Chapter 3: Precision and Accuracy of Chemical Analyses Flashcards

1
Q

The ______ of two or more
measurements is their average value

A

mean

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2
Q

mean is also called as

A

arithmetic mean or average

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3
Q

it is often caused by faulty calibrations or standardization or by random variations and uncertainties in results.

A

errors

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4
Q

TRUE OR FALSE
it is impossible to perform a
chemical analysis that is totally free of errors or uncertainties

A

TRUE

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5
Q

refers to the difference between a
measured value and the “true” or
“known” value

A

ERROR

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6
Q

often denotes the estimated uncertainty in
a measurement or experiment.

A

error

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7
Q

is the middle value in a set of data that has been arranged in numerical order. The ________ is used advantageously when a set of data contain an outlier, a result that differs significantly from others in the set.

A

median

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8
Q

can have a significant effect on the mean of the set but has no effect on the median.

A

outlier

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9
Q

describes the reproducibility of measurements. is the closeness of results to others obtained in the same
way.

A

precision

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10
Q

three terms widely used to describe the precision of a set or replicate data,

A

standard deviation
variance
coefficient of variation

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11
Q

These three (standard deviation, variance, and coefficient of variation) functions of how much an individual result differ from the mean, called

A

deviation from the mean

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12
Q

is the closeness of a
measured value to the true or
accepted value and is expressed by the error.

A

accuracy

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13
Q

describes the agreement among several results obtained in the same way. We can determine ______ just by measuring replicate samples

A

Precision

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14
Q

is often more difficult to determine because the true value is usually unknown. An accepted value must be used instead. It is expressed in
terms of either absolute or relative error.

A

Accuracy

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15
Q

TRUE or FALSE
The sign of absolute error tells you whether the value in question is high or low.

A

TRUE

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15
Q

the formula for absolute error

A

E= measured value - true value

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16
Q

The _______________ of a measurement
is the absolute error divided by
the true value. _________ may
be expressed in percent, parts per
thousand, or parts per million,
depending on the magnitude of the
result

A

relative error

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17
Q

Er is often a more useful quantity than the absolute error. Er stands as

A

relative error

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18
Q

causes data to be scattered more or less symmetrically around a mean value

A

random or indeterminate error

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19
Q

causes the mean of a data set to differ from the accepted value

A

systematic or determinate error

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20
Q

is a sample of about the
same size that is carried through an
analysis in exactly the same way.

A

replicate

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21
Q

TRUE or FALSE
the random error
in a measurement is reflected by its precision.

A

TRUE

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22
Q

One of the first questions to answer before beginning an analysis is

A

What maximum error can be tolerated in the result?

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23
Q

TRUE or FALSE
An example of a systematic error is the loss of a volatile analyte while heating a sample

A

TRUE

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24
differ from indeterminate and determinate errors. They usually occur only occasionally, are often large, and may cause a result to be either high or low. They are often the product of human errors.
gross errors
25
Gross errors lead to
outliers
26
have a definite value, an assignable cause, and are of the same magnitude for replicate measurements made in the same way
systematic errors
27
systematic errors leas to ______ in measurement results
bias
28
three types of systematic errors
instrumental errors method errors personal errors
29
are caused by nonideal instrument behavior, by faulty calibrations, or by use under inappropriate conditions
instrumental errors
30
arise from nonideal chemical or physical behavior of analytical systems.
method errors
31
result from the carelessness, inattention, or personal limitations of the experimenter.
personal errors
32
affect measurement precision. They arise from experimental variables that cannot be controlled or determined. Examples include random fluctuations in electrical noise, random inconsistencies in measurment readings, or random changes in laboratory temperature.
random or indeterminate errors
33
affect the accuracy of results. These errors often occur when instruments or measuring devices are uncalibrated or are calibrated improperly. They have the same effect on all samples. Systematic errors can be very subtle and difficult to detect, but finding and eliminating them is an integral part of the measurement process
systematic errors
34
occur infrequently and often result from an experimental blunder such as misreading a scale or interpreting a number incorrectly. If you mistakenly read a 9 as 4 or 3 as
gross error
35
is an occasional result in replicate measurements that differs significantly from the other results
outlier
36
measures the systematic error associated with an analysis. It has a negative sign if it causes the results to be low and a positive sign otherwise
bias
37
determines the relationship between a measured quantity and the analyte concentration. _________ is a very important part of any analytical procedure
calibration
38
eliminates most systematic errors of this type
calibration
39
The nonideal chemical or physical behavior of the reagents and reactions on which an analysis is based often introduce________________. Such sources of nonideality include the slowness of some reactions, the incompleteness of others, the instability of some species, the lack of specificity of most reagents, and the possible occurrence of side reactions that interfere with the measurement process
systematic method errors
40
TRUE or FALSE Errors inherent in a method are often difficult to detect and are thus the most serious of the three types of systematic error
TRUE
41
Many measurements require personal judgments. Examples include estimating the position of a pointer between two scale divisions, the color of a solution at the endpoint in a titration, or the level of a liquid concerning a graduation in a pipet or buret
Personal errors
42
universal source of error is
prejudice or bias
43
these are errors that are independent of the size of the sample being analyzed
constant errors
44
decrease or increase in proportion to the size of the sample.
proportional errors
45
systematic errors can be either _____ or ________
constant or proportional
46
TRUE or FALSE To minimize the effects of constant errors, keep sample size as less as is feasible
FALSE To minimize the effects of constant errors, keep sample size as large as is feasible
47
Detection of systematic method errors
Analysis of Standard Samples Independent Analysis Blank Determinations Variation in Sample Size
48
The best way to estimate the bias of an analytical method is by analyzing ____________________________, materials that contain one or more analytes at known concentration levels.
standard reference materials (SRMs)
49
A _________ solution contains the solvent and all the reagents in an analysis and no analyte. Whenever feasible, _______ may also contain added constituents to simulate the sample matrix.
blank
50
TRUE or FALSE as the size of a measurement increases, the effect of a constant error decrease
TRUE
51
Constant errors can often be detected by varying the _________.
sample size
52
formula for relative error
initial weight- expected weight/ expected weight x 100
53
TRUE or FALSE If the measurement result is low, the sign is negative; if the measurement result is high, the sign is positive.
TRUE
54
TRUE or FALSE method errors are usually the most difficult to identify and correct
TRUE
55
the formula for relative error
Er= measured value- true values/ true value x 100
56
TRUE or FALSE effect of a constant error becomes more serious as the size of the quantity measured decreases.
TRUE
57
The term ______ refers to the quantity measured by an instrument.
response
58
TRUE or FALSE Most personal errors can be minimized by careful, disciplined laboratory work.
TRUE
58
TRUE or FALSE Errors due to limitations of the experimenter can usually be avoided by carefully choosing the analytical method or using an automated procedure.
TRUE
58
are substances sold by the National Institute of Standards and Technology (NIST) and certified to contain specified concentrations of one or more analytes.
standard reference materials
59
The term ________ refers to the collection of all the constituents in the sample.
matrix