Homework Assignment for CHEM 4531
Professor J. M. Weber
Office: JILA A709; Phone 303-492-7841; email: [email protected]
_
Problem Set 8
To be returned before class on Wednesday, March 16, 2011 (give to me in class or
slide under my office door).
1
Homework Assignment for CHEM 4531
Professor J. M. Weber
Office: JILA A709; Phone 303-492-7841; email: [email protected]
_
Problem Set 6
To be returned before class on Wednesday, February 23, 2011 (give to me in class or
slide under my office door)
Homework Assignment for CHEM 4531
Professor J. M. Weber
Office: JILA A709; Phone 303-492-7841; email: [email protected]
_
Problem Set 2
To be returned before class on Wednesday, January 19.
1. Photoelectric Effect:
a.) As seen in class, light has
Homework Assignment for CHEM 4531
Professor J. M. Weber
Office: JILA A709; Phone 303-492-7841; email: [email protected]
_
Problem Set 7
To be returned before class on Friday, March 4, 2011 (give to me in class or slide
under my office door).
1. Th
a portion of the target population is removed at a given time and
location in space. A grab sample, therefore, provides a snapshot of
the target population. Grab sampling is easily adapted to any of the
sampling schemes discussed in the previous section.
learned that the mass of sample needed can be reduced by decreasing
particle size without affecting the relative sampling variance. Both are
important conclusions. Few populations, however, meet the conditions
for a true binomial distribution. Real popula
improve the analysis by decreasing sm provides only a nominal change
in the overall variance. = X ts n Percentage of s2 o due to s2 m sm/ss
0 0.5 1.5 2 0 10 20 30 40 50 60 70 80 1 1400-CH07 9/8/99 4:02 PM Page
180 Chapter 7 Obtaining and Preparing Samples
by a standard deviation for sampling, ss. The standard deviation for the
analytical method, sm, accounts for random errors introduced when
executing the methods procedure. The relationship among so, ss, and
sm is given by a propagation of random error s 2
deviation for sampling, ss,r, is obtained by dividing equation 7.3 by nA.
Solving for n allows us to calculate the number of particles that must be
sampled to obtain a desired sampling variance. 7.4 Note that the
relative sampling variance is inversely pr
A variety of inorganic, organic polymer and carbon sorbents have been
used. Inorganic sorbents, such as silica gel, alumina, magnesium
aluminum silicate, and molecular sieves, are efficient collectors for polar
compounds. Their efficiency for collecting w
concentrations of weak acid and weak base. A general buffer equation
can be derived by considering the following reactions for a weak acid,
HA, and the salt of its conjugate weak base, NaA. NaA(aq) Na+(aq)
+A(aq) HA(aq)+H2O(l) t H3O+(aq)+A(aq) 2H2O(l) t H
may undergo a chemical or physical change. This is a serious problem
since the properties of the sample will no longer be representative of
the target population. For this reason, samples are often preserved
before transporting them to the laboratory for
contribution to the overall variance, sm 2, is determined using the data
on the right and is equal to 7.00 108. The variance due to sampling,
ss 2, is therefore ss 2 = so 2 sm 2 = 4.71 107 7.00 108 = 4.01
107 7B Designing A Sampling Plan A sampling plan
the method. If ss 2 is significantly greater than sm 2 , then the methods
variance can be ignored and equation 7.7 used to estimate the number
of samples to analyze. Analyzing any sample more than once will not
improve the overall variance, since the vari
provides a good illustration of many of the methods used to sample
solutions. The chemical composition of surface waters, such as streams,
rivers, lakes, estuaries, and oceans, is influenced by flow rate and depth.
Rapidly flowing shallow streams and rive
detected in the second canister, then it is safe to assume that the first
canisters capacity has not been exceeded. The volume of gas that can
be sampled before exceeding the sorbents capacity is called the
breakthrough volume and is normally reported wit
SOLUTION To determine Ks we need to know the average mass of the
cereal samples and the relative standard deviation for the %(w/w) ash.
The average mass of the five cereal samples is 1.0007 g. The average %
(w/w) ash and the absolute standard deviation ar
drawn. When the material being sampled is homogeneous, individual
samples can be taken without regard to possible sampling errors.
Unfortunately, in most situations the target population is
heterogeneous in either time or space. As a result of settling, f
problem. For a target population with a relative sampling variance of 50
and a desired relative sampling error of 5%, equation 7.7 predicts that
ten samples are sufficient. In a simulation in which 1000 samples of size
10 were collected, however, only 57%
9/8/99 4:03 PM Page 185 A composite sample consists of a set of grab
samples that are combined to form a single sample. After thoroughly
mixing, the composite sample is analyzed. Because information is lost
when individual samples are combined, it is norm
[NH3] and the mass balance for I is [I] = [Ag(NH3)2 +] Simplifying the
charge balance expression by dropping [H3O+] and [Ag+], and replacing
[Ag(NH3)2 +] with [I] gives K K K sp b w Ag I Ag NH Ag NH NH OH NH H
O OH = = = = = = = = + + + + + [ ][ ] . [ ( )
constant is evalu- 80 0 6 133 . = times *Problem 8 in the end-of-chapter
problem set asks you to consider the relationship between equations
7.4 and 7.5. 1400-CH07 9/8/99 4:03 PM Page 188 Chapter 7 Obtaining
and Preparing Samples for Analysis 189 ated by
7 Obtaining and Preparing Samples for Analysis 193 and that for the
second sampling strategy is As expected, since the relative method
variance is better than the relative sampling variance, a sampling
strategy that favors the collection of more samples a
Interscience: New York, 1973, p. 73. 4. Lambert, W. J. J. Chem. Educ.
1990, 67, 150153. 5. Kolthoff, I. M.; Lingane, J. J. Phys. Chem. 1938, 42,
133140. 6. Davies, C. W. Ion Association. Butterworth: London, 1962.
6P REFERENCES 1400-CH06 9/9/99 7:41 AM Pa
composite sample instead of many individual grab samples, provides an
appreciable savings in time and cost. Composite sampling is also useful
when a single sample cannot supply sufficient material for an analysis.
For example, methods for determining PCBs
sample introduces a source of determinate error that cannot be
corrected during the analysis. If a sample does not accurately represent
the population from which it is drawn, then an analysis that is
otherwise carefully conducted will yield inaccurate res
R
TO
RACAHPARAMETERS THEIR. ETATIONSHIP TENGTHS
AND
B ONDS
AND C OVATENCIES M nz+-A ND F e8+-OXYGEN
OF
tN S tucATES
P. G . M ANNING*
AssrRAcr
The energieso f the Russell-Saunders tates aGand aD in F et+ a nd M n2+ i n a n umber o f
s
silicates have b een
Chemistry 4011/5011
Exam 2
November 17, 2011
(Each question is 10 pts)
1. Metals typically crystallize in either a close-packed unit cell with packing
efficiency of 74% or a body-centered cubic unit cell with packing efficiency of
68%, but rarely in a sim
Homework Assignment for CHEM 4531
Professor J. M. Weber
Office: JILA A709; Phone 303-492-7841; email: [email protected]
_
Problem Set 6
To be returned before class on Wednesday, October 5, 2011 (give to me in class or
slide under my office door).