Exam 1 Flashcards
1
Q
What are the two types of chemical analysis?
A
qualitative - whats in the sample
quantitative - how much
2
Q
What are the two components of a sample?
A
- the analyte and the matrix
3
Q
What are the steps in chemical analysis?
A
- choose analytes
- obtain representative sample
- large sample from multiple sites - prepare the sample
-grind, dissolve, extract to concentrate - perform analytical procedure
- report numerical result
-appropriate units, uncertainty of the measurement
4
Q
Chemical samples can be…..
A
heterogenous or homogeneous
5
Q
What are three types of extraction?
A
Soxhlet extraction, SPE extraction, QuEChERS method
6
Q
What is Soxhlet extraction?
A
- used when a large amount of sample is available
- extraction thimble made of paper that the sample is placed in
- uses boiling solvent to get analyte out of sample into the solvent, then you can let the solvent evaporate and get a very concentrated sample of the analyte
7
Q
What is SPE?
A
- solid phase extraction
- useful when a sample solution is available
- uses an spe cartridge (these are polar)
8
Q
What are the steps in SPE?
A
- Mix the sample with a suitable solvent like methanol, acetonitrile, dimethyl sulfoxide, etc.
- Shake or homogenize to dissolve as much analyte as possible. (To extract the most analyte, a large volume of solution may be required.)
- Pump the solution through an SPE cartridge that binds the analyte (for example, use a nonpolar SPE cartridge to bind a nonpolar analyte).
- Pump a small amount of nonpolar solvent through the cartridge to flush out the analyte.
- Result: a more concentrated analyte solution with many of the original interferents removed.
9
Q
What is QuEChERS?
A
Quick
Easy
Cheap
Effective
Rugged
Safe
10
Q
What are the steps in QuEChERS?
A
Extraction:
1. Homogenize the sample so it is representative.
2. Shake it with liquid acetonitrile (CH3CN) for 1 min.
Sample clean-up:
3. Add aqueous NaCl, MgSO4, and buffer.
4. Shake 1 min and centrifuge to separate the layers.
5. Decant off the top (supernatant) CH3CN layer.
6. Add MgSO4(s) to the CH3CN extract to adsorb H2O.
7. Add other materials such as anion exchange resin, carbon black, etc. to remove other contaminants.
Now measure the extracted analyte by GC, HPLC, etc.
11
Q
What are 3 official analytical methods?
A
From the U. S. Environmental Protection Agency:
- EPA Method 245.2
analysis of mercury in fish by atomic spectrophotometry
- EPA Method 317
analysis of bromate in water by ion chromatography
From the American Society of Testing Materials:
- ASTM Method D-3525
analysis of fuel contaminants in lubricating oil by gas chromatography
12
Q
Look at quizlet with metric prefixes.
A
13
Q
What is the dilution equation?
A
M1V1=M2V2
14
Q
What is molarity?
A
mols of solute / L solution
(M)
15
Q
What is formality?
A
F = number of formula weights / L of solution
A good way to express the concentration of a compound
(F)
16
Q
Why is formality useful?
A
Formality is independent of whether the compound ionizes, forms complexes, remains intact once in solution
17
Q
What is normality?
A
Equivalents of solute / L solution
- 1 equivalent = 1 reacting unit
For redox reactions:
normality = n ∙ molarity,
where n = moles of electrons transferred per mole of substance in the half reaction.
18
Q
How do you calculate parts per hundred, thousand, million, billion?
A
parts per hundred (percent %) = 100 x mass of substance / mass of total solution
parts per thousand = 1000 x mass of substance / mass of total solution
parts per million = 10^6 x “”
parts per billion = 10^9 x “”
19
Q
What is useful to remember about ppm?
A
Since the density of water is very near 1g/mL, a very useful practical definition of ppm for dilute aqueous solutions is 1 mg of substance / L of total solution
20
Q
What is molality and what is useful about it?
A
- (m)
- defined as mol solute / kg solvent
- this is the only concentration unit defined using the solvent instead of the total solution. molality remains constant even if the temperature changes
21
Q
What is weight percent?
A
- 100 x mass analyte / mass total sample
- aka mass percent
- expressed as 10% by weight, or 10% (w/w) or 10% (m/m)
22
Q
What is volume percent?
A
- 100 x volume of substance / total volume of the mixture
23
Q
What is mixed %?
A
- 10g of solute in 1L solution = 1% (w/v)
- 200 mL solute per 1000g total solution = 20% (v/w)
24
Q
What is mass?
A
the amount of matter present in an object, units of grams
25
What is weight?
- weight expresses the force an object exerts on a lab balance
- units of newtons not grams
- weight takes the acceleration of gravity into account f=ma
26
What are some different weighing methods and what do they measure?
