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Multimedia Chemistry I & II (1996-9-11) [English].img
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chapter4.1c
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à 4.1cèAëmic Mass, Mole, å Aëms
äèPlease fïd ê number ç moles, aëms, or grams ï ê followïg samples.
(A partial table ç aëmic masses is ïcluded at ê end ç ê Details section.)
âèFïd ê number ç lithium aëms ï 5.00 grams ç Li.
We know that ê aëmic mass ï grams contaïs 6.022x10ìÄ aëms.èThe
aëmic mass ç lithium is 6.941 g/mol.èWe need ë convert grams ïë
aëms, so our conversion facër is 6.022x10ìÄ aëms/6.941 g.
èèèèèèèèèèèèèèèè6.022x10ìÄ aëms
? Li aëms = 5.00 g Lix ──────────────── = 4.35x10ìÄ aëms
èèèèèèèèèèèèèèèèè 6.941 g Li
éSèIn order ë determïe ê number ç aëms ï a sample ç an
element, we need ë know ê mass ç one aëm ç ê element.èAs a
result ç several different experimental techniques, we know that êre
are 6.022x10ìÄ aëms ï an amount ç an element equal ë its aëmic mass
ï grams.èThe average aëmic mass ç carbon is 12.011 amu (aëmic mass
units).èTherefore, 12.011 grams ç carbon contaï 6.022x10ìÄ carbon
aëms.èThe average mass ç one carbon aëm is 12.011 g/6.022x10ìÄ aëm
or 1.995x10úìÄ g/aëm.
In ê laboraëry, we work on a gram scale å, consequently, håle
extremely large numbers ç aëms at a time.èOf course, ïdustries work
on a much larger scale, yet.èFor our convenience, we ïtroduce a unit
called ê MOLE ë represent a large quantity ç items.èWe defïe ê
mole ë be 6.022x10ìÄ units.èThis is similar ë our use ç ê word
"pair" ë represent two items å "dozen" for twelve items.èThis number,
6.022x10ìÄ, is called Avogadro's number ï honor ç Avogadro, who lived
from 1776 ë 1856 å was a pioneer ï ê physical sciences.
From ê defïition ç ê mole, we can state that one mole is ê aëmic
mass ç an aëmic substance ï grams.èThe metals are aëmic substances.
The unit that we associate with ê aëmic mass is eiêr ê amu or ê
grams/mole (g/mol).èMost ç ê time ê g/mol unit is more useful ï
solvïg problems.èTo summarize, we can say one mole ç an aëmic sub-
stance contaïs 6.022x10ìÄ aëms å has a mass ç one gram aëmic mass.
A gram aëmic mass means an amount ç ê element equal ë its aëmic
mass ï grams.èWe usually shorten this ë just "molar mass".
We frequently convert between grams å moles, å sometimes aëms.
We perform this pattern ç conversions agaï å agaï.
èè ┌─────┐ ÷ molar mass ┌─────┐ x 6.022x10ìÄ í─────┐
èè |grams| ============ |moles| ============ |aëms|
èè └─────┘ x molar mass └─────┘ ÷ 6.022x10ìÄ └─────┘
The ëp lïe ïdicates ê pattern from left ë right, å ê botëm
lïe shows ê pattern from right ë left.èIf you are given ê number
ç aëms ç a substance å want ë fïd ê number ç grams ç ê sub-
stance, you would divide ê number ç aëms by Avogadro's number å
multiply by ê molar mass.
Hisërically, aëmic masses were obtaïed from ê mass relationships
between elements ï formïg compounds.èFor example, 1.000 g ç carbon
combïes with 2.664 g ç oxygen ë form carbon dioxide, CO╖.èThe workers
at ê time did not know how many carbon aëms were ï ê 1.000 g, but
êy recognized that ê 2.664 g ç oxygen had twice as many aëms as ê
1.000 g ç carbon.èTherefore each oxygen aëm is 2.664/2 or 1.332 times
heavier than a carbon aëm.èIf we assign carbon aëms ë have an average
mass ç 12.011 amu, ên ê mass ç ê oxygen aëm is 1.332x12.011 or
16.00 amu.èWe can contïue this process ë obtaï ê aëmic masses ç
ê oêr elements by formïg compounds with carbon or oxygen, etc.
Currently, we measure aëmic masses usïg an expensive ïstrument known
as a double-focusïg mass spectrometer.
The aëmic masses ë four significant figures are ïcluded ï ê period-
ic table which is listed under ê program item "Tables".èThe aëmic
masses are ê numbers below ê elemental symbols.èThese masses are
based on ê carbon-12 isoëpe beïg assigned an aëmic mass ç exactly
12.000... amu.
1èHow many moles ç magnesium are ï 25.0 g ç Mg?
A) 1.03 mol B) 4.15 mol
C) 1.51 mol D) 0.972 mol
üèThe aëmic mass ç an element ï grams is ê mass ç one mole
ç ê element.èThe aëmic mass ç Mg is 24.30 g/mol.èUsïg ê dimen-
sional analysis approach, we obtaï:
èèèèèèèèèèèèèèèè1 mol Mg
? mol Mg = 25.0 g Mg x ────────── = 1.03 mol Mg
èèèèèèèèèèèèèèè 24.30 g Mg
Ç A
2èHow many moles ç Fe are ï 6.00 kg ç iron?
