Quiz #1 (Notes 5/18 - 6/1)

Question 1

Analytical Chemistry:

Answer 1

Identification, characterization, and quantitative measurement of the chemical composition of a sample

Question 2

Qualitative Analysis -

Answer 2

“what is in a sample?”

Question 3

Quantitative Analysis -

Answer 3

“how much is in a sample?”

Question 4

another name for target compound:

Answer 4

analyte

Question 5

General Steps in a Quantitative Chemical Analysis:

Answer 5

1) Formulate a Question 2) Select an Analytical Procedure (to answer the question) 3) Sampling 4) Sample Preparation 5) Measurement Step (5a: classical or stoichiometric methods; 5b: physical-chemical methods) 6) Calibration and Standardization of Methods 7) Interpretation and Conclusions Based on Analysis

Question 6

1) Formulate a Question:

Answer 6

Translate general questions about the material to be examined into a specific question that can be answered by chemical analysis (or measurements) What information is needed (amount, type, etc.)? What level of accuracy (precision) is required?

Question 7

2) Select an Analytical Procedure (to answer the question):

Answer 7

obtain as much advance information about the sample as possible (size, composition, homogeneity, analyte, concentration, interferences).

Question 8

3) Sampling:

Answer 8

Obtain a laboratory size sample which is representative of the bulk specimen

Question 9

4) Sample Preparation:

Answer 9

Hit the literature! includes… drying dissolving sample pre-concentration isolation of analyte

Question 10

Drying:

Answer 10

removal of excess water or solvent results often based on dry weight. Why? - reproducibility

Question 11

Dissolving Sample:

Answer 11

H2O, acids, bases, organic solvents

Question 12

Pre-concentration:

Answer 12

when the concentration of analyte is too small to detect.

Question 13

Isolation of Analyte:

Answer 13

remove interfering substances by separation method: Distillation, precipitation, and chromatography.

Question 14

When the isolation step is sufficiently selective, then a very ____________ method of measurement may be used.

Answer 14

non-selective

Question 15

5A) Classical or Stoichiometric Methods

Answer 15

Based on chemical reactions with well defined stoichiometry

Question 16

Gravimetric methods:

Answer 16

Analyte reacts quantitatively with excess reagent to produce an insoluble product that is weighed (slow, but high precision, 0.01%)

Question 17

Volumetric Methods:

Answer 17

Analyte is reacted with a standard solution of reagent, changing color or other property of the solution (faster, precision ~0.1%)

Question 18

What is more precise, gravimetric or volumetric? What is faster?

Answer 18

Gravimetric is more precise, volumetric is faster

Question 19

5B) Physical-chemical methods:

Answer 19

Measurement of a physical property related to the concentration of analyte; include light absorption, refractive index, emission, density, conductivity, electrical potential… These methods are generally fast, selective, very sensitive but less precise than classical methods

Question 20

6) Calibration and Standardization of Methods:

Answer 20

Accuracy of a result checked by analyzing a sample of known concentration (a “standard”) Calibration of the instrument response with standards> Loss of sample during preparation checked by standard-addition method or pre-analyzed sample. Sampling erros are checked by replicate random samples

Question 21

How is loss of sample during preparation checked?

Answer 21

by standard-addition method or pre-analyzed sample

Question 22

How is sampling errors checked?

Answer 22

By replicate random samples

Question 23

7) Interpretation and Conclusions Based on Analysis:

Answer 23

Statistical analysis of the results Report the quantitative findings with uncertainties! confidence intervals

Question 24

Define absolute error (ei):

Answer 24

= measured value - true value

Question 25

Define relative error (RE):

Answer 25

ei/true value

Question 26

% Relative Error =

Answer 26

= RE * 100

Question 27

The magnitude of the error in a measurement is an ___________________.

Answer 27

inverse indication of its accuracy

Question 28

Deviation (di) =

Answer 28

xi - xbar

Question 29

xbar =

Answer 29

xbar = 1/N*sum(xi) (see notes 2 page 1)

Question 30

The magnitude of deviations in a series of measurements is an inverse indication of the measurement of ________.

Answer 30

precision

Question 31

Determinate Errors:

Answer 31

correctable errors operating according to known laws (however often difficult to determine). Determinate errors affect accuracy, but when they are constant, do not impact precision.

Question 32

List four types of determinate errors and their explanations

Answer 32

1. Instrument errors - tools, calibration, reagent purity 2. Operator errors - mistakes, blunders 3. Personal errors - color blindness, prejudice 4. Errors of method - side rxns, interferences

Question 33

Random Errors:

Answer 33

errors brought about by the effects of uncontrollable variables, due to the limits of the instrument or observation. You can never see effect of random error unless the measurement approaches the limits of precision of a particular method.

Question 34

Determinate errors effect _______ (accuracy or precision?), but do not effect _______ (accuracy or precision?)

Answer 34

accuracy, precision

Question 35

Dealing with determinate errors:

Answer 35

Careful control of equipment, reagents, technique operator bias, and the method of analysis will minimize these errors.

Question 36

How to check determinate errors?

Answer 36

standardization

Question 37

Standardization technique:

Answer 37

analysis of a sample whose true value is known

Question 38

Gaussian distribution results from ____ ______ __ _______ ______ with each contributing to the ____ ____

Answer 38

Gaussian distribution results from many sources of small errors with each contributing to the total error.

Question 39

Confidence intervals:

Answer 39

determined by the area under the distribution curve within specified limits

Question 40

Z-score:

Answer 40

(X-mu)/sigma

Question 41

Z:

Answer 41

reduced variable indicating how many sigma’s separate a result from the mean.

Question 42

95% confidence

Answer 42

mu +/- 1.96

Question 43

sigma(Xbar) =

Answer 43

sigma(Xbar) = sigma(x)/sqrt(N)

Question 44

σ=

Answer 44

σ=lim(n→infi)[1/NΣ(x_i-μ)²]^1/2​

Question 45

Sample standard deviation (s):

Answer 45

standard deviation based on a small number of samples of the parent population

Question 46

s=

Answer 46

s = [(1/(N-1))Σ(x_i-xbar)²]^1/2

Question 47

How do we estimate confidence intervals based on s?

Answer 47

“Student’s” t Distribution, for small N, the distribution is wider than a Gaussian distribution

Question 48

Confidence interval equation, μ=

Answer 48

μ=x_i​+/-ts

Question 49

Confidence interval for averages: μ=

Answer 49

μ=xbar +/- 1.96σ/sqrt(N)
at 95% confidence, sigma is known

Question 50

confidence interval (sigma unknown):

Answer 50

μ = xbar +/- ts/(sqrtN)

Question 51

Ended notes 3 page 6