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CHM 145 Prof. Jessa > Exam 2 > Flashcards

Exam 2 Flashcards

1
Q

What is the pH of a weak acid solution influenced by?

A

pKa and the [A^-]/[HA] ratio (from Henderson Hasselbalch equation)

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2
Q

Solution of a weak acid reaction equation

A

HA(aq) + H2O(l) = H3O^+ + A^-

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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

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4
Q

How are buffers made?

A
  1. Partially titrating the buffer acid(HA) with a strong base
  2. Partially titrating the buffer base (A^-) with a strong acid
  3. Combining the weak buffer acid and a salt of the buffer acid in a known ratio
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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^-

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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

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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

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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

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9
Q

How to find the range in which a buffer is effective

A

pH=pKa +/- 1

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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

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11
Q

Process for making buffer solutions in volumetric flasks

A
  1. add known amounts of K2HPO4 and KH2PO4 to flask
  2. rinse neck of flask with distilled water
  3. Fill flask halfway with distilled water and swirl to mix
  4. Dilute to etched line
  5. parafilm and invert to mix
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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]

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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

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14
Q

What is quantitatively transferred in the buffer lab?

A

CH3CO2H(acetic acid)

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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

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16
Q

What formula is used to calculate buffer capacities?

A

the molarity formula

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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

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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

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19
Q

What is given about the unknown solutions in the qualitative lab before you analyze them?

A
  1. they each have 2-3 cations and 2-3 anions
  2. they don’t contain more than one halide
  3. they dont contain Fe^3+
  4. They won’t contain BOTH Ca^2+ and Mg2+
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20
Q

what type of reactions do we use in the qualitative analysis lab?

A

mostly precipitation reactions

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21
Q

Solubility of (NO3)^-

A

salts of all metal cations are soluble

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22
Q

Solubility of CH3COO^-

A

salts of all metal cations are soluble

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23
Q

What are the halides in the qualitative analysis lab?

A

Cl-, Br- and I-

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24
Q

Halide solubility

A

soluble except those with Hg2^2+, Ag^2+ and Pb^2+

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25
Q

Sulfate solubility

A

soluble except with Sr^2+, Ba^2+, Hg2^2+ and Pb^2+

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26
Q

Carbonate solubility

A

insoluble except with NH4^+ and alkali metal ions

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27
Q

Phosphate solubility

A

insoluble except with NH4^+ and alkali metal ions

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28
Q

Hydroxide solubility

A

insoluble except with NH4^+, alkali metal cations, Ba^2+, Ca^2+, and Sr^2+

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29
Q

Sulfides solubility

A

insoluble except with NH4^+, alkali metal cations, Ba^2+, Ca^2+, and Sr^2+

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30
Q

Color and form of AgCl

A

white precipitate

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31
Q

Ksp

A

the solubility product constant, also and equilibrium constant, indicates the extent to which dissolution occurs, indicates the ability of precipitate to dissolve,

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32
Q

Ksp of inorganic compounds

A

product of the equilibrium concentration ions raised to the power of their coefficients in the equilibrium reaction

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33
Q

What does the relationship between Q and K tell you

A

Q>K - precipitate forms

Q

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34
Q

pKsp trends

A

larger Ksp=less soluble and more stable precipitate

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35
Q

Characteristics of pKsp’s less than 5, 6-7 and greater than 10

A

<5: difficult to form, unstable and easily dissolved

6-7: easily formed and stable

> 10: considered quantitative

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36
Q

What makes a precipitation equilbria pH dependent?

A

many anions are bases in aq solutions with pH values from 1-13 and they can protonated

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37
Q

What happens when an anionic based is titrated with an acid

A

they are replaced in solution by an anion that generates soluble salts

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38
Q

What does Le Chatlier’s principle say will when the concentration of the anion is reduced by prontonation?

A

the precipitation reaction will be reversed

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39
Q

How do NH3 and thiosulfate dissolve reagents?

A

they form complexes with them

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40
Q

complex ion

A

an ion that consists of a central metal cation covalently bonded to one or more anions one or more ligands

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41
Q

Calgamite

A

an indicator dye used to identify metal precipitates

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42
Q

How can Zn^2+ be selectively precipitated?

