stoichiometry …like a beautiful sunset on a serene lake – not!

Stoichiometry …like a beautiful sunset on a serene lake – NOT!

What the heck is Stoichiometry• Stoichiometry is the study of quantities of

materials produced and consumed in chemical reactions. Or, in a way normal people (not honors students) could understand…

…solving problems using relationships found in balanced, chemical equations!

• A mole is a counting number used by chemists …Kind of like 1 dozen eggs – 12 eggsAvogadro defined the mole as the number of Carbon

atoms in exactly 12 grams of pure Carbon – 12 (think the graphite in a pencil!) It was determined that

1 mole = 6.02 x 1023 unitsThe units are usually atoms or molecules

This number is called AVOGADRO’S NUMBER!

MOLE (a mole is a unit or have you heard containing 6 x 10 to the 23rd)

Just How Big is This Number? (not as big as your hips or mouth!!)

• How big is 6.02 x 1023 ? A huge number!• 1 mole of marbles would cover the entire earth a depth of

50 miles!• 1 mole sheets of paper would go to the moon and back 80 billion times.• 1 mole of pennies- each of us on earth could spend 1 million dollars each hour (day and night) and have ½ of it left if we died at age 70!(Much too big a number to comprehend)

Comparisons of 1 mole of different elements

Aluminum 6.02 x 1023 atoms 26.98 grams

Gold 6.02 x 1023 atoms 196.97 grams

Boron 6.02 x 1023 atoms 10.81 grams

So… what is the connection between moles and atomic weights?

• The atomic weights (or the mass numbers or the atomic masses) of the elements on the periodic table are equal

to 1 mole of that substance. That’s why we

LOVE the periodic table!

You have 10 g of Al. Calculate the number of moles AND the number of

atoms .• Remember: the mass of 1 mole of Al equals 26.98 or 27 grams (the at. wt.)

Mole Map

A silicon chip used in your cell phone has a mass of 5.68 mg (which is 0.00568g). How

many Si atoms are present in this chip?

Co is a metal added to steel (a mixture) to help the steel resist corrosion. You have 5.00

x 1020 atoms of Co. Calculate:

a. The number of moles:

b. The mass of Cobalt:

Molecular or Formula Weights

• A formula weight gives the mass of 1 mole of a molecule.

• Find the formula weight of CH4 (a byproduct of Chipotle)

Calculate the formula weight of NaCl

Calculate the formula weight of C10 H6 O3 , a natural herbicide (weed killer) and dye in

Rit Dye.

Now, nerds of the world…calculate the number of moles of C10 H6 O3 in 0.0156 g

Isopentyl acetate (C7H14O2) is the compound responsible for the scent of bananas. Bee’s also release this same compound when they sting in order to attract other bees (to the buffet) and join in the attack. (smell you bee sting next time!) They release 1 microgram in each bee sting.

1. How many molecules of C7H14O2 are released in one bee sting?

How many atoms of C are present in one bee sting? (a fact you have always wanted to know)

Percent Composition

• Ethanol, an additive to gas (and alcoholic drinks) has the formula C2H5OH. Find the % of each element in ethanol.

STEP 1:

STEP 2:

Find the mass % of each element in C10H14 O

Penicillin, an antibiotic, has the formula C14H20N2SO4.

1. Find the % of Nitrogen in penicillin.

2. Find the number of moles of penicillin if you give a patient a 250 mg tablet.

Empirical Formulas• A empirical formula is the simplest, whole

number ratio of atoms. Most formulas are empirical. What is the key? If it CAN’T be reduced, it is empirical. If it CAN be reduced, it is a molecular formula. What are these?

Na2SO4 C12H22O11 C6H12O6 N4O12

To find the Empirical Formula (from %)

1. Convert % data to grams (% means “parts per hundred”).

2. Convert the grams to moles (÷ by the atomic weight)

3. Divide by the smallest to the get the simplest, whole-number ratio.

The junior scientist was given 38.67% C, 16.23% H and 45.1% N. Find the empirical formula.

A white powder was found near where mole died and was found to contain 43.64% P and 56.36% O. Find the

empirical formula and name this molecule.

