Unit 4: Formula Unit 4: Formula StoichiometryStoichiometry

What is What is stoichiometrystoichiometry??

Deals with the Deals with the quantitativequantitative information in information in chemical formula or chemical reaction.chemical formula or chemical reaction.

Using our reference tables we will learn Using our reference tables we will learn how to calculate the following:how to calculate the following:

Mass of an atomMass of an atom Mass of a compoundMass of a compound Moles of an atom or compound (molar mass)Moles of an atom or compound (molar mass) Percent Composition in a compoundPercent Composition in a compound

Key Terms ReviewKey Terms Review

SubscriptsSubscripts - tell you the amount of - tell you the amount of atoms in a formulaatoms in a formula

CoefficientsCoefficients - tell you the amount of - tell you the amount of molecules of that substancemolecules of that substance

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:

2813

Counting Atoms PracticeCounting Atoms Practice

CaCrOCaCrO44

Ca:Cr: O:

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

Example #2:

114

Counting Atoms PracticeCounting Atoms Practice

CaCa33(PO(PO44))22

Ca:P: O:

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

Example #3:

328

MassMassAtomic Mass Molecular Mass Formula MassMass of one atom of one element from periodic table.

Mass of all of the atoms in a “Covalent” or “Molecular” compound (non-metal atoms only)

Mass of all of the atoms in an ionic compound (metal + non-metal ions)

- All measured in amu (atomic mass units)

Ex)Ex) NeNe Ex)Ex) CHClCHCl33 Ex) CaClEx) CaCl22

Gram Formula Mass & Gram Formula Mass & Gram Molecular massGram Molecular mass

The same as before, but instead of “amu” The same as before, but instead of “amu” Mass expressed in grams Mass expressed in grams (g)(g)

H2O

=O H + H

+ +

+

Molecular Mass = 15.9994 amu + 1.00794 amu +1.00794 amuGram Molecular Mass = 18.0 g

How to calculate molecular or How to 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 by number of atoms, Multiply mass by number of atoms, then get the total of all elements.then get the total of 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 35Cl- 1 x 35 Formula Mass = 58 gFormula Mass = 58 g

Percent Composition by MassPercent Composition by Mass Used to calculate how much Used to calculate how much

of a substance’s mass is of a substance’s mass is made up of a particular made up of a particular element…element…

Formula is given on Table T in Formula is given on Table T in reference tablereference table

Mass of PartMass of PartMass of WholeMass of Whole

X 100 = % Composition “by Mass”

Example 1Example 1:: In a sample of calcium carbonate, what is the In a sample of calcium carbonate, what is the

percent composition by mass of calcium?percent composition by mass of calcium?

Formula = CaCOFormula = CaCO33

Ca: 1 C: 1 O: 3

x 40 = 40 gx 12 = 12 gx 16 = 48 g } total = 100 g

% Composition Calcium = 40 g100 g

X 100 = 40 % Ca

Example 2Example 2:: A compound containing carbon and A compound containing carbon and

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

} total = 16 g

% Composition Carbon = 12 g16 g

X 100 = 75 % Carbon

Formula = C?H?

C = 12 gO = __ g

Hydrated Crystals:Hydrated Crystals: Some ionic compounds have surrounding water Some ionic compounds have surrounding water

molecules.molecules.

These are called These are called Hydrated CrystalsHydrated Crystals..

Percent water hydrationPercent water hydration can be found using can be found using the formula for % composition…the formula for % composition…

Example:Example: CuSOCuSO44 .. 5H5H22OO

55 HH22OO moleculesmolecules are attached to are attached to 11 CuSOCuSO44

Percent Water HydrationPercent Water Hydration (cont.)(cont.)

% Hydration = % Hydration = Total mass of WaterTotal mass of Water x 100 x 100Formula MassFormula Mass

Formula = Formula = CuSOCuSO44 .. 5H 5H22OO

Cu = 1 x 64 = 64 gCu = 1 x 64 = 64 gS = 1 x 32 = 32 gS = 1 x 32 = 32 gO = 4 x 16 = 64 gO = 4 x 16 = 64 g

Separately H2O:

O = 1 x 16 = 16 gH = 2 x 1 = 2 g_________________

H2O = 18 x 5 = 90 g

g

✔

The Mole Pt. 1The Mole Pt. 1 A A molemole is essentially one complete unit of is essentially one complete unit of

any particular substanceany particular substance

Think of it like the term “dozen”Think of it like the term “dozen”

Meaning 12 items of any substance make Meaning 12 items of any substance make up a dozen of that substanceup a dozen of that substance

1 mole 1 mole is always equal to is always equal to 6.02 x 106.02 x 1023 23

particles of a substanceparticles of a substance

The Mole Pt. 2The Mole Pt. 2 Avagadro’s Number Avagadro’s Number

is 6.02 x 10is 6.02 x 102323

Avagadro’s NumberAvagadro’s Number is the number of: is the number of:

- - Atoms in Atoms in 1 mole1 mole of any elementof any element

- Molecules in - Molecules in 1 mole1 mole of any compoundof any compound

- Particles in - Particles in 1 mole 1 mole of any substanceof any substance

The Mole Pt. 3The Mole Pt. 3 The mass of 1 mole will be different for all The mass of 1 mole will be different for all

substances.substances.

