Formula StoichiometryFormula Stoichiometry

What is What is stoichiometrystoichiometry??

Deals with the specifics of QUANTITY Deals with the specifics of QUANTITY in chemical formula or chemical in chemical formula or chemical reaction.reaction.

Review: Counting Atoms Review: Counting Atoms

Remember Remember subscriptssubscripts tell you how tell you how many atoms…many atoms…

… … CoefficientsCoefficients tell you how many tell you how many TOTAL molecules compoundsTOTAL molecules compounds

How many oxygen atoms in each?How many oxygen atoms in each?

NHNH44NONO33

CC88HH88OO4 4

OO33

CC33HH55(NO(NO33))33

(3)

(4)

(3)

(9)

Counting Atoms PracticeCounting Atoms Practice

(NH(NH44))22COCO33

N:

H:

C:

O:

* First list the types of atoms and then count each.

Example #1:

2

8

1

3

Counting Atoms PracticeCounting Atoms Practice

CaCrOCaCrO44

Ca:

Cr:

O:

* First list the types of atoms and then count each.

Example #2:

1

1

4

Counting Atoms PracticeCounting Atoms Practice

CaCa33(PO(PO44))22

Ca:

P:

O:

* First list the types of atoms and then count each.

Example #3:

3

2

8

**Do the rest on your own!**

Atomic MassAtomic Mass

Mass of an atom of one element Mass of an atom of one element based on based on percent abundances and percent abundances and masses of all isotopes (on PT)masses of all isotopes (on PT)

Units: amuUnits: amu

Molecular MassMolecular Mass

Mass of a Mass of a molecular compound (ex: molecular compound (ex: HH22O, COO, CO22))

Units: amuUnits: amu

Formula MassFormula Mass

Mass of an Mass of an ionic compound (ex: NaCl, ionic compound (ex: NaCl, MgIMgI22))

Units: amuUnits: amu

Gram Formula MassGram Formula Mass

Mass expressed in gramsMass expressed in grams

Gram Molecular MassGram Molecular Mass

H2O

=O H + H

+ +

+

Molecular Mass = 15.9994 amu + 1.00794 amu +1.00794 amu

Gram Molecular Mass = 18.0 g

How do I calculate molecular or How do I calculate molecular or formula mass?formula mass?

First:First: Identify and count atoms in the Identify and count atoms in the compound.compound.

Second:Second: Locate atomic mass of each Locate atomic mass of each element.element.

Third:Third: Multiply mass x number of Multiply mass x number of atoms and total all elements.atoms and total all elements.

Molecular Mass Example Molecular Mass Example (Covalent Compound)(Covalent Compound)

Example: HExample: H22OO H- 2 x 1H- 2 x 1 O- 1 x 16O- 1 x 16 Molecular Mass = 18 gMolecular Mass = 18 g

Formula Mass Example Formula Mass Example (ionic compound)(ionic compound)

Example: NaClExample: NaCl Na- 1 x 23Na- 1 x 23 Cl- 1 x 35.5Cl- 1 x 35.5 Formula Mass = 58.5 gFormula Mass = 58.5 g

Percent Composition by MassPercent Composition by Mass

Experimentally-Experimentally- Use masses given in problemUse masses given in problem

Mass of ElementMass of Element x 100 x 100

Mass of CompoundMass of Compound

Example:Example:

A compound containing carbon and A compound containing carbon and hydrogen has a mass of 16 grams. hydrogen has a mass of 16 grams. When decomposed, 12.0 grams are When decomposed, 12.0 grams are found to be carbon. What is the found to be carbon. What is the percent by mass of carbon in the percent by mass of carbon in the compound? compound?

Theoretically-Theoretically- Use Atomic MassesUse Atomic Masses FormulaFormula

(#atoms of element) x (atomic mass)(#atoms of element) x (atomic mass) x100 x100

Formula mass of compoundFormula mass of compound

Percent Composition by Atomic Percent Composition by Atomic MassMass

Hydrated CrystalsHydrated Crystals

Some ionic compounds are found to have Some ionic compounds are found to have surrounding water molecules.surrounding water molecules.

These compounds are called These compounds are called Hydrated Hydrated CrystalsCrystals..

The The percent water hydration percent water hydration can be can be found.found.

ExampleExample

CuSOCuSO44 .. 5H5H22OO

The dot shows that The dot shows that 5 H5 H22O molecules are O molecules are attached to 1 CuSOattached to 1 CuSO4 4 moleculemolecule. .

