1. Plan, Ppt10(PS4) Finish EF (Tro, 3.10) From % masses
From combustion analysis data
Chemical Reaction (Tro, 3.11)
Process (takes time!); original subs  new subs
Fixed mass ratio of substances react and form
Whole-number ratio of FUs react and form
Chemical Equation (Balanced) [later]
Coefficients represent ratio of FUs when reaction occurs
Various problem types (“stoichiometry”) [later] (Tro, 4.2)
Ppt10(PS4)

2. Empirical Formula (EF) Quick Review
Need to find the subscripts
 lowest whole-number ratio of atoms
 lowest whole-number ratio of MOLES (of
atoms)
Ppt10(PS4)

3. 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)

4. From Green Handout Sheet (PS4 & 5 Practice)
What is the empirical formula of tetraethyllead, the “lead” in gasoline?
The composition is: % C, 64.06% Pb, 6.23% H (These percentages are by mass)
Ppt10(PS4)

5. Ppt10(PS4)

6. PS Sign-Posting
The concepts and skills related to problems 10 – 12 on Written PS4 have been covered now in this (and prior) PowerPoint(s). Give those problems a try now!
[the only problem on Written PS4 whose concepts / skills have not yet been covered in PowerPoints is #13, on combustion analysis]
Ppt10(PS4)

7. EF 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”)
Ppt10(PS4)

8. EF From Combustion Analysis (C.A.) Data (flowchart)
Sample of Compound CaHbXc is subjected to C.A.
g CaHbXc -
g CO2 → mol CO2 → mol C
→ g C
g H2O → mol H2O → mol H
→ g H
mol X ←
g X
Subscript ratio
Empirical Formula
Ppt10(PS4)

9. Combustion Analysis Figure 3.13
e.g., excess O2,
sulfur oxides, nitrogen oxides, halogen-oxides , etc.
e.g., Mg(ClO4)2
e.g., NaOH
Ppt10(PS4)

10. 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)

11. 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 g sample yields g of CO2 and g of H2O. What is the empirical formula of coniine?
Ppt10(PS4) 11

12. Coniine/Combustion Analysis Problem
Ppt10(PS4) 12

13. PS Sign-Posting
The concepts and skills related to problem 13 on Written PS4 have been covered now in this PowerPoint. Give that problem a try now!
[end of PS4 material]
Ppt10(PS4)

14. 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
Initial substances (reactants) turn into new substances (products)
Takes time (a process)
Involves actual amounts
Can be expressed in a variety of ways: grams, moles, L, etc.
Chemical Equation
Represents (a type of) chemical reaction (but not “literally”)
Symbols (not “reality”), on paper
Formulas, coefficients, an arrow
Does not involve actual amounts! Only RATIO of combination and formation
Ppt10(PS4)

15. Discussion of Meaning of a Balanced Chemical Equation (Example)
Chemical Reaction
4 g of methane react with 16 grams of oxygen to produce 11 grams of carbon dioxide and 9 grams of water, all as gases.
Chemical Equation
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g)
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.
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.
Ppt10(PS4)

16. 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?
Ppt10(PS4)