Exam 1b (PS2 and PS3 material) Comments

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Exam 1b (PS2 and PS3 material) Comments
Ppt07(PS4)

• Material starting with this Ppt is not on the
  exam. Ppt06 is the end of PS3 (and thus Exam 1b)
  material)

• You need to be able to distinguish between the
  mass of a single atom (isotopic mass) and the
  mass number (both of these apply to a particular
  isotope of an element)

• You need to be able to distinguish between the
  mass of a single atom (isotopic mass) and the
  atomic mass of an element (i.e., weighted
  average of the isotopic masses)
• The atomic mass is that decimal number on the
  periodic table

• Learn the meaning of complete atomic symbols,
  subscripts, superscripts (left and right), etc.

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Exam 1b Comments (cont.)

• You must memorize/learn the names of elements and
  ions (and charges on ions) on the handout sheet
• But ALSO learn how to use the neutrality
  principle and understand the meaning of
  subscripts and superscripts (charges)! There is
  a lot more to nomenclature than memorization of
  names!
• Dont forget that binary molecular compounds have
  a different set of naming rules
• Learn the way to recognize and name acids

• This listing is not even close to being
  exhaustiveanything related to any part of any
  problem on either PS2 or PS3 is fair game.

• Practice Exam for 1b its key are posted

• 1st portfolio check (grade) will be during exam

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Plan (Application of the Counting By Weighing
Idea From Ppt05)

1. Example Determining the ratio of atoms in a
   compound (subscripts in formula)
2. Example Determining the number of FU in a
   sample of H2O
3. A mole of a chemical substance is a certain
   number of FUs. later

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I. Example Determining the ratio of atoms in a
compound (subscripts)

• A (hypothetical, teensy-weensy) sample of a
  compound (lets pretend we did not know about
  ions yet) is separated chemically to form 258000
  amu of Li(s) and 384000 amu of P(s).

0. Are there more atoms of Li or P in the sample?

• If youve already answered, youre probably
  wrong!!
• ? Remember amu is a mass unit it doesnt (by
  itself) tell you how many items!

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Apply counting by weighing idea!

• lbs divided by lbs per BB BBs

• amus divided by amus per atom atoms!

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.258000 amu of Li(s) and 384000 amu of P(s)
1. How many atoms (really ions) of each?
More Li atoms (even though less mass)!
Why? A Li atom has much less mass than a P atom!
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2. What is the ratio of Li atoms P atoms in the
compound?
3. What is the (reduced) chemical formula?
? Li3P
? subscripts tell you exact composition of one
FU, and thus the ratio in any-sized sample!!
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4. How many FUs of Li3P were in the original
sample?

• Reminder (what we have already determined)
• 37170.4 37200 Li atoms
• 12399.0 12400 P atoms
• Formula of compound is Li3P

Recall FUs?
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___ FU of FeCl3

• Answer 12400 FU of Li3P!

• The FUs of Li3P P atoms in this case
  because there is one P atom per FU

• Trike analogy trikes seats (but not
  wheels!)

----- Really ions
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5. Another way? (to get the number of FU of
compound in the sample)

• What is the (average) mass of one FU of Li3P
  (called the Formula MassSee Exp 6)?

? (3 x 6.941 amu) (1 x 30.97 amu) 51.793
amu (per FU)

• Mass of original sample?
• 258000 amu of Li and 384000 amu of P
• ? Sum 642000 amu (of compound)

• Bowling ball idea all over again!

(Same as before)
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Take a step back

• We can
• Determine the number of atoms from total mass of
  atoms if we know mass per atom
• If two sets of atoms came from a sample of a
  compound, we can find
• the number of atoms of each and then find
• the ratio of atoms, which tells us the
• (Reduced) Formula of the compound subscripts
• Determine the number of FUs of a substance from
  total mass if we know the
• mass per FU

? Called the formula mass (technically should
be FU mass)
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Summary Table (Generalized Mines FU and
Formula Mass Examples)
Formula Basic Units making up the substance (Composition of) 1 Formula Unit Formula Mass (Average mass of one Formula Unit)
Fe Atoms 1 atom of Fe 55.85 amu
O2 Molecules 2 O atoms (together as one molecule) 32.00 amu
CO2 Molecules 2 O atoms and one C atom (together as one molecule) 44.01 amu
CoCl2 (an anhydrous compound) Ions 1 Co2 ion and 2 Cl- ions 129.84 amu
CoCl2?6H2O (a hydrate!) Ions and Water Molecules 1 Co2 ion, 2 Cl- ions, and 6 H2O molecules 237.93 amu
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II. Example Determining the number of molecules
(FU) in a sample of H2O (without moles or
molar mass)

• How many FU (molecules, here) of H2O are in 150.
  g of water?

Careful! g ? amu!!!
? 1 amu 1.6605 x 10-24 g
formula mass (like lb/BB)
conversion
Plan g ? amu
? FU
(bowling balls)
(lbs)
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The number of molecules (FU) in 150. g of H2O
(continued)
for H2O?
formula mass (like lb/BB)
amu ? FU
(bowling balls)
(lbs)
Thats a LOT of molecules (in a fairly small
glass of water)!!!
5,010,000,000,000,000,000,000,000
8,000,000,000,000,000,000
Two estimates of the of sand grains on Earth!!
5,000,000,000,000,000,000,000