1. Zumdahl • Zumdahl • DeCoste
World of
CHEMISTRY

2. Chapter 6
Chemical Composition

3. Goals of Chapter 6 Average mass/counting by weighing
Experimental determination of atomic mass
Moles and Avogadro's number
Molar Mass
Convert between moles and mass
Mass percent of elements in compounds
Empirical formulas
Calculating molecular formulas

4. One of most important chemical activities: Synthesis of new substances
Nylon, aspartame, Kevlar (bulletproof vests), PVC, Teflon
All originated in chemist's laboratory
Once they make it – they must determine what it is
What is it's composition?
What is it's chemical formula?

5. Answer: Counting by weighing
Question: How can you determine exactly how many M&M's are in your cup without counting every one?
Answer:
Counting by weighing

6. Answer: No, each is slightly different
Question: Does every M&M weigh exactly the same as all of the other M&M's?
Answer:
No, each is slightly different

7. How can you determine a mass that is typical of each M&M?
Weigh several M&M's and determine the average mass

8. How to determine number of M&M's:
Determine average mass of an M&M by weighing several
Average mass = sum of the masses divided by the number of M&M's weighed
Weigh entire cup of M&M's and divide that weight by the average mass to get the number of M&M's

9. Mathematical Equations:
Av Mass = m1 + m2 + m3 + m4 +m5 5
# M&M's = __total mass (g)__
av mass (g/M&M)

10. Atomic Masses: Counting Atoms by Weighing
Solid carbon reacts with gaseous oxygen to form gaseous carbon dioxide:
C(s) + O2(g) → CO2(g)
1 atom C reacts with 1 molecule O2 to yield 1 molecule of CO2

11. If you have a pile of solid carbon and want to know how much Oxygen you need for the reaction to occur – what do you do?
Need to count carbon atoms – but atoms are too small to count
Use same principle applied to the M&M's and count by weighing

12. Atomic Mass Unit (amu)
Gram & kilogram too large to use because atoms are so tiny
Atomic mass unit is used – much smaller unit than a gram
1 amu = 1.66 x grams
Atoms exist as isotopes
Use average atomic mass (bottom # on periodic table)

13. To find oxygen needed to react with carbon:
Weigh the pile of carbon = 3.00 x 1020 amu
From periodic table: 1 C atom weighs amu
3.00 x 1020 amu x C atom = 2.50 x 1019 C atoms
12.01 amu

14. What is another name for 12 donuts?
A dozen
Other Examples:
Ream of paper
Gross of pencils

15. The Mole
Definition: the number equal to the number of carbon atoms in grams of carbon
Used precise counting techniques to determine this number to be 6.02 x 1023 atoms
Avogadro's number = 6.02 x 1023 atoms
One mole of something consists of 6.02 x 1023 units of that substance

16. 1 dozen eggs = 12 eggs 1 mole eggs = 6.02 x 1023 eggs 1 mole water = 6.02 x 1023 H2O molecules 1 mole Ag = 6.02 x 1023 Ag atoms

17. What if you were offered $1 million to count from 1 to 6 x 1023 at a rate of one number each second? What would be your hourly wage? Would you do it? Could you do it?

18. 6 x seconds is about 2 x 1020 hours or 2 x 1016 years Your hourly wage would by $ per hour If would take hundreds of millions of years to earn a penny In other words – it's impossible

19. A sample of an element with a mass equal to that element's average atomic mass expressed in grams contains 1 mole of atoms.
1 mole Al = 27 grams = 6.02 x 1023 atoms
1 mole Au = 197 grams = 6.02 x 1023 atoms
1 mole Fe = 55.6 grams = 6.02 x 1023 atoms
1 mole S = 32 grams = 6.02 x 1023 atoms

20. This is the molar mass of methane
Chemical compound is a collection of atoms
Methane = CH4 = 1 C atom + 4 H atoms
Mass 1 mol of C = 1 x g = g
Mass 4 mol of H = 4 x g = _4.032 g
Mass of 1 mol of CH4 = g
This is the molar mass of methane

