Authored by Fred J. Grieman

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Chem 1a Lecture 4 Average Relative
Mass Avogadros Number Moles Volume Density
HW Assignment Announced in Class
Authored by Fred J. Grieman
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Connection between Macroscopic and Atomic Worlds

• Last time Relative Masses from Chemical Formula,
  Experiment
• Atomic Theory Interpretation
• Discovery of Atom Components
  _____________________
• Need Connection btwn Macro- and micro-scopic
  domains
• Modern Mass Work ____________________

Atoms of Different Masses Separated (remember
positive ions created by losing e-)
Discovery of __________ Atoms of same element
(__) chemically the same, but different
masses (__) ? must be different __
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For example, ________ of carbon (C), one isotope
found much more than other. Major isotope has
_____ and _____ ? _______ called _______,
symbol _____ We have relative masses so we need
to start somewhere Relative Atomic Mass
_________________________ Using this scale
_______________ 13.003354 So what if we pick
up a bunch of carbon?? What mass are we talking
about? ____________________ (pi) of each isotope
remains constant in natural carbon ______________
_________ m(C) ________________________
_____________
______________________________________
11.867
0.1441 12.011 Another example
Si m(Si) p(28Si) A(28Si)
p(29Si) A(29Si) p(30Si)
A(30Si) 0.9221 (27.97693)
0.0470 (28.97649) .0309 (29.97376)
25.80
1.36 0.926
28.09
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Still RELATIVE MASSES! Havent Connected to
ATOMS (Discrete s) Must Start Somewhere
Define ________________________ N0
_________________________ _________________
atoms !!!! Mass connected to Atom
!!!! Mass of N0 of other substances?? Take 13C
m(N0 13C atoms) ___________________ (12C
atoms/N0)
13.003354/12 12 g
13.003354 g !!!! True
for any substance m(N0 C atoms) ___________
g m(N0 Si atoms) ________ g m (N0
Element atoms) A (Element) What about molecules
(compounds)?? Butane C4H10 relative
molecular mass ________________________

_______________________ 58.123 m(N0
C4H10 molecules) ?
______________ g
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Balance (laboratory) what we can deal with -
Atomic Theory (what we want to explore)

• Tired of writing N0 element atoms or N0 compound
  molecules
• Define ______ _____ __ particles 6.022 x
  1023 particles a chemical amount
• 1 mole(substance) _______________
• m(1 mole C) 12.011 g m(1 mole C4H10)
  58.123 g
• These amounts have _______________ of atoms and
  molecules
• These are called _________________ Symbol
  _____ or _____
• M(C) Mc 12.011 g M(C4H10)
  MC4H10 58.123 g
• How does this work? Easy connection between
  atomic macroscopic world
• Mi 2.0156 31.998
  18.015
• 2H2 O2
  ? 2H2O
• 
  ?
  molecules (ATOMIC)
• 
  ?
  molecules
• 
  ?
  moles
• 
  ? 2(18.015) grams
• 
  ? 36.030 grams
  (LAB)

N0
Mi
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Usually work the other way Suppose 100 g (C)
to make C4H10 What mass of hydrogen??

(Balanced Reaction!!! Conservation of
Mass) 100.0 g ? m(H2) ? M (C4H10)
___ atoms ___ molecules ___ molecule
___ moles ___ moles ___
mole 8.326 mol ? mol ?
mol n(H2) ___________________________
10.41 mol (H2) m(H2) _____________________
(10.41 mol) (2.0156 g/mol) 20.98 g If
rxn ? 100, ? n(C4H10) m(C4H10) ?
(student exercise)
m(C) / M(C) 100.0 g / 12.011gmol-1
8.326 mol
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Mass of 1 atom or molecule? m(C atom) m(1 mol
(C)) / N0 12 g / 6.022 x 1023 1.993 x
10-23 g !!! Difficult to Deal With When atomic
scale use atomic mass unit amu symbo u
___________________________________ So,
___________________________________ Makes things
very easy mol (large scale) ? amu (atomic
scale) _____________________________
______
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Volume convenient measure of substances in
lab Connection between Volume and Mass
?? density ____________ mass /
Volume units g / L or g / mL or kg /
m3 Complication Volume function (T,P) ?
__________ must know d at T P Example
d(H2O) 0.9970 g / mL at 25C
d(H2O) 0.9997 g / mL at 10C Suppose
100.0 mL H2O ? mass _________________________
__ (0.9970) (100.0) 99.70 g at
25C (0.9997) (100.0) 99.97 g
at 10C A 0.3 3 ppt difference Conversion
connections
_________________