- equal arm balance - measures mass
- spring - measures weight
27
What are the two general types of methods of chemical analysis?
Classical (wet) methods, and instrumental methods
28
What are the classical methods?
Gravimetric methods (by weight)
Volumetric methods (titrations)
29
What are the instrumental methods?
Electrochemistry (potentiometry; voltammetry; coulometry)
Spectroscopy (molecular; atomic)
Separations (chromatography; electrophoresis; extractions)
Others (mass spectrometry; microscopy; thermal analysis, etc.)
30
What is the servo mechanism?
- used in modern single pan balances
- the electric correction current required to raise the pan back to its original null position is measured and displayed in units of grams - current through coil generates magnetic field
31
What are typical errors in weighing? (most serious to least serious)
1. adsorbed moisture in the sample (place in oven, cool in desicator)
2. poor balance calibration
3. air currents (hot samples appear to weigh less)
4. drift due to static electricity (in dry air)
5. ignoring bouyancy (objects displace some air, we can calculate this)
6. building vibrations - vibrations of foundation
32
What is the buoyancy equation?
- allows you to correct a sample weight for bouyancy in air
33
What is the coefficient of expansion for glass?
- borosilicate - least expansion and contraction
- coefficient of thermal expansion = 0.000010 deg ^-1
34
What are the classes of laboratory weights?
class 1 and class 2, class 1 is better
35
What is the coefficient of expansion for water?
- coefficient of thermal expansion = 0.00025 deg^-1
36
What equation do you use to calculate the effect of solvent thermal expansion on solution concentration?
concentration 1 / density 1 = concentration 2 / density 2
37
What are some of the labels on class A glassware?
A = class a
$ (with line over it) = standard taper 19 stopper
KIMAX = brand name
500mL = volume
TC 20 degrees C = 20 degrees celcius
38
What do TD and TC mean?
calibrated to deliver or to contain
39
What are some approximate errors of class A glassware?
graduated beaker or flask = 10%
graduated cylinder = 1-2%
buret, volumetric flask, volumetric pipet = 0.1%
40
What are two types of pipets?
Volumetric (transfer pipet) - drain to the tip to do not blow out
Graduated or "Mohr" pipet - drain only to the bottom calibration mark; the tip is not calibrated
41
What are the 4 types of water?
Tap ,Type 3: Reverse Osmosis, Type 2: Distilled, Type 1:Deionized Water
42
Tap Water
Too impure for making any chemical solutions. Great for washing & preliminary rinsing.
43
Type 3: RO water
-Piped through the building to white plastic taps.
-About half as pure as distilled water, and cheaper.
-Use it to rinse tap water out of glassware.
-Suitable for making some chemical solutions.
-Made by putting pressure on a column of tap water to force it through a semi-permeable membrane
44
Type 2: Distilled Water
-Pure enough for making many chemical solutions.
-Theory that most impurities don't distill/ aren't the same volatility as water
- boil water allow it to pass through condenser
45
Type 1: Deionized Water
Very pure, but expensive. Suitable for making any chemical solution.
46
What are properties of primary standard chemicals?
- very pure (ACS reagent grade at least)
- stable (doesnt decompose, react w air, ect)
- not hygroscopic (hydroscopic things adsorb water)
- easy to dry if necessary
47
Which can be primary standard chemicals? NaCl? NaOH? conc HCl? conc H2SO4?
- NaCl - yes
- NaOH - no, adsorbs water, not easy to dry, reacts w H2CO3
- conc HCl - too volatile, not stable
- conc H2SO4 - adsorbs water
48
What is an example of a primary standard?
KHP - potassium hydrogen phthalate
49
What are two types of errors?
Random/indeterminate - non reproducible
Determinate/systematic - reproducible
50
How do you reduce determinate error?
- use high quality accurate equipment and maintain it well
- use the equipment correctly
51
How do you reduce indeterminate error?
- indeterminate (random) errors cannot be avoided completely but we can minimize and quantify them
- minimize them by developing good reproducible lab technique
- quantify them by using statistical methods to estimate the uncertainty of the measurement
52
Error is a measure of ______ and deviation is a measure of _______
- Error is a measure of accuracy and deviation is a measure of precision
53
What is important to remember about propagation of errors?
- in addition and subtraction calculate the absolute error, then the relative error
- in multiplication and division calculate the relative error first, then the absolute error
54
Absolute error
xi - actual value
55
Relative absolute error
(xi - actual value) / actual value
56
Absolute deviation
xi - mean
57
Average deviation
58
Relative Average Deviation
59
Standard Deviation
60
Variance
s^2
61
Coefficient of variation
- aka relative standard deviation
- (s/m) x 100%
- can also be expressed in ppt, ppm
62
How are sig figs maintained with logs and exponents?
log(5.403x10^-8) =
10^6.142 =
63
What is S.E.M?
standard error of the mean
64
What is a confidence limit?
calculated using t values
65
Why are variances useful?
