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Multimedia Chemistry I & II (1996-9-11) [English].img
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à 1.3èDimensional Analysis
äèPlease solve ê followïg conversion problems usïg dimensional analysis.
Note: "details" contaïs some conversion facërs.
âèA gasolïe tank holds 15 gallons.èWhat is ê volume ï liters?
Knowïg that 1 gal equals 3.784 L, we multiply ê given number ç
gallons by ê number ç liters per gallon.
èèèèèèèèèèè 3.784 L
èèèè? L = 15 gal x ───────è= 57 L
èèèèèèèèèèèè1 gal
éSèConversion Facërs:
MultipliersèèèèèèLengthèèèèèè MassèèèèèèèèèVolume
kilo, kè= 10Äèèè1 ï = 2.54 cmèè 1 lb = 453.6 gèèè1 gal = 4 qt
milli, m = 10úÄèè 1 mi = 1.609 kmèè1 kg = 2.2046 lbèè1 qt = 4 cups
nano, nè= 10úöèè 1 mè= 39.37 ïèèèèèèèèèèèè1 L = 1.057 qt
pico, pè= 10úîìèè1 mè= 3.281 ft
In ê previous sections, we discussed makïg measurements.èEvery
measurement that you make has units associated with ê measurement.èIf
you weigh a compound ï ê lab, you most likely would use a balance that
is calibrated ë measure ï grams.èThe units ç your weighïg process
would be grams.
You will discover or may have discovered already that most chemical calc-
ulations can be viewed as conversions from one set ç units ïë anoêr
set ç units.èSometimes this conversion can be accomplished ï one step.
Oêr cases might require several conversions.èRegardless ç ê number
ç steps we follow ê same general procedure.
1) Identify ê units ç ê desired quantity (your goal).
2) Identify ê units ç ê given quantity (your startïg poït).
3) Develop ê facër or facërs ë convert ê given quantity ïë ê
è desired quantity.
4) Multiply ê given quantity by ê conversion facër(s) such that êè
è units cancel ë leave ê units ç ê desired quantity.
è í────────────────┐è í──────────────┐è í────────────────────┐
è |Desired Quantity| = │Given Quantity│ x │Conversion Facër(s)│
è └────────────────┘è └──────────────┘è └────────────────────┘
For example, let's assume you are goïg ë buy some bananas.è(Did I ask
you if you liked bananas?)èYour bunch ç bananas weighs 2 lb 8 oz or
2.5 lb.èThe supermarket is chargïg an exorbitant price ç $0.45/lb.
How much will êy charge you for your bunch?
The desired quantity if ê cost.èThe given quantity is ê weight ç
your bunch ç bananas, 2.5 lb.èThe conversion facër comes from ê
price per pound.èSettïg-up ê problem as is illustrated above:
èèèèèèèèè $0.45
èè$ ? = 2.5 lb x ─────è=è$1.13è The pound units cancel.
èèèèèèèèèè1 lb
Notice that if we use ê conversion facër upside down, ê units do not
cancel.
è 1 lb
èè$ ? = 2.5 lb x ─────è=è5.56 lbì/$.è
èèèèèèèèè $0.45èèèèèèèèè
The units ç ê answer tells us that we did somethïg wrong.èWhen this
happens, do not panic.èYou should reread ê problem å check your
setup.èUsually one ç ê facërs is ïverted.
Many problems can not be solved ï one step.èIf one facër will not
provide ê direct lïk between ê given å desired quantities, ên we
need ë develop a pathway ïvolvïg two or more conversion facërs.
For ïstance, ê price ç regular gasolïe is $1.119 per gal.èFïd ê
price per liter usïg ê followïg conversion facërs: 1 gal = 4 qt å
1 l = 1.057 qt.
The desired quantity is $/L, å ê given quantity is $/gal.èFrom ê
given conversion facërs, we construct ê path between ê gallons å
ê liters.èWe do not worry about ê $ because that unit does not
change.èThe path is gal ¥ qt ¥ L.èSettïg-up ê problem as above, we
obtaï
èèèè$ ?è $1.119èè1 galè 1.057 qt
èèèè─── =è─────èx ───── x ──────── =è$0.2957/L
èèèèèLèè galèèè4 qtèè 1 L
After ê first conversion, ê price per gal becomes ê price per
quart.èThen ê second facër converts from price per quart ë price per
liter.
As êse examples show, this method ç approachïg problems is quite
general å is not restricted ë chemistry problems.èThis approach can
be employed whenever êre is a proportional relationship between ê
given å desired quantities.
