Harris QCA 8e Chapter 04
(Try all)
1 of 17
When an analytical chemist uses the word precision when referring to the data
obtained from an analysis, the chemist is talking about
a) how close the mean obtained from a set of data is to the true value of the
composition of the sample analyzed.
b) relative error calculated from the mean of a set of data.
c) how close the individual data points obtained from a set of data are to each other.
2 of 17
A government chemist has performed a gravimetric analysis for chloride ion on a
sample submitted to the laboratory. The following results were obtained:
Sample
1
2
3
4
Percent Chloride Ion
27.46
27.52
27.48
27.10
When reporting the standard deviation, the analyst said that
a) approximately 68.3% of the data points lie between 27.19 and 27.59 percent
chloride.
b) approximately 75% of the data points lie between 27.46 and 27.52 percent
chloride.
c) approximately 68.3% of the data points lie between 27.29 and 27.49 percent
chloride.
3 of 17
A government chemist has performed a gravimetric analysis for chloride ion on a
sample submitted to the laboratory. The following results were obtained:
Given the following data:
What is the 98% confidence interval for this set of data?
a) 0.37%
b) 0.91%
c) 0.45%
4 of 17
The following data was collected when performing a spectrophotometric analysis for
chromium:
Using the least squares method of linear regression, the following equation was
determined to define the line generated by plotting concentration vs. absorbance:
y = 0.0196x - 0.008
a) This equation represents the statistical best fit of a line drawn through the data
points.
b) The amount of unknown in the sample may be determined by substituting the
absorbance value of the unknown for x in the equation and solving for the y value.
c) The value 0.0196 represents the value on the y axis where the line crosses this
axis.
5 of 17
The following data was collected when performing a spectrophotometric analysis for
chromium:
Using the least squares method of linear regression, the following equation was
determined to define the line generated by plotting concentration vs. absorbance:
y = 0.0196x - 0.008.
The concentration of chromium in the sample was found to be
a) 13.11 mg Cr/L.
b) 2.81 mg Cr/L.
c) 13.93 mg Cr/L.
6 of 17
If one has a Gaussian distribution of data points,
a) approximately two thirds of the data points lie above the mean, and one third of
the data points lie below the mean accounting for all of the data points.
b) approximately 2/3 of the data points lie within plus or minus two standard
deviations of the mean.
c) Approximately 2/3 of the data points lie within plus or minus one standard
deviation of the mean.
7 of 17
A Gaussian distribution of data is symmetric if
a) 4.5% of measurements lie outside the range defined by 2 standard deviations
above the mean, and two standard deviations below the mean.
b) 4.5% of measurements lie outside the range defined by 2 standard deviations
above the mean, and two standard deviations below the mean with 2.25% of the
values above 2 standard deviations from the mean and 2.25% of the values below 2
standard deviations from the mean.
c) 68.3% of the measurements lie within plus or minus one standard deviation from
the mean.
8 of 17
When analyzing the iron content of a well drilled for use in a new housing
development, water from the well was analyzed on three different days. The
following data was obtained:
The pooled estimate of the standard deviation of the analysis is
a) 0.14 mg Fe/L.
b) 0.072 mg Fe/L.
c) 0.076 mg Fe/L.
9 of 17
The degrees of freedom used to calculate the confidence interval are
a) the number of data points which are being evaluated.
b) the number of data points - 1.
c) the value associated with the number of data points used to determine the value
of t.
10 of 17
A spectrometer was used to measure the light absorbed by three standard solutions
containing different concentrations of K2Cr2O7. The following data was obtained:
The equation of the straight line generated using the method of least squares is
a) y = 0.0331x - 0.0079
b) y = 30.24x + 0.242
c) y = -0.0079x + 0.0331
11 of 17
A spectrometer was used to measure the light absorbed by three standard solutions
containing different concentrations of K2Cr2O7. The following data was obtained:
The equation of the straight line generated using the method of least squares is y =
0.0331x - 0.0086. Use this information to find the standard deviation of the slope of
the line expressed to the proper number of significant figures.
a) m = 0.033(± 0.0004)
b) m = 0.0331(± 0.0045)
c) m = 0.0331(± 0.0001)
12 of 17
The following standardization curve was prepared by plotting light absorbed on the y
axis and ppm Cr on the x axis.
An unknown sample of 100 mL of solution containing K2Cr2O7 was diluted 2 to 100
mL with 0.5 M H2SO4. This mixture was further diluted, 10 to 100 mL with the same
H2SO4 solution. The resulting solution had an absorbance of 0.20. How many
milligrams of chromium were in the original sample?
a) 100 ppm Cr
b) 5000 mg Cr
c) 500 mg Cr
13 of 17
Given the following titration data, correctly report the average and standard
deviation for the volume of NaOH: (Trial 1) 25.60 mL, (Trial 2) 25.75 mL, and (Trial
3) 25.85 mL.
a) 25.73 ± 0.1 mL
b) 25.7 ± 0.13 mL
c) 25.7 ± 0.1 mL
14 of 17
Police have a hit-and-run case and need to identify the brand of red auto paint. What
statistical test might they perform?
a) Q test
b) t test
c) F test
15 of 17
Police have a hit-and-run case and need to identify the brand of red auto paint. The
percentage of iron oxide, which gives paint its red color, found during analysis is as
follows: 43.15, 43.81, 45.71, 43.23, 42.99, and 43.56%. What is the average
percentage of the iron oxide in the paint sample? (Hint: use a 90% confidence level)
a) 43.3%
b) 43.7%
c) 43.6%
16 of 17
Police have a hit-and-run case and need to identify the brand of red auto paint. The
percentage of iron oxide, which gives paint its red color, was determined to be 43.3 5
± 0.33% by one method of analysis using five measurements. The color reference
provide by the automotive manufacture is 44.21%. What can you conclude about
whether the sample matches the reference at a 95% confidence level?
a) The sample matches the reference.
b) The sample does not match the reference.
c) There are an insufficient number of trials to determine if the sample is a match.
17 of 17
Police have a hit-and-run case and need to identify the brand of red auto paint. The
percentage of iron oxide, which gives paint its red color, was analyzed by two
different methods. The results were 43.3 ± 0.33% and 43.6 ± 0.14% with five
measurements each. Are the standard deviations "significantly" different from each
other a 95% confidence level?
a) Yes
b) No
c) Possibly