When adding or subtracting means their variances can be added
66
What is the F test used for?
- to test to prove the standard deviations for two groups are not equal
- if F(calculated) > F(table) then there is a <5% chance that the two data sets came from populations with the same population standard deviation and the difference is considered to be significant
- null hypothesis is that the standard deviations are equal
67
How do you perform an F test?
68
What is the F test asking?
are the standard deviations the same for two data sets
69
What is the T test asking?
- is the difference between means statistically significant
- do the means agree within a specific confidence limit
70
What are the required assumptions for a T test?
- data sets are gaussian
- have uncorrelated means
- have equal standard deviations
71
What is the language for the conclusion of a t-test?
If t(calculated) > t(table) the null hypothesis is rejected and m1 does not equal m2 with ____% confidence
72
What is used to test the means of more than 2 data sets?
the t test is only valid for testing the means between two data sets, for more data sets use anova instead
73
What is the grubbs equation?
- used to exclude an outlier
- the mean and s are calculated before the outlier is excluded
74
What are some desirable characteristics of analytical methods?
- good linearity, high sensitivity, low limit of detection (LOD), low limit of quantitation (LOQ)
75
What is good linearity?
the amplitude of an analytical signal should be directly proportional to the quantity of analyte present
Signal A = Ka x Quantity of A
76
What is high sensitivity?
- sensitivity is the slope (m) of a signal vs quantity plot
- high sensitivity means the signal changes a lot for a small change in analyte quantity
77
What is low detection limit?
- a small quantity of analyte can be detected
- to calculate it analyze a low conc sample multiple times and find the sd
- then calculate LOD = 3 x s/m
- an alternative method is to ignore the slope (m) and just calculate LOD = 3 x s
78
What is the limit of quantitation?
- at the LOD the signal is too unstable to use for good quantitation, so the LOQ is LOQ = 10 x s/m
- LOQ = 3.33 x LOD
79
What are the axes on a direct calibration plot? How are these plots used?
x = concentration
y = absorbance
Example procedure:
1. Make several standards of known concentration (c).
2. Measure their absorbances (A).
3. Measure the absorbance of the unknown.
80
Where are calibration lines most certain?
linear calibration plots are least uncertain near their centroid
81
What is the matrix effect?
occurs when non-target analytes and or physical/chemical characteristics of a sample prevent quantification of the analytes of interest
82
What are the benefits and drawbacks of the direct calibration method?
- works for small samples
- in the plot calibration points can be placed on both sides of the unknown point
- works for both linear and curved calibration plots
- cannot detect the prescence of matrix effects
83
What are the benefits and drawbacks of the standard addition method?
- requires large samples
- in the plot, the region below added analyte lacks any calibration points
- fails if the response is non-linear
- small slope errors cause inaccuracies
- corrects from matrix effects and also detects them if the results are compared with those from direct calibration
84
What is the procedure for the standard addition method?
1. prepare solutions that contain identical matrices
2. place 5ml of unknown solution in each flask
3. then add 0, 5, 10, 15, or 20mL of very pure analyte dissolved in very pure solvent
4. finally fill each flask to the 50mL mark with very pure solvent and mix
5. run each solution and make a standard addition plot
85
What is gravimetric analysis?
- analysis based on weight
- old, slow, usually requires large samples
- example is determination of weight % h2o in moist KBr
(weigh, heat, cool, weigh again)
86
Describe a gravimetric analysis experiment to determine weight percent of Cl- in soluble chloride.
- weigh sample of unknown
- dissolve sample in water
- add aqueous AgNO3 solution to provide excess Ag+
- digest mixture at 90 degrees C for 10 min to increase the size of AgCl particles in the precipitate
- cool and filter the mixture to collect the AgCl
- wash dry and weigh the AgCl
87
What are some properties of precipitates?
- digestion at elevated temperature provides time for particles to expel impurities and to aggregate
- solvent inclusion - containing contaminants
- coprecipitate - wrong substance
- ions on surface - same charge inhibits aggregation of particles
88
What is a combustion train? What are the 3 traps?
- a type of gravimetric analysis for C/H compounds
89
What is TGA? What does its graph look like?
- measures the sample mass while it is being heated
90
What is TGA useful for?
-thermal behavior of new materials and catalysts
-chemical reactions at surfaces
-phase behavior of polymers
-thermal stability of polymers
91
What is the law of mass action? When is it rigorously true?
aA + bB <-> cC + dD
Keq = ([C]^c[D]^d) / ([B]^b[A]^a)
Rigorously true only at infinite dilution
92
What are the rules for manipulating Keqs (equilibrium constants)?
