Exam 2 Flashcards
(133 cards)
1
Q
What is the pH of a weak acid solution influenced by?
A
pKa and the [A^-]/[HA] ratio (from Henderson Hasselbalch equation)
2
Q
Solution of a weak acid reaction equation
A
HA(aq) + H2O(l) = H3O^+ + A^-
3
Q
Buffer definition
A
an aqueous solution of a weak acid (HA) and its conjugate base (A^-). With sufficient concentration of A^- and HA, it can resis drastic changes in pH upon addition of small amounts of a weak acid or base
4
Q
How are buffers made?
A
- Partially titrating the buffer acid(HA) with a strong base
- Partially titrating the buffer base (A^-) with a strong acid
- Combining the weak buffer acid and a salt of the buffer acid in a known ratio
5
Q
Why is a buffer able to resist drastic pH change?
A
It can react with a strong acid to produce HA or a strong base to produce A^-
6
Q
When will the pH change dramatically in a buffer solution?
A
when the acid or base component of the buffer is consumed from reacting with a strong acid or base that was added to the solution
7
Q
Buffer capacity
A
the amount of strong acid or strong base that can be consumed by the buffer before the pH changes drastically, it is an amount in moles
8
Q
What limits buffer capacity?
A
The capacity of the buffer to consume acid is limited by the concentration of A^- present and the capacity of the buffer to consume base is limited by the concentration of HA present in the original buffer
9
Q
How to find the range in which a buffer is effective
A
pH=pKa +/- 1
10
Q
what determines how big of a range a buffer is effective over?
A
whether it is monoprotic, diprotic, triprotic, etc. The amount of H+ the acid has to give away is how many different ranges are covered by the buffer
11
Q
Process for making buffer solutions in volumetric flasks
A
- add known amounts of K2HPO4 and KH2PO4 to flask
- rinse neck of flask with distilled water
- Fill flask halfway with distilled water and swirl to mix
- Dilute to etched line
- parafilm and invert to mix
12
Q
What 2 equations do you use when setting up a system of equations when calculating concentrations for the target buffer?
A
Henderson hasselbalch equation and 0.05=[A^-]+[HA]
13
Q
what does it mean to quantitatively transfer something?
A
Pour it from location A to location B then follow with 2 rinses of distilled water from location A to location B
14
Q
What is quantitatively transferred in the buffer lab?
A
CH3CO2H(acetic acid)
15
Q
Which part of the buffer lab included the use of water as a buffer?
A
The portion following the K2HPO4/KH2PO4 buffer, before the acetic acid buffer
16
Q
What formula is used to calculate buffer capacities?
A
the molarity formula
17
Q
in general, how are cations and anions identified in the qualitative analysis lab?
A
they are identified by their reactions in aq solutions with a small group of common reagents
18
Q
In the qualitative lab, what property must the reactions have and why?
A
the must have a visual effect because the lab is qualitative and we cant measure anything
19
Q
What is given about the unknown solutions in the qualitative lab before you analyze them?
A
- they each have 2-3 cations and 2-3 anions
- they don’t contain more than one halide
- they dont contain Fe^3+
- They won’t contain BOTH Ca^2+ and Mg2+
20
Q
what type of reactions do we use in the qualitative analysis lab?
A
mostly precipitation reactions
21
Q
Solubility of (NO3)^-
A
salts of all metal cations are soluble
22
Q
Solubility of CH3COO^-
A
salts of all metal cations are soluble
23
Q
What are the halides in the qualitative analysis lab?
A
Cl-, Br- and I-
24
Q
Halide solubility
A
soluble except those with Hg2^2+, Ag^2+ and Pb^2+
25
Sulfate solubility
soluble except with Sr^2+, Ba^2+, Hg2^2+ and Pb^2+
26
Carbonate solubility
insoluble except with NH4^+ and alkali metal ions
27
Phosphate solubility
insoluble except with NH4^+ and alkali metal ions
28
Hydroxide solubility
insoluble except with NH4^+, alkali metal cations, Ba^2+, Ca^2+, and Sr^2+
29
Sulfides solubility
insoluble except with NH4^+, alkali metal cations, Ba^2+, Ca^2+, and Sr^2+
30
Color and form of AgCl
white precipitate
31
Ksp
the solubility product constant, also and equilibrium constant, indicates the extent to which dissolution occurs, indicates the ability of precipitate to dissolve,
32
Ksp of inorganic compounds
product of the equilibrium concentration ions raised to the power of their coefficients in the equilibrium reaction
33
What does the relationship between Q and K tell you
Q>K - precipitate forms
| Q
34
pKsp trends
larger Ksp=less soluble and more stable precipitate
35
Characteristics of pKsp's less than 5, 6-7 and greater than 10
<5: difficult to form, unstable and easily dissolved
6-7: easily formed and stable
>10: considered quantitative
36
What makes a precipitation equilbria pH dependent?