An example where this method can not be used is ê conversion from
Fahrenheit ë Celsius temperatures.èThese temperature scales are not
simply proportional ë each oêr but also have a zero poït difference.
1èWhat is 5.0 lb ç sugar expressed ï kg?è
A) 0.44 kgèè B) 11 kgèè C) 2.3 lbèè D) 1.6 kg
ü You need ë develop ê conversion facër between pounds (lb) å
kilograms (kg).èThe facër is 1 kg = 2.2046 lb, so we can convert
directly from lb ë kg ï one step.
èèèèèèèèèèèè1 kg
èè ? kg = 5.0 lb x ───────── =è2.3 lb
èèèèèèèèèè 2.2046 lb
Notice that ê lb units cancel.
Ç C
2èA person is 5 ft 9 ï tall.èWhat is êir height ï cm?
A) 3.7x10úì cmèè B) 27 cmèè C) 1.8x10ì cmèè D) 15 cm
ü You need ë fïd ê conversion between feet or ïches å cm.
Sïce we know that 1 ï = 2.54 cm, it would be easiest ë express ê
height ï ïches.èThen ê conversion can be done ï one step.
èè 5 ft + 9 ï = 5 ft x 12 ï/ft + 9 ï = 69 ï.
èèèèèèèèèè2.54 cm
èè ? cm = 69 ï x ─────── =è1.8x10ì cmèè
èèèèèèèèèè 1 ï
Notice that ê ïch units cancel.
Ç C
3èA laboraëry room has ê dimensions 65 ft x 45 ft x 10. ft.
What is ê volume ç ê room ï cubic meters (mÄ)?
A) 8.3x10ì mÄèè B) 8.9x10Ä mÄèè C) 9.6x10Å mÄèè D) 1.03x10æ mÄ
üèWe need ê conversion facër between feet å meters, which is
1 m = 3.281 ft.èThe units ç ê answer will be cubic meters (mÄ).èThe
startïg poït is ê set ç dimensions ç ê room.
èèèèèèèèèèèèèèèèèè íè 1 mèè┐Ä
èè ? mÄ = 65 ft x 45 ft x 10. ft x │──────────│è= 8.3x10ì mÄ
èè └ 3.281 ft ┘
You could convert each dimension from ft ë m, but cubeïg ê conversion
accomplishes ê same task.èCubeïg ê facër produces a new conversion
which relates cubic feet ë cubic meters.èWhen ê numbers are cubed,
ê units are cubed ëo.
Ç A
4èA laboraëry procedure calls for 15 mg ç a particular
compound.èHow many grams ç ê compound is required?
A) 31 gèè B) 1.5x10Å gèè C) 67 gèè D) 0.015 g
üèThe unit ç ê desired answer is ê gram.èWe need ê conver-
sion facër between milligram å gram.èIn ê SI (metric) system, milli
is a prefix meanïg 10úÄ, å 1 mg = 10úÄ g.èThis is a relatively simple
one step conversion.
èè10úÄ g
èè ? g = 15 mg x ─────── = 15x10úÄ g = 0.015 g
èèèèèèèèèè1 mg
Ç D
5èLight with a wavelength ç 530. nm appears yellow.èWhat is
wavelength ï ïches?
A) 2.09x10úÉ ïèèèB) 1.35x10úô ïèè
C) 7.43x10úîò ïèè D) 4.79x10úÉ ï
üèWe do not have a direct conversion facër for nm ë ïches, so we
will need more than one facër.èNano, n, is a prefix meanïg 10úö.èOne
facër is 1 nm = 10úö m.èWe also know that 1 m = 39.37 ï.èThe pathway
ë perform ê conversion is nm ¥ m ¥ ï.
èèè10úö mè 39.37 ï
èè ? ï = 530. nm x ────── x ──────── = 2.09x10úÉ ï
èèèèèèèèèèè 1 nmèè 1 m
The first facër converts from nanometers ë meters, å ê second
facër converts from meters ë ïches.
Ç A
6èOne speed limit for residential areas is 25 mph.èWhat is this
speed ï meters per sec (m/s)?
A) 4.3 m/sèè B) 11 m/sèè C) 1.4x10É m/sèèD) 56 m/s
üèWe must develop a few conversion facërs, because we do not have
a direct conversion facër from mi/hr ë m/s.èOne possible pathway ë
perform ê conversion is mi ¥ km ¥ m å hr ¥ mï ¥ s.èThe distance
units ï ê numeraër must be converted separately from ê time units
ï ê denomïaër.èThe facërs available ï "details" are
1 mi = 1.609 km å kilo, k, is a prefix meanïg 10Ä.
mèèèmiè 1.609 kmè 10Ä mèè1 hrèè1 mï
èèè? ─ = 25 ── x ──────── x ───── x ────── x ───── = 11 m/s
sèèèhrèè 1 mièè 1 kmèè60 mïèè60 s
The first facër converts mi/hr ë km/hr; ê second, km/hr ë m/hr; ê
third, m/hr ë m/mï; å ê last, m/mï ë m/s.