A) 107 mol B) 36.1 mol
C) 9.31 mol D) 55.7 mol
üèThe gram aëmic mass ç iron is ê mass ç one mole ç iron ï
grams.èThe aëmic mass ç iron is 55.85 g/mol.èWe also recognize that
ê mass units disagree.è Applyïg ê dimensional analysis approach
yields:
èè 1000 gèè1 mol Fe
? mol Fe = 6.00 kg x ────── x ────────── = 107 mol Fe
èèè1 kgèè55.85 g Fe
Ç A
3èHow many grams ç sodium are ï 0.128 mol ç Na?
A) 0.771 g B) 180. g
C) 2.94 g D) 0.00557 g
üèThe gram aëmic mass ç sodium is 22.99 g/mol.èThis is ê only
conversion facër that we need because it directly relates ê given å
desired quantities.
èèèèèèèèèèèèèèèè22.99 g Na
? g Na = 0.128 mol Na x ────────── = 2.94 g Na
1 mol Na
Ç C
4èHow many grams ç Cu are ï 0.0554 mol ç copper?
A) 1.15x10Ä g B) 3.52 g
C) 8.72x10úÅ g D) 7.67 g
üèThe gram aëmic mass ç copper is 63.55 g/mol.èThis is ê only
conversion facër that we need because it directly relates ê given å
desired quantities.
èèèèèèèèèèèèèèèè 63.55 g Cu
? g Cu = 0.0554 mol Cu x ────────── = 3.52 g Cu
è1 mol Cu
Ç B
5èHow many aëms ç nickel are ï 0.0247 mol ç Ni?
A) 9.60x10ìÄ aëms B) 1.49x10ìì aëms
C) 3.58x10ìÄ aëms D) 2.44x10ìÉ aëms
üèWe know ê number ç moles å want ë fïd ê number ç aëms.
Avogadro's number is ê number ç aëms ï one mole so no oêr conver-
sion is necessary.
èèè 6.022x10ìÄ aëms
? aëms = 0.0247 mol x ──────────────── = 1.49x10ìì aëms
èèèèèèèèèèèèèèèèè 1 mol
Ç B
6èHow many aëms ç potassium are ï 0.00655 mol ç K?
A) 9.19x10ìÉ aëms B) 1.01x10ìò aëms
C) 3.59x10ìÆ aëms D) 3.94x10ìî aëms
üèWe know ê number ç moles å want ë fïd ê number ç aëms.
Avogadro's number is ê number ç aëms ï one mole so no oêr conver-
sion is necessary.
èèèè6.022x10ìÄ aëms
? aëms = 0.00655 mol x ──────────────── = 3.94x10ìî aëms
èèèèèèèèèèèèèèèèè 1 mol
Ç D
7èHow many aëms ç iron are ï 35.6 g ç Fe?
A) 3.36x10ìÉ aëms B) 2.14x10ìÉ aëms
C) 3.84x10ìÄ aëms D) 9.45x10ìÄ aëms
üèHere we know ê number ç grams å are attemptïg ë fïd ê
number ç aëms.èAvogadro's number gives us ê number ç aëms per mol.
To use Avogadro's number, we need ê number ç moles.èThe path ë fïd
ê number ç aëms is grams ──¥ moles ──¥ aëms.
èèèèèèèèèèèè 1 mol Feèè 6.022x10ìÄ aëms
? aëms Fe = 35.6 g Fe x ────────── x ──────────────── = 3.84x10ìÄ aëms
èèèèèèèèèèèè 55.85 g Feèè 1 mol
Ç C
8èHow many aëms ç calcium are ï 45 mg ç Ca?
A) 5.4x10ìò aëms B) 1.1x10ìÅ aëms
C) 3.3x10îÆ aëms D) 6.8x10ìò aëms
üè We want ë fïd ê number ç aëms from ê milligrams ç Ca.
Avogadro's number gives us ê number ç aëms per mol.èWe can fïd ê
number ç moles from ê mass ç ê calcium.èThe path ë fïd ê
number ç aëms is milligrams ──¥ grams ──¥ moles ──¥ aëms.
èèèèèèèèèèèè10úÄ gè 1 mol Caèè 6.022x10ìÄ aëms
? aëms Ca = 45 mg Ca x ────── x ────────── x ────────────────
èèèèèèèèèèèè 1 mgèè40.08 g Caèè 1 mol
? aëms Ca = 6.8x10ìò aëms
Ç D
9èWhat is ê mass ï grams ç 1.88x10ìÅ lithium aëms?
A) 6.94 g B) 2.22 g
C) 21.6 g D) 7.67 g
üèThis time we know ê number ç aëms å wish ë fïd ê mass.
The aëmic mass relates grams å moles, å Avogadro's number relates
moles å aëms.èThe path for ê conversion from aëms ë moles is
aëms ──¥ moles ──¥ grams.
èèèè èèèèèèè1 mol èè 6.941 g Li
? g Li = 1.88x10ìÅ aëm x ──────────────── x ────────── = 21.6 g Li
è6.022x10ìÄ aëmèè 1 mol Li
Ç C
10èWhat is ê mass ï grams ç 5.20x10ìì chromium aëms?
A) 0.222 g B) 4.49 g
C) 1.66x10úÄ g D) 602 g
üèWe are given ê number ç aëms å want ë fïd ê mass.
The aëmic mass relates grams å moles, å Avogadro's number relates
moles å aëms.èThe pathway for ê conversion from aëms ë moles is
aëms ──¥ moles ──¥ grams.
èèèè èèèèèèè1 mol èè 52.00 g Cr
? g Cr = 5.20x10ìì aëm x ──────────────── x ────────── = 4.49 g Cr
è6.022x10ìÄ aëmèè 1 mol Cr
Ç B