1
CHEM 119 H
Quiz 1
09/27/07
____________________________________
Show work neatly on these pages. Use backs as needed.
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If we cannot follow your work QUICKLY, it is wrong.
taking this quiz
USE PENCIL & ERASE CLEANLY. Pay attention to decimals in answers.
1. (5) What is meant by “confidence interval”?
The confidence interval is the range around the average value in which we believe that the true
value lies.
2. (35) The analyses of trace amounts of material are inherently more difficult than analyses of samples
with concentrations in the range we are using. There will almost always be a “relatively large”
uncertainty in the results. Harris reports the following results for the analysis of concentrations of
xylenes (isomeric C
8
H
10
compounds) in air in two places: average, standard deviation, number of
measurements. As you can see, there is a wide variation (large standard deviation) for these
measurements. A microgram of material per cubic meter of air is not much.
Place
C{xyl,
µ
g/m
3
}
#measurements
Turnpike
31.4 ± 30.0
32
Tunnel
52.9 ± 29.8
32
e. (10) Use the appropriate statistical equations
to determine whether or not these average
values differ at the 95 % confidence level
()
12
1
2
22
11 22
XX
n
*
n
t =
sn
+
n
n1s +
(n1
)s
s =
n+n2
⎞
⎛
⎞
⎛
−
⎟
⎜
⎟
⎜
⎝
⎠
⎝
⎠
⎞
⎛
⎟
⎜
⎝
⎠
s = 29.9
XX
21
.5
=
2
21.5
32
t{calc}=
2.88
29.9
64
⎞
⎛
⎞
⎛
=
⎟
⎜⎟
⎜
⎝
⎠
⎝
⎠
t{table, 60}
≅
t{62} = 2.00
t{calc} > t{table}; therefore, the two results are
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 Fall '07
 Munson
 Chemistry, Normal Distribution, Handbook of Chemistry and Physics, 1.628 M, 208.24 g

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