Chapter 10: : Precision and Accuracy of Chemical Analyses

Question 1

Measurement data can only give us an ____ of the “true” value

Answer 1

Estimate

Question 2

In order to improve the reliability and to obtain information about the variability of results

Answer 2

Replicates

Question 3

How many replicates are usually needed to carried out an entire analytical procedure

Answer 3

Several (often two to five) portions (replicates)

Question 4

The best estimate of the true value

Answer 4

Central value for the set

Question 5

• The most widely used measure of central value
• also called the arithmetic mean or the average

Answer 5

Mean

Question 6

is the middle result when replicate data are arranged in increasing or decreasing order

Answer 6

Median

Question 7

indicates the closeness of the measurement to the true or accepted value and is expressed by the error

Answer 7

Accuracy

Question 8

is a measure of how close a series of measurements are to one another

Answer 8

Precision

Question 9

Three terms widely used to describe the precision

Answer 9

• standard deviation
• variance
• coefficient of variation

Question 10

the difference between the experimental value and the accepted value.

Answer 10

Error

Question 11

is the correct value based on reliable references

Answer 11

Accepted value

Question 12

is the value measured in the lab.

Answer 12

Experimental value

Question 13

Error formula

Answer 13

Error= Experimental value - accepted value

Question 14

where xi is a measurement of the quantity and xt is the true or accepted value of the quantity.

Answer 14

Absolute error

Question 15

Absolute error formula

Answer 15

E= xi - xt

Question 16

• often a more useful quantity than the absolute error
• also expressed in parts per thousand (ppt)

Answer 16

Relative error

Question 17

Relative error formula:

Answer 17

Er = xi-xt/xt x 100

Question 18

Types of errors in experimental data

Answer 18

• Random
• Systematic
• Gross error

Question 19

also known as inderterminate error
- causes data to be scattered more or less symmetrically around a mean value
- usually small in values and not avoidable

Answer 19

Random error

Question 20

• also known as determinate error
• causes the mean of a data set to differ from the accepted value

Answer 20

Systematic error

Question 21

Examples of systematic error

Answer 21

• Miscalibration of pH meter
• Miscalibration of analytical balance
• Evaporation of liquids
• Spattering chemicals

Question 22

differ from indeterminate and determinate errors

Answer 22

Gross error

Question 23

are often the product of human errors

Answer 23

Gross error

Question 24

Gross errors lead to ___

Answer 24

Outliers

Question 25

results that appear to differ markedly from all other data in a set of replicate measurements.

Answer 25

Gross errors lead to outliers

Question 26

• have a definite value
• Have an assignable cause

Answer 26

Systematic errors

Question 27

Systematic errors may be either ____ or ____

Answer 27

Constant of proportional

Question 28

• The absolute error is
  constant with sample size
  -The relative error varies when the sample size is changed.

Answer 28

Constant error

Question 29

increase or decrease according to the size of the sample taken for analysis

Answer 29

Proportional errors

Question 30

• the absolute error varies with sample size
• relative error stays constant when the sample size is changed

Answer 30

Proportional errors

Question 31

Example of proportional error

Answer 31

Measured amount of water in a bucket by filling beakers. If the beaker is 5% smaller, you final volume will be off by 5%

Question 32

Detection of systematic: Intrument and personal errors

Answer 32

• Periodic calibration of equipment
• Careful, discipline laboratory work

Question 33

Detection of systematic: Method errors

Answer 33

• Analysis of Standard Samples (or SRMs)
• Blank determination
• Variation of sample size

Question 34

are materials that contain one or more analytes at known concentration levels

Answer 34

SRMs (Standard Reference Materials)

Question 35

contains the reagents and solvents used in a determination but no analyte

Answer 35

A blank

Question 36

many of the sample constituents are added to simulate the analyte environment, which is called

Answer 36

Sample matrix

Question 37

As the size of a measurement increases, the effect of a constant error decreases

Answer 37

Variation of Sample size

Question 38

contains the solvent and all the reagents in an analysis

Answer 38

Blank solution

Question 39

refers to the collection of all the constituents in the sample.

Answer 39

Matrix