StoichiometryCalculations with Chemical Formulas and

EquationsChapter 3 BLB 12th

Expectations Balance chemical equations. g ↔ moles ↔ molecules ↔ atoms Find empirical and molecular problems. Calculate amounts of reactants and products. Calculate theoretical and percent yield.

StoichiometryQuantity relationships based on

chemical equations3 Main Concepts:1. Chemical formula – molar ratio of

atoms2. Chemical equations – molar ratio of

compounds3. Law of Conservation of Mass:

mass of reactants = mass of products

3.1 Chemical EquationsComponents: reactants → products Physical states (s, l, g, aq) Reaction conditions (heat Δ, light, solvents, etc.) Coefficients determine molar ratios. The number

of moles of each type of atom must be the same on each side.

Balancing: By inspection Use coefficients; don’t change chemical formulas

Coefficients vs. Subscripts

Fe2S3(s) + HCl(aq) → FeCl3(s) + H2S(g)

KClO3(s) → KCl(s) + O2(g)

HNO3(l) + P4O10(s) → (HPO3)3(l) + N2O5(g)

3.2 Some Simple Patterns of Chemical Reactivity

Combination and Decompositioncombination: A + B → C

4 Fe(s) + 3 O2(g) → 2 Fe2O3(s)

decomposition: C → A + B2 NaN3(s) → 2 Na(s) + 3 N2(g)

3.2 Some Simple Patterns of Chemical Reactivity

Combustion burning of a fuel in the presence of

oxygen products of complete combustion: CO2,

H2O exothermic (produces heat)

Molecular view of methane combustion

3.2 Some Simple Patterns of Chemical Reactivity

CombustionC3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g)

2 CH3OH(g) + 3 O2(g) → 2 CO2(g) + 4 H2O(g)

Each C atom in fuel produces 1 mol CO2

Each H atom in fuel produces ½ mol H2O

3.3 Formula WeightsFormula and Molecular Weights (amu)

formula weight – generalmolecular weight – moleculesformula unit weight – ionic compound

- sum of the atomic masses of each atom in chemical formula

% Composition

% composition by mass Mass of one type of atoms over mass of all

atoms

%100wholepartpercent

3.4 Avogadro’s Number and the Mole

Word association:pair –dozen –case –ream –

amu impractical for lab use (too small) Avogadro’s number: 6.0221421 x 1023

mol-1 The number of atoms in exactly 12 g of

12C For conversions: 6.022 x 1023 ?/mol, where

? can equal atoms, molecules, ions, etc. 1 mole = Avogadro’s number of

anything molar mass – mass in grams of one

mole of a substance, which is equal to the atomic mass in amu; g/mol

3.4 Avogadro’s Number and the Mole

Atoms & compounds have different masses, thus the mass of 1 mole of atoms & compounds are different.

3.4 Avogadro’s Number and the Mole

Conversions: g → mol divide by molar mass mol → g multiply by molar mass

Abbreviations: mole – molmolarity – M

3.4 Avogadro’s Number and the Mole

Practice with Avogadro’s # & the Mole

3.5 Empirical Formulas from Analyses

Empirical formula – smallest whole number ratio of atoms

Molecular formula – actual ratio of atoms in a compound; multiple of the empirical formula; must know molar mass of compound

Use % composition to find formula

Problems

A once-used gasoline additive contains 49.5% C, 3.2% H, 22.0% O, and 25.2% Mn. Determine the emipirical formula of this compound.

Azulene, a hydrocarbon, contains 93.71% C. Its molar mass is ~128 g/mol. Determine the emipirical and molecular formulas for azulene.

3.5 Empirical Formulas from AnalysesSummary:

% data → grams Grams → moles Moles → molar ratio → empirical

formula Empirical formula → molecular

formula

3.5 Empirical Formulas from Analyses

Combustion analysis 1 mol C in fuel → 1 mol CO2

2 mol H in fuel → 1 mol H2O

Problems

The combustion of propane, a hydrocarbon, produces 2.641 g CO2 and 1.442 g H2O. Determine the emipirical formula of propane.

3.6 Quantitative Information from Balanced Equations

3 Main Concepts:1. Chemical formula – molar ratio of

atoms2. Chemical equations – molar ratio

of compounds3. Law of Conservation of Mass:

mass of reactants = mass of products

What’s balanced in a balanced equation?

Stoichiometry ProblemsUse these 4 steps as a guide: (p. 97) Write & balance chemical equation. Convert to moles. Apply molar ratio. Convert from moles to quantity

desired (mass, volume, etc.)

Stoichiometry Problems

How many grams of CaCl2 is produced from taking 2 antacid tablets, each containing 500. mg of CaCO3?

CaCO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + CaCl2(s)

From Sample Exercise 3.16, p. 98

How many grams of HCl are needed to react with 1000 mg of CaCO3?

CaCO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + CaCl2(s)

3.7 Limiting Reactants Limiting reactant – reactant that is

completely consumed; limits the amount of product that can be formed

Theoretical yield – calculated yield of a product based on limiting reactant

Percent yield

100%yieldltheoreticayieldalexperimentyield%

78. Calculate the theoretical yield (in grams) of NO when 2.00 g of NH3 react

with 2.50 g of O2. NH3(g) + O2(g) → NO(g) + H2O(g)

Silver metal reacts with elemental sulfur according to the reaction below. If 2.0 g each of

silver and sulfur react, what is the theoretical yield (in grams) of silver(I) sulfide? How many

grams are left over? 16 Ag(s) + S8(s) → 8 Ag2S(s)

How many grams are left over? 16 Ag(s) + S8(s) → 8 Ag2S(s)

84. When hydrogen sulfide gas is bubbled into a solution of sodium hydroxide, the reaction forms sodium sulfide and water. How many grams of sodium sulfide are formed if 1.25 g of hydrogen sulfide is bubbled into a solution containing 2.00 g of sodium hydroxide, assuming that the sodium sulfide is made in 92.0% yield?

When 0.750 g iron(III) chloride hydrate is heated, 0.300 g of steam is produced. What is

the value of x ?

FeCl3·x H2O(s) → FeCl3(s) + x H2O(g)Δ