Percent Water HydrationPercent Water Hydration

% Hydration = % Hydration = Total mass of WaterTotal mass of Water x 100 x 100

Formula MassFormula Mass

CuSOCuSO44 .. 5H 5H22OO

Cu – 1 x 63.5 = 63.5Cu – 1 x 63.5 = 63.5

S – 1 x 32 = 32S – 1 x 32 = 32

O – 4 x 16 = 64O – 4 x 16 = 64

HH22O – 5 x 18 = 90O – 5 x 18 = 90

Formula Mass = 249.5Formula Mass = 249.5

% Hydration = (90/249.5) x 100% Hydration = (90/249.5) x 100

= 36% H= 36% H22OO

The MoleThe Mole

Avagadro’s NumberAvagadro’s Number is the number is the number of atoms in of atoms in 1 mole1 mole of any substance. of any substance.

Avagadro’s Number is 6.02 x 10Avagadro’s Number is 6.02 x 102323

That means that in 1 mole of any That means that in 1 mole of any sample of matter there are 6.02x10sample of matter there are 6.02x102323 particles.particles.

The MoleThe Mole There are 6.02 x 10There are 6.02 x 102323 particlesparticles in one in one

molemole of any substance of any substance

One particle can be one One particle can be one atomatom or one or one moleculemolecule..

The mass of 1 mole will be different for all The mass of 1 mole will be different for all substances.substances.

Atomic / Formula / Molecular Mass = Atomic / Formula / Molecular Mass = Molar MassMolar Mass because it is the mass of 1 because it is the mass of 1 mole of any substance.mole of any substance.

Example: Example: 1 mole of H1 mole of H22O = 18gO = 18g

Mole Relationships or EqualitiesMole Relationships or Equalities 1 mole = formula mass in grams1 mole = formula mass in grams

1 mole = 6.02 x 101 mole = 6.02 x 102323 particles particles

1 mole = 22.4L (for gases)1 mole = 22.4L (for gases)

Formula mass = 6.02x10Formula mass = 6.02x102323 particles particles

22.4L of a gas = 6.02x1022.4L of a gas = 6.02x102323 particles particles

Formula mass = 22.4L of a gasFormula mass = 22.4L of a gas

Example ConversionsExample ConversionsConverting Grams to Moles

given grams XGFM

1 moles

grams

Converting Moles to Grams

given moles Xmoles

grams

1

GFM

Table TTable T For problems that involve mass mole For problems that involve mass mole

conversions you can use the “Mole conversions you can use the “Mole Calculation” formula on Table T.Calculation” formula on Table T.

You will usually have to calculate the You will usually have to calculate the gram formula mass of the substance gram formula mass of the substance unless it is given.unless it is given.

See Table TSee Table T

ExamplesExamples

Example #1: What is the mass of 1.75 Example #1: What is the mass of 1.75 moles of oxygen gas (Hint: oxygen is moles of oxygen gas (Hint: oxygen is diatomic)?diatomic)?

1st find the molecular mass of oxygen remember oxygen is a diatomic element (O2).

O: 2 x 16 g= 32 g

Convert from moles to grams using factor label method

Factor Label Setup

1.75 mol x __32 g _ = ? g

1 mol

56 g

ExamplesExamples

Example #2: How many moles are in Example #2: How many moles are in 42 g of water?42 g of water?

1st find the molecular mass of H2O.

H: 2 x 1 = 2

O: 1 x 16 = 16

2 + 16 = _18 g_

1 mol

Convert from grams to moles…

42 g x __ 1 mol _ = ? mol

18 g

2.3 mol

ExamplesExamplesExample #3: How many moles of Example #3: How many moles of

potassium chromate are in a 500 g potassium chromate are in a 500 g sample?sample?

1st find the molecular mass of potassium dichromate (K2CrO4).

K:2 x 39 = 78

Cr: 1 x 52 = 52

O: 4 x 16 = 64

78+104+112 = _194 g_

1 mol

Convert from grams to moles…

500 g x __ 1 mol _ = ? mol

194 g

2.6 mol

Empirical MassEmpirical Mass

Step 1: CHStep 1: CH22O Empirical MassO Empirical Mass

C – 1 x 12 = 12C – 1 x 12 = 12

H – 2 x 1 = 2H – 2 x 1 = 2

O – 1 x 16 = O – 1 x 16 = 1616

3030

Step 2: Calculate Multiple (Formula Step 2: Calculate Multiple (Formula mass/Empirical mass)mass/Empirical mass)

180/30 = 6180/30 = 6

Step 3: Multiply Empirical Formula by Step 3: Multiply Empirical Formula by Multiple Multiple

6(CH6(CH22O) = CO) = C66HH1212OO66