Just like a dozen eggs being different from a Just like a dozen eggs being different from a dozen tires (size and weight)dozen tires (size and weight)

Gram Atomic / Formula / Molecular Mass = Gram Atomic / Formula / Molecular Mass =

Molar Mass Molar Mass - - because it is the mass of one because it is the mass of one UnitsUnits(g/mol)(g/mol) mole of any substance.mole of any substance.

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

Molar Mass of Water = 18 g/molMolar Mass of Water = 18 g/mol

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)So…So…

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

Conversion ProblemsConversion Problems Some problems involve mass-mole Some problems involve mass-mole

conversions conversions

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

You will usually have to calculate the You will usually have to calculate the gram formula mass of the gram formula mass of the compound…compound…

# of Moles = given mass (g)gram formula mass

Example 1Example 1- What is the mass of 1.75 moles of oxygen - What is the mass of 1.75 moles of oxygen

gas (Hint: oxygen is diatomic)?gas (Hint: oxygen is diatomic)?1st find the molecular mass of oxygen remember oxygen is a diatomic element (O2)O: 2 x 16 g= 32 g

? = 56 g

Then use formula on table T:

Moles = Given Mass (g)Gram Formula Mass

1.75 = ?32 g

✔

- How many moles are in 42 g of water?- How many moles are in 42 g of water?1st find the molecular mass of H2O: H: 2 x 1 g = 2 gO: 1 x 16 g = 16 g

2 + 16 = 18 g

Convert from grams to moles…? Moles = 42 g

18 g? =2.3 mol

Example 2Example 2

✔

- How many moles of potassium chromate - How many moles of potassium chromate are in a 500 g sample?are in a 500 g sample?

K:2 x 39 g = 78 gCr: 1 x 52 g = 52 gO: 4 x 16 g = 64 g

78+52+64 = 194 g194 g

? = 2.6 mol

Example 3Example 3

1) Find the formula of Potassium Chromate: K2CrO42) Calculate formula Mass:

3) Calculate the Moles using the formula on table T: ? Moles = 500 g

194 g

Empirical Formula ReviewEmpirical Formula Review

In a question you are given the In a question you are given the molecular mass molecular mass and and empirical formulaempirical formula……

11stst you must find the you must find the empirical mass empirical mass

Then divide Then divide empirical mass empirical mass by by molecular massmolecular mass

Multiply all subscripts in the Multiply all subscripts in the empirical formulaempirical formula by your answer to get by your answer to get molecular formulamolecular formula

Empirical MassEmpirical Mass - Is the mass of the empirical formula of a compound

Empirical Mass ExampleEmpirical Mass Example

Step 1: CHStep 1: CH22O O Empirical MassEmpirical MassC – 1 x 12 = 12C – 1 x 12 = 12H – 2 x 1 = 2H – 2 x 1 = 2O – 1 x 16 = O – 1 x 16 = 1616

30 g30 gStep 2: Divide (Step 2: Divide (Molecular massMolecular mass//Empirical massEmpirical mass))

180/30 = 6180/30 = 6

Step 3: Multiply Step 3: Multiply Empirical Formula Empirical Formula by Step 2by Step 26(CH6(CH22O) = CO) = C66HH1212OO66

Molecular Mass = 180 Empirical Formula = CH2O

Moles in Chemical EquationsMoles in Chemical Equations In problems involving chemical reactions In problems involving chemical reactions

we balance chemical reactions using we balance chemical reactions using coefficientscoefficients

These coefficients also represent the These coefficients also represent the mole ratios in a reactionmole ratios in a reaction

Ex) Ex) Moles C2H6 Moles O2 Moles CO2 Moles H2O

2 7 4 61 3.5 2 3

After balancing we can determine the After balancing we can determine the amount of a reactant or product involved amount of a reactant or product involved in the reactionin the reaction

As long as your given the amount of one As long as your given the amount of one ingredient…ingredient…

Ex)Ex)

Moles in Chemical EquationsMoles in Chemical Equations Pt. 2Pt. 2

What is the Minimum # of moles of Oxygen gas needed to make 1 mole of aluminum oxide?

The The mole ratio mole ratio in a balanced reaction can in a balanced reaction can also be used to calculate the also be used to calculate the massmass of of reaction ingredientsreaction ingredients

Ex) Based on the reaction below a Ex) Based on the reaction below a student calculates 20g of Hstudent calculates 20g of H22 will react will react with 20g Clwith 20g Cl22 to form 40g HCl, is this true? to form 40g HCl, is this true?

Moles in Chemical EquationsMoles in Chemical Equations Pt. 3Pt. 3

We can use these We can use these mole ratios mole ratios to calculate to calculate the the volumevolume of reaction ingredients also of reaction ingredients also (only in gas reactions)(only in gas reactions)

Ex) Ex)

Moles in Chemical EquationsMoles in Chemical Equations Pt. 4Pt. 4

Mole ratio = 2 : 7 : 4 : 6

How many Liters of CO2 (g) will be produced from the combustion of 30 Liters of C2H6 (g) ?

30 L2

= 15 L Then, 15 L x 4 = 60 L

A pound of meth is worth A pound of meth is worth 40,000 $40,000 $ 1 lb = 1 lb = ~454 g~454 g Balance the reaction and figure out the Balance the reaction and figure out the

amount of reactants needed to amount of reactants needed to synthesize 454 g of meth…synthesize 454 g of meth…

? g + ? g 454 g