How many grams of potassium can be produced when 150.6 g of potassium sulfate is completely decomposed?

First, find the % of K in K2SO4

Next, multiply the % (decimal) by the mass given.

How many grams of Cu are needed to make 16.6 g of copper II phosphate?

First, find the % of Cu:

Second, multiply the % by the mass given:

Molecular Formulas• A molecular formula is NOT the simplest

formula (it can be reduced…but don’t do it!)• Formula:

– W

–Where X is an integer

A compound with an empirical formula of CH5N has a molecular weight of 93 g/mole. Find the

molecular formula.

Find the empirical and molecular formulas (normal kids don’t get to do

this….you’re lucky!) for a compound that is 71.65% Cl, 24.27% C, and 4.07% H. The molecular formula weight is 98.96 g/mol.

Four Steps to find the Molecular Formula:1. Find the empirical formula:

2. Find the empirical formula weight:

3. Find X (X = Molec.fwt/Emp. Fwt)

4. Multiply X by the empirical formula to get the molecular formula.

Chemical Equations• CH4 + O2 → CO2 + H2O• In a chemical reaction, atoms have been rearranged

but have NOT been created or destroyed. This is why we have to balance every chemical reaction/equation.

• Balance CH4 + O2 → CO2 + H2O

• Balance HCl + NaHCO3 → CO2 + H2O + NaCl

• When balancing equations, the formulas must NEVER be changed. We may just add coefficients.

Balance the following reactions:

• C2H5OH + O2 → CO2 + H2O

• (NH4 )2Cr2O7 → Cr2 O3 + N2 + H2O

• C3H8 + O2 → CO2 + H2O

• NH3 + O2 → NO + H2O

Diatomic Elements

• N2 , Cl2 , O2 , H2 , I2 , Br2 , F2 • Memory aid: I Bring Clay For Our New House• Or Never Collect Our House Insurance Before

Friday (this one is ghetto!) • Diatomic elements always combine with a like

atom when they are a free element. • An arrow pointing up (↑) indicates a gas. The

abbreviation ppt. or a down arrow (↓) indicates a precipitate.

Chemical equations indicate a…

• Chemical change. Remember, they must obey the Law of Conservation of Mass – the number of atoms of the reactants MUST equal the number of atoms of products.

• The coefficients from the balanced equation tell us the mole ratios of reacting substances (this will be important later!!!)

• 2K + S → K2S

• 2FeCl3 + 3Ca(OH)2 → 2Fe(OH)3 + 3CaCl2

Five Types of Chemical Reactions

• 1. Composition (or Combination) Reactions• A + B → AB• Sodium and sulfur yields _____________

1. complete the word equation2. write the correct formulas3. Balance that bad boy!

Aluminum and iodine yields ??

2. Decomposition Reactions (6 types)

• Generic equation AB → A + B• These are the opposite of Composition

Reactions.• Here, one thing is HEATED and it breaks down

into two things.• The best way to learn these six types of

equations is to MEMORIZE them!!!

1. Binary Compound into Elements

• Arsenic III oxide yields when heated ______________ + ________________

• Water yields with electricity _____________ + ____________.

2. Metal Carbonates form metal oxides and carbon dioxide

• Magnesium carbonate yield when heated _________________+ _________________

• Sodium carbonate yields when heated _________________ + _________________

3. Metal hydroxides yield metal oxides and water

• Potassium hydroxide yields when heated __________________ +________________

• Calcium hydroxide yields when heated _________________ + _________________

4. Metal chlorates yield metal chlorides and oxygen

• Potassium chlorate yields when heated _________________ + ___________________

5. Metallic bromates yields metal bromides and oxygen

• Calcium bromate yields when heated _________________ + _________________

• Iron II bromate yields when heated _________________ + _________________

6. Acids when heated form nonmetal oxides and water

• Carbonic acid when heated yields _________________ + __________________

3. Single Replacement Reactions (two types)

• A. Positive Replacement (3 types)

• AX + B → BX + A• The single element will replace the

positive element in the compound

1. Replacement of a Metal by a more active metal

• Magnesium and calcium nitrate yields

2. Replacement of hydrogen in water by a metal (to form a metallic hydroxide and

hydrogen gas)