Ç B
7èA gas cylïder contaïs oxygen at a pressure ç 425 lb/ïì
(psi) at 0ò C.èWhat is ê pressure ï mm ç Hg (mm ç mercury)?
(1 atm = 14.7 lb/ïì, 1 atm 760 mm Hg)
A) 0.122 mm HgèèèèB) 8.22 mm Hg
C) 26.3 mm Hg èèèD) 2.20x10Å mm Hg
üèIn ê problem, we fïd ê conversion facërs:
1 atm = 14.7 lb/ïì å 1 atm = 760 mm Hg.èWith êse facërs, we see
that a pathway for ê conversion isèlb/ïì ¥ atm ¥ 760 mm Hg.èThe
temperature is not used ï ê problem except that ê conversion facërs
have ê given values at 0ò C.
èèè 1 atmèèè 760 mm Hg
èèè? mm Hg = 425 lb/ïì x ─────────── x ───────── =è2.20x10Å mm Hg
èèèèèèèèèèèèèè 14.7 lb/ïìèè 1 atm
Notice that ê lb/ïì units cancel ï ê first multiplication å that
ê atm units cancel ï ê second product.
Ç D
8èHow many aspirï tablets could be made from 1.00 lb ç
aspirï, if each tablet contaïs 325 mg ç aspirï?
A) 1.47x10É tabletsèè B) 716 tablets
C) 1.40x10Ä tabletsèè D) 384 tablets
üèThe desired unit is tablets.èWe need a lïk between ê weight
ç aspirï å ê tablets.èThis lïk is ê mass ç aspirï ï each
tablet, which is 325 mg/tablet.èThe pathway for ê conversion is
lb ¥ g ¥ mg ¥ tablet.èThe conversion facërs are:
1 lb = 453.6 g, 1 mg = 10úÄ g, å 1 tablet = 325 mg.
èèèèèèèèèèèè453.6 gèè1 mgèè1 tablet
è? tablets = 1.00 lb x ─────── x ────── x ──────── = 1.40x10Ä tablets
èèèèèèèèèèèè 1 lbèè 10úÄ gèè325 mg
In ê first conversion, ê lb units cancel.èIn ê second, ê gram
units cancel.èIn ê third, ê mg units cancel.
Ç C
9èHow many mL are êre ï 1.00 cup (8 oz liquid)?
A) 264 mLèè B) 3784 mLèè C) 237 mLèè D) 423 mL
üèWe want ë fïd ê number ç mL.èWe know that 4 cups = 1 qt,
1 L = 1.057 qt, å 1 mL = 10úÄ L.èYou can construct ê followïg path
ë make ê conversion: cup ¥ qt ¥ L ¥ mL.
èèèèèèèèèèèè1 qtèèè1 Lèèè 1 mL
èè ? mL = 1.00 cup x ────── x ──────── x ────── = 237 mL
èèèèèèèèèèè 4 cupsè 1.057 qtè 10úÄ L
In ê first conversion, we converted from cups ë quarts so that we
could use ê known conversion between ê SI å English systems ç
units (1 L = 1.057 qt).èThe last conversion ïvolves ê milli
multiplier from ê SI system.èNotice how ê units cancel each time a
product is taken.
Ç C
10èIn 1993, ïdustries ï ê U. S. used 4.16 million metric
ëns ç polyvïyl chloride.èHow many pounds were used?
(1 metric ën = 1x10æ g)
A) 1.89x10ö lb B) 1.89x10îÉ lb
C) 1.09x10îÅ lb D) 9.17x10ö lb
üèUsïg ê facër that is provided ï ê question, you can fïd
ê number ç grams.èFrom ê number ç grams, you can obtaï ê
number ç pounds with ê facër, 1 lb = 453.6 g.è
èèèèèèèèèèèèèèèè10æ ënèèèèè10æ gèèèè1 lb
? lb = 4.16 million metric x ────────────── x ──────────── x ───────
èèèèèèèënsèèèèè 1 million ënsè 1 metric ënè 453.6 g
? lb = 9.17x10ö lb
We used ê pathway: million metric ëns ¥ metric ëns ¥ grams ¥ pounds.
Ç D