21. Figure 6.3: Various numbers of methane molecules.
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22. Copyright © Houghton Mifflin Company
Molar Mass
Defined as the mass (in grams) of one mole of a substance
Obtain molar mass by summing the masses of the component atoms
For ionic compounds, sometimes referred to as formula weight
Used to be called molecular weight
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23. Example 6.5: Calculate the molar mass of sulfur dioxide – SO2 (exhaust gas that can produce acid rain)
1 mol SO2 molecules = 1 mol S + 2 mol O
Mass of 1 mol S = 1 x g = g
Mass of 2 mol O = 2 x g = g
Mass of 1 mol SO2 = g
The molar mass of sulfur dioxide is g
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24. How many grams of sulfur dioxide do you have if you have 5 moles?
5 mol SO2 x grams = grams SO2
1mol SO2
Used molar mass from previous slide
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25. If you have 800 grams of SO2, how many moles is this?
800 g SO2 x mol SO2 = 12.5 mol SO2
64.07 g
How many molecules is this?
12.5 mol x x 1023 molecules = 7.53 x 1024
1 mole SO SO2 molecules
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26. Percent composition of compounds
Mass fraction mass of element in 1 mol of cmpd
for a given =
element mass of 1 mole of compound
To convert mass fraction to mass percent, multiply mass fraction by 100%
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27. Mass percent: Ethanol (C2H5OH)
Mass of C = 2 mol x g/mol = g
Mass of H = 6 mol x g/mol = g
Mass of O = 1 mol x g/mol = g_
Mass: 1 mol C2H5OH = molar mass = g
To find mass percent, divide total mass of each element by molar mass of ethanol (x 100%)
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Mass % X = mass of X in 1 mol ethanol x100% mass of 1 mole of ethanol Mass % C = g x 100% = 52.14% g Mass % H = g x 100% = 13.13% g Mass % O = g x 100% = 34.73% g Mass percentages of all of the elements should add up to 100% (52.14% % % = %)
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Formulas of Compounds
Sometimes a chemical reaction gives a product that has never been formed before:
Identify compound by determining which elements are present and how much of each is there
Previously, we used formula to determine the mass of each element present in a mole – to obtain the formula of an unknown compound – do the opposite
Use measured masses of elements present to determine the formula
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Determining Formula
Formula represents relative numbers of the various types of atoms present
To determine formula – count the atoms
How do we count atoms? - By weighing!
Change mass into moles
Change moles into atoms
Use ratios to determine formula
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If we have a compound that contains only carbon, hydrogen, and oxygen and we weigh out grams for analysis. The sample is analyzed and found to contain g C, g H, and g O. What is the compound? Step 1: Convert masses of elements to moles Step 2: Convert moles of elements to atoms
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Calculations show the same number of C and O atoms Calculations show twice as many H atoms as C atoms and O atoms Can write formula as CH2O But formula could also be C2H4O2 Must be in 1:2:1 ratio
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Empirical Formula
Formula of a compound that expresses the smallest whole number ratio of the atoms present (simplest formula)
In a class the girl to boy ratio is 1:8
This could mean the class has 1 girl & 8 boys OR
This could mean the class has 2 girls & 16 boys OR
This could mean the class has 3 girls & 24 boys
etc.
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34. Steps for Determining Empirical Formula of a Compound
Obtain mass of each element present (grams)
Determine # moles present for each atom
Divide # of moles by smallest # of moles (converts smallest # to 1). If all numbers are integers, these will be the subscripts, if not, go to step 4
Multiply numbers in step 3 by the smallest integer that will convert them to whole numbers. These whole numbers are the subscripts
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Molecular Formula
The actual formula of a compound
Gives the composition of the molecules that are present
Need molar mass to determine
Our mystery compound could be glucose:
C6H12O6
(C to H to O ratio is 1:2:1)
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36. Determining Molecular Formula
Always a multiple of empirical formula
(Empirical formula)n = molecular formula
n = molar mass______
empirical formula mass
for glucose n = 6: (CH2O)6 = C6H12O6
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37. Figure 6.5: Ball-and-stick model of P4O10.
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