• Aluminum and water yields

3. Replacement of H in an acid by a metal

• Aluminum and hydrochloric acid yields

B. Negative Replacement - Replacement of Halogens (1 type)

• A + MB → MA + B• The halogens are Br-1 , Cl-1 , F -1 ,

and I-1 and they are all diatomic when they stand alone (Br2 , Cl2 , F2 , and I2 )

Hydrochloric acid and bromine yields…

4. Ionic/Exchange/Double Replacement Reactions

• AB + CD → AD + CB• The product MAY produce a solid or a

precipitate• PRECIPITATE: an insoluble substance

formed in solution. •a liquid + liquid → solid • but only one of the products may be a

precipitate

Solubility Rules

• See chart in your book, the chart in your handouts section or on the back of your Chem Helper.

Sodium hydroxide + iron III chloride →

Lead II nitrate + potassium iodide →

Sodium sulfate + lead II nitrate →

5. Combustion Reactions

• Combustion reactions are the burning of a fuel (usually a hydrocarbon like CH4) with oxygen (O2 ) to form CO2 and H2O.

• Methane gas (CH4 ) is burned in a furnace

Propane gas (C3H8) is used to cook in most campers.

First 10 Hydrocarbons

• Methane CH4 • Octane C8H18

• Ethane C2H6 • Nonane C9 H20

• Propane C3H8 • Decane C10 H22

• Butane C4H10

• Pentane C5H12

• Hexane C6H14

• Heptane C7H16

MASS/MASS Problems

• Five Steps:1. Balance the equation2. Find the formula weights of given and

wanted3. Convert the known mass to moles4. Use the mole ratio to get from the given to

the wanted5. Convert the moles to grams of the wanted

Lithium hydroxide is used in the space shuttle to remove exhaled carbon dioxide from the living environment to form

lithium carbonate and water. What mass of CO2 can be absorbed by one Kg of lithium hydroxide?

Sodium bicarbonate is often used as an antacid because it neutralizes HCl secreted by the stomach to form sodium chloride, water and carbon dioxide. How much baking soda is required to

neutralize 6.8 g of HCl?

Summer Job: Preparing lunch at Boy Scout camp

• 400 hungry boy scouts need lunch.• You have 35 loaves of bread, 17 jars of jelly and 2 jars

of peanut butter. • During the sandwich-making process (the “reaction”)

which “reactant” will you run out of first (bread, jelly, or pb?)

• The “reactant” that you run out of 1st will stop the sandwich-making process.

• And, 300 boy scouts are going to tie you up in the knots they just learned to tie and hang you in an oak tree (they just learned to identify trees)

Limiting Reagent Problems

X + Y → XY3g + 4g → both are used up

completely

What would happen if we reacted 3g of X with 5 grams of Y?

Limiting Reactant

• The reactant that is completely consumed when a reaction is run to completion. (think of the scout problem when you ran out of peanut butter)

• Three Steps:1. Find moles you have(÷ grams by formula wt.)2. Find the moles needed (use mole ratio)3. Work a mass/mass problem starting with the

limiting reagent as the given

Ammonia is an important fertilizer used by farmers. It is produced by a famous reaction called the Haber process. Suppose 25,000

grams of N2 and 5000 grams of H2 are mixed to form NH3 . Find the limiting reactant and calculate the amount of ammonia produced.

First, write the balanced equation:

Next, find the moles of each reactant:

Next, use the mole bridge to find the moles NEEDED.

Now that you know the limiting reactant, use it to work a mass/mass problem to find grams of ammonia

Ammonia and copper II oxide yields nitrogen gas, copper, and water vapor. If 18.1 g of ammonia is reacted with 90.4 g of copper II oxide,

which is the limiting reagent? How many g of nitrogen will be formed?

Wood alcohol is methanol (CH3OH). If hydrogen + carbon monoxide yields methanol, write the balanced

equation.

If 68.5 Kg of CO is reacted with 8.60 Kg of hydrogen, find the limiting reagent:

Now, calculate the mass of methanol produced.

If the actual yield is 35,700 g, calculate the % yield of CH3OH.