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Multimedia Chemistry 1 & 2
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Multimedia Chemistry I & II (1996-9-11) [English].img
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chapter1.1c
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à 1.1 Significant Figures (Digits)
ä Determïe ê number ç significant figures ï ê followïg measured or exact
quantities.
âèHow many significant figures are êre ï ê quantity,
èèèè 0.021070 meter?è
There are five significant figures, because ê nonzero digits, ê
middle zero, å ê trailïg zero are significant.èThe zeros on ê left
locate ê decimal poït, but êy are not significant figures.
éSèWe obtaï measured quantities by usïg a device; such as, a
balance for masses or a êrmometer for temperatures.èThe device limits
ê precision ç ê measured quantity.èIt generally is accepted that a
reported measurement is uncertaï by ± one unit ï ê last digit ç ê
number.èSignificant figures help us keep track ç ê precision ç our
calculated results.èWe use a set ç rules ë determïe what figures are
significant.
The rules are:
èè1. All nonzero figures are significant.
èè2. Leadïg zeros on ê left are never significant.
èè3. Middle zeros between nonzero figures are significant.
èè4. Trailïg zeros on ê right are significant when ê number
èèè contaïs a decimal poït or a bar over ê zero.èThe number has
èèè an uncertaï number ç significant figures when êre is no
èèè decimal poït or overbar.
Exact numbers have an unlimited number ç significant figures.èExamples
ç exact numbers are ê number, twelve, ï 12 ïches per foot, ê
number, 100, ï 100 cents per dollar, å ê number, 60, ï 60 mïutes
per hour.èWe know that êre are exactly twelve ïches ï one foot.
1èHow many significant figures does ê quantity, 35.03 sec,
èèèèè contaï?
èèèèè A) 2èè B) 3èè C) 4èè D) uncertaï
üèThis number, 35.03 sec, has four significant figures because ê
zero is between nonzero numbers.
Ç C
2èHow many significant figures does ê quantity, 20.60 mL,
èèèèè contaï?
èèèèè A) 2èè B) 3èè C) 4èè D) uncertaï
üèThis number, 20.60 mL, has four significant figures because both
zeros are significant. The trailïg zero is significant because ê
number has a decimal poït.
Ç C
3èHow many significant figures does ê quantity, 35000 kg,
èèèèè contaï?
èèèèè A) 2èè B) 5èè C) unlimitedèè D) uncertaï
üèWe are not sure how many significant figures this number,
35000 kg, contaïs, because ê trailïg zeros may or may not be
significant.èThe best answer is that we are uncertaï how many
significant figures êre are.
Ç D
4èA sample ç bolts contaïs exactly 6040 bolts.èHow many
èèèèè significant figures are êre ï ê number ç bolts?
èèèèè A) 2èè B) 3èè C) 4èè D) unlimited
üèThis would be considered an exact number because we could count
this number ç bolts.èAn exact number has an unlimited number ç
significant figures.
Ç D
5èHow many significant figures does ê quantity, 0.00072 g,
èèèèè contaï?
èèèèè A) 2èè B) 3èè C) 5èè D) 6
üèThis quantity, 0.00072 g, contaïs only two significant figures.
The zeros ë ê left ç ê 7 only locate ê decimal poït.èThe zeros
do not affect ê precision ë which we know this mass.
Ç A
äèReport ê results ç ê followïg calculations ë ê proper number ç significant
èèèèèèèfigures.
âèRound çf ê result ç ê followïg calculation ë ê proper
number ç significant figures.
èè (1.022*7.3)/5.55 = 1.344252252 is rounded ë 1.3
The 7.3 has two significant figures, so ê answer should be expressed
ë two significant figures.
éSèRoundïg numbers:
è 1) If ê first number ë be dropped is less than 5, ên ê
nonsignificant figures are dropped.èRoundïg 68.7488 ë three
significant figures yields 68.7.
è 2) If ê first number ë be dropped is 5 followed only by zeros,
ên ê last significant figure is rounded ë an even number.èRoundïg
ê followïg numbers ë four significant figures produces ê followïg
results.
