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Stoichiometry
Chemical Equations
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Short hand way to represent chemical reactions
H2 + Cl2 → HCl
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Symbols
+ = reacts with
→ = produces, yields
Δ = adding energy (usually heat)
↔ = reversible
aq - aqueous
s - solid
l - liquid
Balancing Reactions
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Law of Conservation of mass
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Change coefficients not subscripts why?
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Start balancing with elements other than H and
O
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Next balance H and then O last
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Trial and Error
Balancing Practice
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__Al +__HCl → __AlCl3 + __H2
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__H3PO4 +__ HCl → __PCl5 + __ H2O
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__ CO(g) + __H2(g) → __C8H18(l) + __H2O
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__C2H6 + __O2 → __H2O + __CO2
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__(NH4)3PO4 + __Pb(NO3)4 → __Pb3(PO4)4 +
__NH4NO3
Types of Reactions
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Combination – A + B → C
N2(g) + 3H2(g) → 2NH3(g)
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Decomposition – C → A + B
2KClO3(s) → 2KCl(s) + 3O2(g)
Combustion – oxidation reaction that is
exothermic
Most often involves O2
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
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Single Replacement – AB + C → A + BC
Use activity series
Formula Weights
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Definition – sum of the atomic weights of each
atom
Molecular weight – formula weight of a
molecule
Calculate the formula weight of:
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Ca(MnO4)2
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KH2PO4
Percent Compostion
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Definition - percent of mass contributed by each
element
% Comp = # of a particular atom x atomic mass
Total mass of the compound
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Calculate the percentage of phosphorus in
P4O10.
Mole
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Avogadro's # - 1 mole = 6.02x1023 objects
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Representative Particles
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Formula Units
Molecules
Atoms
Calculate the number of oxygen atoms in 1.5
moles of sodium carbonate.
Mole
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Molar mass – mass of one mole of a compound
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Check if you are working with an isotope
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Find the sum of the masses of the atoms
Unit is g/mol
Grams ← use g/mol → Moles ← use
Avogadro's # → Representative particles
Mole Practice
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What is the mass, in grams of 6.33 mol of
NaHCO3?
How many nitric acid molecules are in 4.20
grams of HNO3? How many oxygen atoms?
Empirical and Molecular Formula
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Finding Empirical Formula
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Assume 100 g
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Convert from grams to moles
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Divide by smallest # of moles
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Use results as subscripts
Convert Empirical to Molecular
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Divide molar mass by empirical mass
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Multiply empirical formula by result
Molecular Formula Example
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What is the molecular formula of a compound
that is 71.65% Cl, 24.27% C, 4.07% H and has
a molar mass of 98.96g/mol?
Molecular Formula Practice
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Caffeine contains 49.48% C, 5.15% H, 28.27%
N, 16.49% by mass and has a molar mass of
194.2 g/mol. Determine the molecular formula
of caffeine.
Molecular Formula Example
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Isopropyl alcohol is made up of C,H, and O.
Combustion of 0.255g of this alcohol produces
0.521g CO2 and 0.306g H2O. What is the
molecular formula if the molar mass is 60 g/mol.
Molecular Formula Practice
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Caproic acid, which is responsible for the foul
odor of dirty socks, is composed of C, H, and O
atoms. Combustion of a 0.255g sample of this
compound produces 0.512 g CO2 and 0.209g
H2O. What is the molecular formula is the molar
mass is 116 g/mol?
Calculations from Balances
Equations
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First ensure that equation is balanced.
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Use mole ratios
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Grams A → Moles A → Moles B → Grams B
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When potassium chlorate is heated it
decomposes in potassium chloride and oxygen.
How many grams of oxygen can be prepared
from 4.50g of potassium chlorate.
Calculations practice
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Milk of magnesia (Mg(OH)2) is often used as an
antacid. It neutralizes excess hydrochloric acid
secreted by the stomach. How many grams of
milk of magnesia are needed to remove 150 mg
of hydrochloric acid?
Limiting Reagents
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Limiting Reagents – reactant that runs out
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Excess Reagents – reactant that is left over
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Limiting reagent gives you the least amount of
product
Limiting Reagent Example
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A strip of zinc metal weighing 2.00g is placed in
an aqueous solution containing 2.50g of silver
nitrate. Silver and zinc nitrate are produced.
Which reactant is limiting? How many grams of
each product will form? How grams of excess
reagent will be left over?
Limiting Reagent Practice
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Nitrogen gas can prepared by passing gaseous
ammonia over solid copper (II) oxide at high
temperatures. The other products of the
reaction are solid coper and water vapor. If a
sample containing 18.1g NH3 is reacted with
90.4g CuO, which is the limiting reagent? How
many grams of N2 are made? How much
excess reagent is left?
Yield
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Theoretical Yield – amount of product formed
when all of the LR is used up
Actual Yield – amount of product formed during
an experiment
Percent Yield –
actual yield
theorectical yield
x 100%
Yield Example
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When iron (III) oxide react with carbon
monoxide to form iron and carbon dioxide. If
you start with 150g of iron (III) oxide as the
limiting reagent, what is the theoretical yield? If
the actual yield of iron was 87.9g, what was the
percent yield?
Yield Practice
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Methanol (CH3OH) is used as a fuel in race
cars. It can be manufactured by a combination
of gaseous carbon monoxide and hydrogen. If
68.5 kg of CO is reacted with 8.60 kg H2.
Calculate the theoretical yield of methanol. If
3.57x104g of methanol is produced, what is the
percent yield of methanol?
Homework
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6, 8, 18, 30, 46, 48, 56, 58, 60, 70, 74, 76, 78
Homework
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6, 8, 18, 30, 46, 48, 56, 58, 60, 74, 76, 78