The S-wordStoichiometry

Chapter 3

By definition: 1 atom 12C “weighs” 12 amu

On this scale

1H = 1.008 amu

16O = 16.00 amu

Atomic mass is the mass of an atom in atomic mass units (amu)

Micro Worldatoms & molecules

Macro Worldgrams

3.1

Natural lithium is:

7.42% 6Li (6.015 amu)

92.58% 7Li (7.016 amu)

(7.42% x 6.015) + (92.58% x 7.016)100

= 6.941 amu

3.1

Average atomic mass of lithium “weighted average of all naturally occurring

isotopes of an element:

The mole (mol) is the amount of a substance that contains as many elementary entities as there

are atoms in exactly 12.00 grams of 12C

3.2

1 mol = NA = 6.0221367 x 1023

Avogadro’s number (NA)

Molar mass is the mass of 1 mole of in gramseggsshoes

marblesatoms

1 mole 12C atoms = 6.022 x 1023 atoms = 12.00 g

1 12C atom = 12.00 amu

1 mole 12C atoms = 12.00 g 12C

1 mole lithium atoms = 6.941 g of Li

For any element

atomic mass (amu) = molar mass (grams)

3.2

One Mole of:

C S

Cu Fe

Hg

3.2

1 g = 6.022 x 1023 amu

3.2

M Or MM = molar mass in g/mol

NA = Avogadro’s number

1 amu = 1.66 x 10-24 g

Do You Understand Molar Mass?

How many atoms are in 0.551 g of potassium (K) ?

3.2

Molecular mass (or molecular weight) is the sum ofthe atomic masses (in amu) in a molecule.

SO2

1S 32.07 amu

2O + 2 x 16.00 amu SO2 64.07 amu

For any molecule

molecular mass (amu) = molar mass (grams)

1 molecule SO2 = 64.07 amu

1 mole SO2 = 64.07 g SO2 3.3

Do You Understand Molecular Mass?

How many H atoms are in 72.5 g of C3H8O ?

3.3

KE = 1/2 x m x v2

v = (2 x KE/m)1/2

F = q x v x B

3.4

Ligh

t

Ligh

t

Hea

vy

Hea

vy

Percent composition of an element in a compound =

n x molar mass of elementmolar mass of compound

x 100%

n is the number of moles of the element in 1 mole of the compound

C2H6O

3.5

Types of FormulasTypes of Formulas• Empirical FormulaEmpirical Formula

The formula of a compound that The formula of a compound that expresses the expresses the smallest whole number smallest whole number ratioratio of the atoms present. of the atoms present.

Ionic formula are always empirical formulaIonic formula are always empirical formula

• Molecular FormulaMolecular Formula

The formula that states the The formula that states the actualactual number of each kind of atom found in number of each kind of atom found in one one moleculemolecule of the compound. of the compound.

To obtain an To obtain an Empirical Empirical FormulaFormula

1.1. Determine the mass in grams of each Determine the mass in grams of each element present, if necessary.element present, if necessary.

2.2. Calculate the number of Calculate the number of molesmoles of of each each element.element.

3.3. Divide each by the smallest number of moles Divide each by the smallest number of moles to obtain the to obtain the simplest whole number ratio.simplest whole number ratio.

4.4. If whole numbers are not obtainedIf whole numbers are not obtained** in step in step 3), multiply through by the smallest number 3), multiply through by the smallest number that will give all whole numbersthat will give all whole numbers

** Be careful! Do not round off numbers prematurelyBe careful! Do not round off numbers prematurely

A sample of a brown gas, a major air pollutant, A sample of a brown gas, a major air pollutant, is found to contain 2.34 g N and 5.34g O. is found to contain 2.34 g N and 5.34g O. Determine a formula for this substance.Determine a formula for this substance.

Calculation of the Molecular FormulaCalculation of the Molecular Formula

A compound has an empirical formula of NOA compound has an empirical formula of NO22. . The colorless liquid, used in rocket engines The colorless liquid, used in rocket engines has a molar mass of 92.0 g/mole. What is the has a molar mass of 92.0 g/mole. What is the molecular formula molecular formula of this substance?of this substance?

Empirical Formula from % Empirical Formula from % CompositionComposition

A substance has the following composition by mass: 60.80 % Na ; 28.60 % B ; 10.60 % H What is the empirical formula of the substance?

3.6

Combust 11.5 g ethanol

Collect 22.0 g CO2 and 13.5 g H2O

Example – Determining a Molecular FormulaMany homes in rural America are heated by propane gas, a compound that contains only carbon & hydrogen. Complete combustion of a sample of propane produced 2.641 g of carbon dioxide and 1.442g of water. Find the empirical formula of propane.

1. Write balanced chemical equation

2. Convert quantities of known substances into moles

3. Use coefficients in balanced equation to calculate the number of moles of the sought quantity

4. Convert moles of sought quantity into desired units

Mass Changes in Chemical Reactions

3.8

Other units

• Molarity– Moles solute / L solution

• Gases– 22.4 L = 1 mole of ANY GAS at STP

Methanol burns in air according to the equation

2CH3OH + 3O2 2CO2 + 4H2O

If 209 g of methanol are used up in the combustion, what mass of water is produced?

grams CH3OH moles CH3OH moles H2O grams H2O

3.8

Example – StoichiometryBaking soda (NaHCO3) is often used as an antacid. It neutralizes excess hydrochloric acid secreted by the stomach:

Milk of magnesia, which is an aqueous suspension of magnesium hydroxide, is also used as an antacid:

Which is more effective per gram?

)(2)(2)()(3 glaqaq COOHNaClHClNaHCO

)(2)(2)()(2 22)( aqlaqs MgClOHHClOHMg

6 green used up6 red left over

Limiting Reagents

3.9

Method 1

• Pick A Product

• Try ALL the reactants

• The lowest answer will be the correct answer

• The reactant that gives the lowest answer will be the limiting reactant

Method 2

• Convert one of the reactants to the other REACTANT

• See if there is enough reactant “A” to use up the other reactants

• If there is less than the GIVEN amount, it is the limiting reactant

• Then, you can find the desired species

Limiting Reactant• 10.0g of aluminum reacts with 35.0 grams of chlorine gas

to produce aluminum chloride. Which reactant is limiting, which is in excess, and how much product is produced?

2 Al + 3 Cl2 2 AlCl3

LimitingLimitingReactantReactant

Do You Understand Limiting Reagents?In one process, 124 g of Al are reacted with 601 g of Fe2O3

2Al + Fe2O3 Al2O3 + 2Fe

Calculate the mass of Al2O3 formed.

3.9

3.9

Finding Excess Practice• 10.0g of aluminum reacts with 35.0 grams of

chlorine gas 2 Al + 3 Cl2 2 AlCl3

• We found that chlorine is the limiting reactant, and 43.8 g of aluminum chloride are produced. How much aluminum is left unreacted?

Percent YieldTheoretical Yield is the amount of product that wouldresult if all the limiting reagent reacted.

Actual Yield is the amount of product actually obtainedfrom a reaction.

% Yield = Actual Yield

Theoretical Yieldx 100

3.10

Example – Calculating % YieldMethanol (CH3OH), also called methyl alcohol, is the simplest alcohol. It is used as fuel in race cars and is a potential replacement for gasoline. Methanol can be manufactured by combining gaseous carbon monoxide & hydrogen. Suppose 68.5 kg CO(g) is reacted with 8.60 kg H2(g). Calculate the theoretical yield of methanol. If 3.57x104 g of methanol is actually produced, what is the percent yield of methanol?