many anions are bases in aq solutions with pH values from 1-13 and they can protonated
37
What happens when an anionic based is titrated with an acid
they are replaced in solution by an anion that generates soluble salts
38
What does Le Chatlier's principle say will when the concentration of the anion is reduced by prontonation?
the precipitation reaction will be reversed
39
How do NH3 and thiosulfate dissolve reagents?
they form complexes with them
40
complex ion
an ion that consists of a central metal cation covalently bonded to one or more anions one or more ligands
41
Calgamite
an indicator dye used to identify metal precipitates
42
How can Zn^2+ be selectively precipitated?
with ferrocyanide
43
Role of H2PO4^-
it can complex with Fe^3+ and break up the Fe(salicylate)^+ complex
44
role of flame test
used to detect Na^+
45
What lab technique was used in the qualitative analysis lab?
microscale laboratory technique
46
What do you do to help you see the color of solutions in the qualitative analysis lab?
place the microplate on a white paper towel
47
Which solutions is calgamite exclusively added to?
the hydroxide (OH^-) solutions
48
What do you do if a precipitate forms in the microplate?
Perform the metal-oxalate test, count and record the number of drops of 1 M HNO3 that are required to dissolve the precipitate
49
Steps for performing the flame test
1. Fill a test tube 1/3 of the way with stock solution and soak the wood splint in it for 5 minutes
2. Wave the stick in and out of a bunsen burner flame
3. Record observations and dip the splint in the water to extinguish the flame
50
What were halides tested with in the qualitative analysis lab?
1M AgNO3
51
How was the thiosulfate performed?
1. 10 drops of each halide was added to a separate test tube
2. 10 drops of AgNO3 was added to each tube and observations were recorded
3. add 5-10 drops of thiosulfate to each tube, record observations then keep adding drops until no further changes are observed
52
Purpose of Fe(salicylate)^+ test
test for H2PO4^- ion
53
Plan for unknown solution and rationale behind each step
1. Sulfate to test for Ba^2+ because it is the only thing that formed a white precipitate
2. Oxalate test for Ca^2+ because it is the only thing that made a white and cloudy precipitate
3. OH^- test to test for Mg^2+ because only Mg^2+ reacted with calgamite
4. Flame test for Na^+
54
What must be done before the procedure is applied to experimental water samples?
we must analyze artificial water samples in which the known contaminant level is in the range expected for experimental samples
55
How are ion concentrations determined in the analysis of surface waters lab?
ion-selective electrode(ISE)
56
units used to report nitrate levels in water
mg/L N
57
How are the units of mg/L N interpreted
1 L of solution contains enough solute to contaminate 1 mg of N
58
How was glassware cleaned in the analysis of surface waters lab?
1. wash each piece with detergent and rinse with tap water 3-4 times
2. Rinse at least twice with about 20mL of 1% HCl
3. Rinse 3+ times with deionized water, fill the glassware 1/6 of it's volume with deionized water
Clean the 600mL and 250mL beaker first and fill them with deionized water/HCl to clean the rest of the glassware with
59
What precaution must be taken with the nitrate stock solution during the water analysis lab and why?
it is not stable for extended periods of time so it is acidified, 12mL of 3.0M aqueous H2SO4 solution is added before the flask is brought to 1L
60
General purpose of each week of the water analysis lab
Week 1: devise a group hypothesis to test nitrate concentration
Week 2: Run the complete analytical process for nitrate contamination on two artificial, validated water samples NA and NB and become familiar with the experimental process
Week 3: Carry out the experimental process from week 2 on the collected environmental samples
61
In what pattern are water samples analyzed and why?
