Chapter 3: Precision and Accuracy of Chemical Analyses

Question 1

The ______ of two or more
measurements is their average value

Answer 1

mean

Question 2

mean is also called as

Answer 2

arithmetic mean or average

Question 3

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

Answer 3

errors

Question 4

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

Answer 4

TRUE

Question 5

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

Answer 5

ERROR

Question 6

often denotes the estimated uncertainty in
a measurement or experiment.

Answer 6

error

Question 7

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.

Answer 7

median

Question 8

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

Answer 8

outlier

Question 9

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

Answer 9

precision

Question 10

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

Answer 10

standard deviation
variance
coefficient of variation

Question 11

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

Answer 11

deviation from the mean

Question 12

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

Answer 12

accuracy

Question 13

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

Answer 13

Precision

Question 14

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.

Answer 14

Accuracy

Question 15

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

Answer 15

TRUE

Question 15

the formula for absolute error

Answer 15

E= measured value - true value

Question 16

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

Answer 16

relative error

Question 17

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

Answer 17

relative error

Question 18

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

Answer 18

random or indeterminate error

Question 19

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

Answer 19

systematic or determinate error

Question 20

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

Answer 20

replicate

Question 21

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

Answer 21

TRUE

Question 22

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

Answer 22

What maximum error can be tolerated in the result?

Question 23

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

Answer 23

TRUE

Question 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.

Answer 24

gross errors

Question 25

Gross errors lead to

Answer 25

outliers

Question 26

have a definite value, an assignable cause, and are of the same magnitude for replicate measurements made in the same way

Answer 26

systematic errors

Question 27

systematic errors leas to ______ in measurement results

Answer 27

bias

Question 28

three types of systematic errors

Answer 28

instrumental errors method errors personal errors

Question 29

are caused by nonideal instrument behavior, by faulty calibrations, or by use under inappropriate conditions

Answer 29

instrumental errors

Question 30

arise from nonideal chemical or physical behavior of analytical systems.

Answer 30

method errors

Question 31

result from the carelessness, inattention, or personal limitations of the experimenter.

Answer 31

personal errors

Question 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.

Answer 32

random or indeterminate errors

Question 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

Answer 33

systematic errors

Question 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

Answer 34

gross error

Question 35

is an occasional result in replicate measurements that differs significantly from the other results

Answer 35

outlier

Question 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

Answer 36

bias

Question 37

determines the relationship between a measured quantity and the analyte concentration. _________ is a very important part of any analytical procedure

Answer 37

calibration

Question 38

eliminates most systematic errors of this type

Answer 38

calibration

Question 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

Answer 39

systematic method errors

Question 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

Answer 40

TRUE

Question 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

Answer 41

Personal errors

Question 42

universal source of error is

Answer 42

prejudice or bias

Question 43

these are errors that are independent of the size of the sample being analyzed

Answer 43

constant errors

Question 44

decrease or increase in proportion to the size of the sample.

Answer 44

proportional errors

Question 45

systematic errors can be either _____ or ________

Answer 45

constant or proportional

Question 46

TRUE or FALSE To minimize the effects of constant errors, keep sample size as less as is feasible

Answer 46

FALSE To minimize the effects of constant errors, keep sample size as large as is feasible

Question 47

Detection of systematic method errors

Answer 47

Analysis of Standard Samples Independent Analysis Blank Determinations Variation in Sample Size

Question 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.

Answer 48

standard reference materials (SRMs)

Question 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.

Answer 49

blank

Question 50

TRUE or FALSE as the size of a measurement increases, the effect of a constant error decrease

Answer 50

TRUE

Question 51

Constant errors can often be detected by varying the _________.

Answer 51

sample size

Question 52

formula for relative error

Answer 52

initial weight- expected weight/ expected weight x 100

Question 53

TRUE or FALSE If the measurement result is low, the sign is negative; if the measurement result is high, the sign is positive.

Answer 53

TRUE

Question 54

TRUE or FALSE method errors are usually the most difficult to identify and correct

Answer 54

TRUE

Question 55

the formula for relative error

Answer 55

Er= measured value- true values/ true value x 100

Question 56

TRUE or FALSE effect of a constant error becomes more serious as the size of the quantity measured decreases.

Answer 56

TRUE

Question 57

The term ______ refers to the quantity measured by an instrument.

Answer 57

response

Question 58

TRUE or FALSE Most personal errors can be minimized by careful, disciplined laboratory work.

Answer 58

TRUE

Question 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.

Answer 58

TRUE

Question 58

are substances sold by the National Institute of Standards and Technology (NIST) and certified to contain specified concentrations of one or more analytes.

Answer 58

standard reference materials

Question 59

The term ________ refers to the collection of all the constituents in the sample.

Answer 59

matrix