Transcript Plan, Ppt10(PS4) 1. Finish EF  From % masses

Plan, Ppt10(PS4)
1. Finish EF (Tro, 3.9)
 From % masses
 From combustion analysis data
2.
Chemical Reaction (Tro, 3.10)
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3.
Chemical Equation (Balanced) [later]
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4.
Process (takes time!); original subs  new subs
Fixed mass ratio of substances react and form
Whole-number ratio of FUs react and form
Coefficients represent ratio of FUs when reaction occurs
Various problem types (“stoichiometry”) [later] (Tro, 4.2)
Ppt10(PS4)
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Empirical Formula (EF) Quick Review
•
Need to find the subscripts
 lowest whole-number ratio of atoms
 lowest whole-number ratio of MOLES (of
atoms)
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Determining the Empirical Formula of a
Compound
• From mass data (of each element A, B, C, …) of a sample:
masses (of A, B, C, …)  moles (of A, B, C, … atoms)  ratio of moles
 reduced (whole-number) ratio of moles  formula (subscripts)
• From % mass data (of each element A, B, C, …)
– Get mass data, and do as above!
– Assume 100 g of compound for convenience*
• 20.4 % of 100 g = 20.4 g! (etc.)
 *NOTE: If molar mass is also given, it may be easiest to use that mass
(rather than 100 g). You will then get the molecular formula first, which
is perfect, if that is asked for. If only EF is asked for MF can be reduced.
Ppt10(PS4)
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From Green Handout Sheet
(PS4 & 5 Practice)
• What is the empirical formula of
tetraethyllead, the “lead” in gasoline?
The composition is: 29.71% C, 64.06% Pb,
6.23% H (These percentages are by mass)
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PS Sign-Posting
• The concepts and skills related to
problems 15, 19, [and 20 and 21] on PS4
have been covered now in this (and prior)
PowerPoint(s). Give those problems a try
now!
Ppt10(PS4)
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From Combustion Analysis Data
• Often cannot easily get the elements out
• So, react with oxygen:
– All C atoms end up in CO2 molecules!
– All H atoms end up in H2O molecules!
• Figure out C in sample from mass of CO2
• Figure out H in sample from mass of H2O
• Figure out a 3rd atom type (O, N, S, …) by mass
difference (as in Exp 4—get Cl “by difference”)
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Combustion Analysis Figure 3.10
e.g., excess O2,
sulfur oxides,
nitrogen oxides,
halogen-oxides
, etc.
e.g., Mg(ClO4)2
Ppt10(PS4)
e.g., NaOH
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From Green Handout Sheet
(PS4 & 5 Practice)
• A hydrocarbon of unknown formula CxHy was
submitted to combustion analysis with the
following results. What is the empirical
formula of the hydrocarbon?
Ppt10(PS4)
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From Green Handout Sheet
(PS4 & 5 Practice)
• Coniine, a toxic substance isolated form
poison hemlock, contains only carbon,
hydrogen, and nitrogen. Combustion analysis
of a 5.024 g sample yields 13.90 g of CO2 and
6.048 g of H2O. What is the empirical formula
of coniine?
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Coniine/Combustion Analysis Problem
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PS Sign-Posting
• The concepts and skills related to
problem 18 on PS4 have been covered
now in this PowerPoint. Give that problem
a try now!
[end of PS4 material]
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Discussion of Meaning of a Balanced
Chemical Equation
• Reaction vs. equation (different!)
– Most chemistry texts tend to discuss this initially but then
use the words interchangeably later on(!) Be careful! 
Chemical Reaction
Chemical Equation
◦ Initial substances (reactants)
turn into new substances
(products)
◦ Takes time (a process)
◦ Involves actual amounts
◦ Represents (a type of) chemical
reaction (but not “literally”)
◦ Symbols (not “reality”), on paper
• Formulas, coefficients, an
arrow
• Can be expressed in a variety
of ways: grams, moles, L, etc.
◦ Does not involve actual
amounts! Only RATIO of
combination and formation
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Discussion of Meaning of a Balanced
Chemical Equation (Example)
Chemical Reaction
Chemical Equation
4 g of methane react with 16
grams of oxygen to produce 11 CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g)
grams of carbon dioxide and 9
grams of water, all as gases.
The reaction occurs over some
period of time, during which
the number of molecules of
methane and oxygen steadily
decrease as the number of
molecules of carbon dioxide
and water increase.
Ppt10(PS4)
• Coefficients represent a ratio
only, and they represent the
ratio of formula units, most
fundamentally.
• Here, a 1 : 2 : 1 : 2 ratio of
molecules of CH4, O2, CO2,
and H2O but only as reaction
occurs.
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Discussion of Meaning of a Balanced
Chemical Equation
• Analogy (hypothetical): Working at Lawson
products
25 boxes packed up → $15 in pocket
• How much money is in my pocket after the
“reaction” occurs?
• The equation alone can’t answer that!
– How much “reaction” occurred? (not indicated by equation!)
– How much was in your pocket before the reaction?
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