They are measured as triplicates so we can analyze the precision/reproducibility of the values
62
What role does nitrate play in the water analysis lab?
nitrate is the analyte
63
Purpose of standard solutions in the water analysis experiment
produce a calibration curve with solutions of known concentration of N. This curve will then be interpolated
64
How are nitrate concentrations usually reported?
in terms of N concentration
65
Limitation of nitrate method
it is applicable to samples with a concentration of 1 mg/L or greater
66
Why must the stock solution be diluted when preparing the standard solutions in the water analysis lab?
nitrate concentration rarely exceeds 10 mg/L in nature and the stock solution is at 100 mg/L so it needs to be diluted
67
What is the stock solution diluted with in the water analysis lab?
deionized water
68
Range of concentration of standard solutions in the water analysis lab
1-10mg/L
69
How were standard solutions prepared in the water analysis lab?
diluting the stock solution with deionized water
70
What is the process of measuring the nitrate concentration in the water samples in the water analysis lab?
An ion-selective electrode(ISE) is used. An ISE develops a potential voltage that is sensitive to the concentration specific to the ion you're looking for. It is insensitive to other ions in the solution
71
Electrode potential formula
Electrode potential=m x log[NO3-] + b
72
What other lab equipment is the ISE similar to
pH electrode
73
What must be true of the slope of a graph of electrode potential vs. log[NO3-]?
it must be linear
74
Purpose of ammonium sulfate/(NH4)2SO4 in the water analysis lab
In practice, the ISE potential is somewhat sensitive to the total concentration of all other ions in the solution, so this error is minimized by adding a large excess of inert ions and the ionic strength adjusting for ammonium sulfate. Therefore, when a small amount of concentrated ammonium sulfate is added to the collected samples, the total ionic concentration is essentially the same.
75
Why are the two 600 mL and the 250 mL beakers cleaned first in the water analysis experiment?
so you can keep deionized water and HCl at your bench to clean the rest of the glassware
76
Use and number of 600mL beakers in the water analysis lab
2, holding deionized water to use at bench
77
Use and number of 250 mL beakers in the water analysis lab
holding 1% HCl at your bench for you to use
78
Use and number of large test tubes in the water analysis lab
11, running nitrate test. 5 tubes are standard solutions, 3 are for NA and 3 are for NB
79
Use and number of 100 mL volumetric flasks in the water analysis lab
6, preparation of 5 standard solutions and 1 ISA
80
Use and number of 125mL Erlenmeyer flasks in the water analysis lab
7, 6 for holding triplicates NA and NB solutions and 1 for the storage of ISA
81
Use and number of 100mL graduated cylinders in the water analysis lab
2, measuring volume of NA and NB
82
Use and number of 10 mL pipets in the water analysis lab
1, for measuring ISA and 1 for making standard solution
83
Use and number of 150mL beaker in the water analysis lab
1, holding stock solution that is poured from storage bottle so you don't contaminate your stock solution
84
Use and number of 400mL beakers in the water analysis lab
2, collecting NA and NB from the storage bench
85
Use and number of 250mL sample bottle in the water analysis lab
collecting environmental samples between weeks 2 and 3
86
How to figure out how much stock solution is needed to make the standard solutions in the water analysis lab
M1V1=M2V2
87
Suggestion concentrations of standard solution concentrations in the water analysis lab
1, 2, 4, 6, and 10 mg/L
88
what is used to transfer the stock solution to make the standard solutions in the water analysis lab?
a 10 mL Mohr pipet
89
What is ISA transferred to and from for storage in the water analysis lab and why?
it is transferred to a 125 mL Erlenmeyer flash since the volumetric flask it is prepared in is used by other lab sections
90
How is ISA mixed with the standard solutions after it is added in the water analysis lab?
inverting the mixture 10 times
91
How is the volume of standard solution being poured in to the test tubes estimated in the water analysis lab?
they are filled halfway, an estimate of 40 mL
92
What is done in between measurements with the ISE?
it is dipped in deionized water and blotted dry
93
What is added to the phosphate buffer and what is added to the unknown buffer?
phosphate: HCl
Unknown: NaOH
94
Purpose of complex ions in qualitative analysis lab
add reagents to dissolve precipitates by form complexes with the metal portion and identifying metal cations
95
Why is the flame testing only actually useful in detecting Na?
the flame color from Na will mask out other alkali metal ions that have their own distinctive color in flame
96
Purpose of glass stirring rod in water analysis experiment
(week 3 only) mix environmental samples after addition of ISA
97
purpose of fluted filter paper
in week 3 of water analysis experiment, use gravity filtration on environmental samples
98
voltaic cell
electrochemical cell in which oxidation-redox reactions reactions occur spontaneously and transfer of electrons occurs through an external pathway.
