.330

0 0 0

Final Moles (mol)

Final Mass (g)

Limiting Reagent How much chlorine is produced in the following reaction

when 47.25 g of manganese (IV) oxide reacts with 48.2 g of hydrochloric acid?

MnO2 + 4HCl → MnCl2 + Cl2 + 2H2OReaction MnO2 4HCl MnCl2 Cl2 2H2O

Molar Mass (g/mol) 86.93 36.46 125.83 70.90 18.02

Initial Mass (g) 47.25 48.2 0 0 0

Initial Moles (mol) 0.5435 1.32 0 0 0

Limiting Reagent (mol)

0.5435 0.330

0 0 0

Final Moles (mol) 0.330 1.32 0.330 0.330 0.660

Final Mass (g)

Limiting Reagent How much chlorine is produced in the following reaction

when 47.25 g of manganese (IV) oxide reacts with 48.2 g of hydrochloric acid?

MnO2 + 4HCl → MnCl2 + Cl2 + 2H2OReaction MnO2 4HCl MnCl2 Cl2 2H2O

Molar Mass (g/mol) 86.93 36.46 125.83 70.90 18.02

Initial Mass (g) 47.25 48.2 0 0 0

Initial Moles (mol) 0.5435 1.32 0 0 0

Limiting Reagent (mol)

0.5435 0.330

0 0 0

Final Moles (mol) 0.330 1.32 0.330 0.330 0.660

Final Mass (g) 28.7 48.1 41.5 23.4 11.9

Excess Reagent If you started with 47.25 g, how much manganese (IV)

oxide is left in excess after the reaction is completed?

47.25 – 28.7 = 18.6 g of MgO2 excess

Reaction MnO2 4HCl MnCl2 Cl2 2H2O

Molar Mass (g/mol) 86.93 36.46 125.83 70.90 18.02

Initial Mass (g) 47.25

48.2 0 0 0

Initial Moles (mol) 0.5435 1.32 0 0 0

Limiting Reagent (mol)

0.5435 0.330 0 0 0

Final Moles (mol) 0.330 1.32 0.330 0.330 0.660

Final Mass (g) 28.7 48.1 41.5 23.4 11.9

Limiting Reagent

Limiting Reagent

Practice Problem – pg. 93

Limiting Reagent

Practice Problem – pg. 95

Limiting Reagent

Practice Problem – pg. 96

Reaction Yield