- if two equations are summed their K values are multiplied
- if an equation is reverse K new = 1/K old
93
Why are Keq values unitless?
- K eq values are always unitless because concentrations are divided by the concentration of the reference state
- Standard state for aqueous solutions is 1 M, solid is the concentration of the solid itself, solvent is the the concentration of the pure solvent, gas is pressure at 1 bar
94
What is Le Chatlier's principle?
If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or total pressure the equilibrium will shift so as to partially counteract the imposed change
95
Common ion effect
A shift in the equilibrium caused by the addition of a substance that provides an ion that takes part in the equilibrium
96
What is a tip for complex equilibrium problems?
if you can find a concentration from any of the equations, it must be the same in all the equations
97
What is the Arrhenius definition of acids and bases?
- acid - forms H+ in aqueous solutions
- base forms OH- in aqueous solutions
98
What is the Bronsted Lowry definition of acids and bases?
Acid - H+ donor
Base - H+ acceptor
99
What is the Lewis definition of acids and bases?
Acid - electron pair acceptor
Base - electron pair donor
100
What is Kw?
the equilibrium between acids and bases
Kw = (Ka)(Kb) = 1.0 x 10^-14 (temperature dependent)
101
What is the pKa equation?
pKa = -logKa
102
Describe the pKa of strong acids and their conjugate bases?
- strong acids have a pKa < = 0
- conjugate acids have a pKa > = 16
103
What must be included in a base hydrolysis reaction?
water must be a reactant
104
Give an example acid dissociation reaction.
HA + H2O -> A- + H3O+
105
Give an example of a base hydrolysis reaction.
A- + H2O -> HA + OH-
106
What are the pH equations?
pH = - log [H+]
pH + pOH = 14.00
Kw = Ka x Kb
107
What do Ka1 and Kb1 represent in the case of polyprotic acids and bases?
Ka1 = the dissociation of the most protonated form
Kb1 = the hydrolysis of the least protonated form
108
What is the difference between absorption and adsorption?
In adsorption, a substance becomes bound to the surface of another substance. In absorption, a substance is taken up inside another substance.
109
What is the difference between an inclusion and an occlusion?
- an inclusion is an impurity that occupies lattice sites in a crystal
- an occlusion is an impurity trapped inside a pocket in a growing crystal
110
Solubility products predict that cation A3+ can be 99.999% separated from cation B2+ by precipitation with anion X−. When the separation is tried, we find 0.2% contamination of AX3(s) with B2+. Explain what might be happening.
It is likely that BX2 coprecipitates with AX3. This means that some BX2 is trapped in or on AX3 during precipitation
111
Why do activity coefficients not appear in the charge and mass balance equations?
charge and number of atoms are proportional to molarity, not to activity
112
List some polyvalent ions.
Ca2+ Al3+ SO42-
113
How do polyvalent ions behave in solution?
often they do not dissociate completely and exist partly as ion pairs
114
Which drying agent is more efficient drierite or phosphorus pentoxide?
Phosphorous pentoxide (really P4O10) is an anhydride of phosphoric acid. Phosphorous pentoxide reacts with water to form phosphoric acid. Phosphoric acid is also hygroscopic and absorbs water to form a solution. Calcium chloride and calcium sulfate (drierite) only continue to absorb water until they reach their maximum hydration.
115
Describe the non-deal behavior of concentrated nitric acid.
- “Concentrated” nitric acid is ~70%(w/w) HNO3 in water. Its concentration is 15.6 F.
- However, nitric acid is only about 20% dissociated
- the solution contains ~1.5 water molecules / HNO3 molecule! H+ and NO3– are stable in solution only if solvated. There are too few H2O molecules to solvate all the potential ions
- Even strong acids dissociate poorly at high concentrations
116
What are the three types of ion pairs?
Contact ion pair - no solvent layer between
Solvent-shared ion pair - 1 layer solvent between
Solvent-separated ion pair - 2 solvent layers between
117
pH meters measure _________ not _________
Activity, not concentration
118
What is Ionic atmosphere and how does it explain the effects of solutes on equilibrium constants?
- The greater the ionic strength of a solution, the higher the charge in the ionic atmosphere. Each ion-plus-atmosphere contains less net charge and there is less attraction between any particular cation and anion.
- Ions with small ionic radii and large charge tend to more strongly bind to solvent molecules (Ion-dipole interactions).
- The result of this binding is a larger hydrated radius, causing diminished interaction with other ions.
119
How do you calculate ionic strengths?
120
How is ionic strength related to activity?
As the ionic strength of a solution increases, the activity coefficient (γ) of an ion decreases according to the Debye–Hückel limiting law