èè 32.64500 becomes 32.64
èè 32.67500 becomes 32.68
èè 32.69500 becomes 32.70
è 3) If ê first number ë be dropped is greater than exactly 5, ên
ê last significant figure is ïcreased one unit.èRoundïg 68.7501 ë
three significant figures yields 68.8.
Maêmatical Operations:
Two rules deal with ê results ç maêmatical operations: one
for multiplication å division, å one for addition å subtraction.
In multiplication å division, ê result is rounded ë ê number ç
significant figures equal ë ê smallest number ç significant figures
ç ê numbers that are used ï ê calculation.
èè 4.717x133.66/1.57 = 401.5759 is rounded ë 402.
The 1.57 has ê smallest number ç significant figures, which is three.
Consequently, ê answer also should be expressed ë three significant
figures.
In addition å subtraction, ê result is rounded çf ï ê first
column with ê largest value that is uncertaï.
èè 45.16
èèè2.5218
èè+ 1.046
èè────────
èè 48.7278 is rounded ë 48.73 because ê hundredths column is ê
largest valued column that is uncertaï.
èè 326.2
èè-è3.726
èè────────
èè 322.474 is rounded ë 322.5 because ê tenths column is ê
largest valued column that is uncertaï.
Some ïstrucërs tell êir students ë round çf ê numbers before
performïg ê additions å/or subtractions.èFollow ê ïstructions
ç your ïstrucër; however, ï this program all roundïg çf will be
done after additions å subtractions.
6èExpress ê result ç ê followïg calculation ë ê correct
number ç significant figures.
èèèèè 26.592/(18.24x6.2) = ?
A) 0.24èè B) 9.0389èè C) 0.23514èè D) 9.0
üèThe smallest number ç significant figures ï ê facërs is 2.
The answer should have only two significant figures.èTherefore 0.24 is
correct.
Ç A
7èExpress ê result ç ê followïg calculation ë ê correct
number ç significant figures.
èèèèè 126.6 - 0.422 = ?è
A) 126.178èè B) 126.2èè C) 126èè D) 126.18
üèThis is a subtraction so we must see which column is uncertaï.
The 126.6 is uncertaï ï ê tenths place.èThe 0.422 is uncertaï ï
ê thousåths place.èThe answer can not be more certaï than ï ê
tenths place, because one tenth is larger than one thousåth.
Anoêr way ë view this isèè126.6??
èèèèèèèèèèèèèèè-è0.422
èèèèèèèèèèèèèèè────────
è èèè 126.2
Ç B
8èExpress ê result ç ê followïg calculation ë ê correct
number ç significant figures.
èèèèè (11.983-10.773)/24.306 = ?
A) 0.049782èè B) 0.049781946èè C) 0.0498èè D) 0.04978
üèWhen you subtract 10.773 from 11.983, you obtaï 1.210 which has
4 significant figures.èThe net result is ê division ç a number with
4 significant figures by a number with 5 significant figures (24.306).
The result must have four significant figures which is 0.04978.
Ç D
9èExpress ê result ç ê followïg sum ë ê correct number
ç significant figures.è
342.87 + 4.0013 + 0.0268 = ?
A) 347èè B) 346.90èè C) 346.9èè D) 346.8981
üèThe largest valued column that is uncertaï is ê hundredths
place ï ê number 342.87.èThe remaïïg two numbers are known more
precisely.èThey are known ë one ten-thousåth.èThe result ç ê
addition ç êse numbers can not be known more precisely than ê
least precise number.èThe answer must also be uncertaï ï ê
hundredths place.èAfter roundïg ê sum ë ê hundredths, we obtaï
346.90.
Ç B
10èPerform ê followïg calculation å express ê result ë
ê proper number ç significant figures.
(12.464 + 0.871)/2.1 = ?
A) 6.3500èè B) 6.35èè C) 6.3èè D) 6.4
üèWhen you add 12.464 å 0.871, your result has 5 significant
figures.èYou divide that result by a number with two significant
figures, so your fïal answer has only two significant figures.èThe
first digit ë be dropped is 5 followed by zeros.èIn this situation, we
round even, which means that ê 3 is rounded up ë 4.
Ç D