99
electrodes
metal strips(solids) in electrochemical cells
100
Which strips go in which solutions?
the metal strip goes in the solution with the matching metal ions in it
101
At which electrodes to oxidation and reduction take place?
Anode: oxidation
Cathode: reduction
102
salt bridge
porous material that provides an electrical connection through which ions can migrate from one compartment to another
103
characteristics of voltaic cell reactions
spontaneous, easily measured electrical potential that can be packaged into batteries, forward reactions, not equilibrium reactions
104
why are voltaic cell reaction forward reactions and not equilibrium reactions?
they are spontaneous, electrical potential is what makes batteries work
105
Standard cell potential
E*cell, depends on standard reduction potentials of cathode and anode
106
standard reduction potential
standard electrode potentials tabulated for reduction reactions
107
How to determine cathode from standard reduction potentials
the electrode with the more positive value is more easily reduced, and therefore the cathode
108
What is true of E*cell in spontaneous reactions?
it is greater than 0
109
what does a negative 🔼G* value indicate?
the reaction is spontaneous
110
Relationship between 🔼G* and E*cell?
inverse relationship, if one is negative, the other is positive and vice versa
111
what R value do you use in the formula related free energery change and standard free energy change?
R=8.314
112
difference between E*cell and Ecell
E*cell refers to the standard state of 1M for all dissolved substances, Ecell refers to different concentrations
113
Under what conditions can you used the Ecell=E*cell-(0.0592/n)logQ formula?
room temperature(298K)
114
Role of KNO3 in the voltaic cell lab
to soak filter paper to form salt bridge
115
Which electrode was connected to the red lead and the black lead?
The cathode is supposed to be connected to the red lead and the anode to the black lead
116
How do you know if you properly matched the cathode/anode to the red/black lead?
the measured potential will be positive
117
purpose of part 2 of the voltaic cell experiment
see the dependence of the potential of voltaic cells ont he salt solution concentrations
118
purpose of part 3 of the voltaic cell experiment
see the dependence of cell potentials upon addition of EDTA
119
Why cant deionized water be used as a standard solution in the water analysis lab?
it has an unpredictable electrode response
120
what can affect the ISE accuracy once the probe in put into the solution?
The probe can not be touching the bottom, have air bubbles, and solution must be up to the white dot
121
What does a precipitate consist of
an insoluble ionic solidp
122
purpose of ferrocyanide test
for colorless metal cations
123
What is the ISA solution transferred with?
a 10 mL mohr pipet
124
What is the ISA in the water analysis lab?
ionic strength adjusting solution, (NH4)2SO4
125
Use of 400 ml beaker in week 3 of water analysis lab
hold filtered environmental samples
126
How many test tubes and needed in week 3 of the water analysis lab and why?
14: 5 for standard and 3 for each of the 3 environmental samples
127
What is used to transfer standard solutions into test tubes in water analysis lab?
a mohr pipet that is separate from then one you used for ISA solutions
128
How much ISA is added to the environmental water samples?
2 mL for every 100 mL of environmental sample
129
process for cleaning metal strips in the voltaic cell lab
clean each strip with sandpaper for 30 sec with a different sandpaper piece for each metal. Rinse each piece with distilled water then quickly dip each electrode strip into dilute 1M HCl and rinse each strip again with distilled water
130
Precautions to be taken with filter paper in voltaic cell experiment
Make sure it reaches both solutions, does not dry out and does not touch leads or salt bridge
131
How do you find the buffer capacity graphically?
the moles at the point at which the pH graph starts to change rapidly
132
purpose of KNO3 in the water analysis lab
it is used to make the stock solution
133
Why must the slope and y intercept of the ISE be measured experimentally?
they differ between each individual ISE
