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8/3/2019 Chem Chaps 2-3
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General Chemistry ICHEM 201, Fall 2011Bucknell UniversityD. Rovnyak
Chapter 2-3, McMurry and Fay
Sept 6, 2011
Quiz today. After quiz, please turn onclickers.
Please remain quiet while others finish.
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Molecular Compounds
Generally compounds of non-metals
Both elements require prefixes A variety of ratios are allowed and have been preparedin labs - no easy rules as with ionic compounds
No roman numerals here How do we decide on the order (why not O3N2?)
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ex: HBrO
problem 2.25 is excellent practice (answer in backof book)
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Practice
H2Cr2O7 is
a. chromous acidb. chromic acid
c. dichromous acidd. dichromic acid
(hint: focus on suffix of the oxoanion
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Practice
P2O5 is widely called phosphorouspentoxide. This is
a. technically incorrect
b. an informal name accepted by chemistsc. ambiguousd. all of the above
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Practice
tin(ii) phosphate is
a. Sn3(PO4)2b. Sn2(PO4)3
c. SnPO4d. Sn(PO4)2
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Chapter 3starts
Balancing Chemical Equations (a.k.a. reactions)
enforce conservation of mass
count and balance number of atoms
look at the composition of compounds (aka - formula unit)
ex: one MgBr2 has one Mg2+ ion and two Br- ions
cant change identity of compounds given
ex: need another oxygen? dont change NO to NO2
simple equations can be balanced by inspection,
otherwise use a system:
1. write unbalanced equation2. introduce coefficients, solving one atom at a time2a. use fractions if they help but multiply them out to whole
numbers at the end3. divide coefficients to lowest whole numbers4. always CHECK - you will always know if you are right
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Balancing by Inspection
- easy to count atoms in your head?- final equation required no more than 2 coefficients?
-probably ok, but always be 110% certain; again - you shouldalways know if you get these right/wrong
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C3H8 + O2 -----> CO2 + H2O
Using a system
C 3 1
H 8 2
O 2 3
(i) write down the atom counts on each side
(ii) fix one atom at a time, ignoring the rest, often starting with aheavy atom like C or O; to balance Cs, multiply CO2 by3
C3H8 + O2 -----> 3 CO2 + H2O
C 3 3
H 8 2
O 2 7
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(ii) proceed rationally through the remaining atoms; wheneverpossible, select whole number options; O still messy, sofix H next by multiplying H2O by 4
C3H8 + O2 -----> 3 CO2 + 4 H2O
C 3 3
H 8 8
O 2 10
(iv) fix O by multiply O2 by 5
C3H8 + 5 O2 -----> 3 CO2 + 4 H2O
C 3 3
H 8 8
O 10 10
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Photosynthesis
a CO2 + b H2O ------> c C6H12O6 + dO2
hint: in some cases (example: when pure elements are available) think
about addition, not just multiplication
in the fully balanced equation, thecoefficients c and dare
a. 1 and 6b. 2 and 6c. 1 and 10
d. 1 and 3
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The mole
Big picture : molecules and elements react in whole numberratios - we want to have an easy way to keep track of the
number of molecules
But first:atomic mass units: can be summed to get molecular mass
atomic mass of H : 1.00 amuatomic mass of Cl: 35.5 amu
molecular mass* of HCl: 36.5 amu
* molecular mass must also be anisotope weighted average since it isbased on atomic masses, which we
already know are isotope weighted
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The mole
Number of atoms in 12 g of carbon-12
6.0221415 0.0000010 x 1023
http://www.americanscientist.org/issues/pub/2007/2/an-exact-value-for-avogadros-number
review: howmany sigfigs in the
currentvalue ofNA?
aka:Avogadrosconstant,Avogadrosnumber,Avogadroconstant, NA
key facts:
this is a measured number
the NUMBER of things, not their mass
named in honor of Avogadro who realizedgases of equal volumes had equal number ofparticles (he had no knowledge of, or ability todetermine, the mole)
amu will be molar mass with this definition
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amu tells you molar mass
36.5 amu is the mass of a virtual HCl molecule based
on natural abundance weighting
36.5 g HCl contains 6.022 x 1023 molecules of HCl
Q: The atomic mass of N is 14.0 amu and that of oxygen
is 16.0 amu; what mass of N2O4 contains one moleof N2O4 molecules?
a. 30.0 gb. 36.0 gc. 44.0 gd. 92.0 g
In this textwe use NA to
4 significantfigures, avery commonpractice
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practice with atomic andmolecular molar masses
how many molecules of sucrose (C12H22O11) are in37.3 grams of sucrose?
(i) read off amu from periodic table and add up for allatoms in your molecule
12 C atoms * 12.01 amu = 144.12 amu22 H atoms * 1.01 amu = 22.2 amu11 O atoms * 16.00 amu = 176.0 amu
342.3 amu
(ii) therefore the molar mass if 342.3 g / mol
37.3 g *1 mol
342.34 g*6.022x10
23molecules
mol= 6.56 x1022molecules
sucrose
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Molarity is a unit ofconcentration
Molarity =moles solute(mol)
volume solution(L)
1. add known mass of solute
2. fill to line with solvent
Ex: prepare 58.44 g of NaCl ina 100.00 ml vol flask
Molarity isindicated with acapital M andwith squarebrackets. If asolution is 1 Min glucose then:
[glucose] = 1 M
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counting vs mass
1 mol of many different substances including pureelements and complex molecules. All masses are
different, but each has the same number of molecules
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Stoichiometry
in the production of ammonia, hydrogen and nitrogenmolecules react in a ratio of 3:1
3H2 + N2 ----> 2NH3
If I dont want to waste the starting materials, then I want toalways use hydrogen and nitrogen in a 3:1 molecular ratio
Q: How much nitrogen (by mass) will react perfectly with 1.000 kgof hydrogen?
rule: must have balanced equations before doing stoichiometry
hint : whenin doubtconvert tomoles
1000 gH2( )1 molH2
(2 x 1.008) gH2
= 496.0 molH2(i) how many molecules
of H2 do I have?
496.0 molH2( )1 molN23 molH2
=165.3 molN2(ii) need 1/3 times asmany molecules of N2
165.3 molN2( )28.0 gN2
molN2
= 4630 gN2(ii) change moles N2 tomass of N
2
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stoichiometry
H2 (g) + O2 (g) -------> H2O (g)
Q: What mass of hydrogen gas would react exactly with24.0 g of oxygen gas?
a. 3.02 gb. 1.51 g
c. 0.75 gd. 6.04 g