A

with ferrocyanide

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43
Q

Role of H2PO4^-

A

it can complex with Fe^3+ and break up the Fe(salicylate)^+ complex

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44
Q

role of flame test

A

used to detect Na^+

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45
Q

What lab technique was used in the qualitative analysis lab?

A

microscale laboratory technique

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46
Q

What do you do to help you see the color of solutions in the qualitative analysis lab?

A

place the microplate on a white paper towel

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47
Q

Which solutions is calgamite exclusively added to?

A

the hydroxide (OH^-) solutions

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48
Q

What do you do if a precipitate forms in the microplate?

A

Perform the metal-oxalate test, count and record the number of drops of 1 M HNO3 that are required to dissolve the precipitate

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49
Q

Steps for performing the flame test

A
  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
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50
Q

What were halides tested with in the qualitative analysis lab?

A

1M AgNO3

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51
Q

How was the thiosulfate performed?

A
  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
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52
Q

Purpose of Fe(salicylate)^+ test

A

test for H2PO4^- ion

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53
Q

Plan for unknown solution and rationale behind each step

A
  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^+
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54
Q

What must be done before the procedure is applied to experimental water samples?

A

we must analyze artificial water samples in which the known contaminant level is in the range expected for experimental samples

55
Q

How are ion concentrations determined in the analysis of surface waters lab?

A

ion-selective electrode(ISE)

56
Q

units used to report nitrate levels in water

A

mg/L N

57
Q

How are the units of mg/L N interpreted

A

1 L of solution contains enough solute to contaminate 1 mg of N

58
Q

How was glassware cleaned in the analysis of surface waters lab?

A
  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
Q

What precaution must be taken with the nitrate stock solution during the water analysis lab and why?

A

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
Q

General purpose of each week of the water analysis lab

A

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
Q

In what pattern are water samples analyzed and why?

A

They are measured as triplicates so we can analyze the precision/reproducibility of the values

62
Q

What role does nitrate play in the water analysis lab?

A

nitrate is the analyte

63
Q

Purpose of standard solutions in the water analysis experiment

A

produce a calibration curve with solutions of known concentration of N. This curve will then be interpolated

64
Q

How are nitrate concentrations usually reported?

A

in terms of N concentration

65
Q

Limitation of nitrate method

A

it is applicable to samples with a concentration of 1 mg/L or greater

66
Q

Why must the stock solution be diluted when preparing the standard solutions in the water analysis lab?

A

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
Q

What is the stock solution diluted with in the water analysis lab?

A

deionized water

68
Q

Range of concentration of standard solutions in the water analysis lab

A

1-10mg/L

69
Q

How were standard solutions prepared in the water analysis lab?

A

diluting the stock solution with deionized water

70
Q

What is the process of measuring the nitrate concentration in the water samples in the water analysis lab?

A

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
Q

Electrode potential formula

A

Electrode potential=m x log[NO3-] + b

72
Q

What other lab equipment is the ISE similar to

A

pH electrode

73
Q

What must be true of the slope of a graph of electrode potential vs. log[NO3-]?

A

it must be linear

74
Q

Purpose of ammonium sulfate/(NH4)2SO4 in the water analysis lab

A

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
Q

Why are the two 600 mL and the 250 mL beakers cleaned first in the water analysis experiment?

A

so you can keep deionized water and HCl at your bench to clean the rest of the glassware

76
Q

Use and number of 600mL beakers in the water analysis lab

A

2, holding deionized water to use at bench

77
Q

Use and number of 250 mL beakers in the water analysis lab

A

holding 1% HCl at your bench for you to use

78
Q

Use and number of large test tubes in the water analysis lab

A

11, running nitrate test. 5 tubes are standard solutions, 3 are for NA and 3 are for NB

79
Q

Use and number of 100 mL volumetric flasks in the water analysis lab

A

6, preparation of 5 standard solutions and 1 ISA

80
Q

Use and number of 125mL Erlenmeyer flasks in the water analysis lab

A

7, 6 for holding triplicates NA and NB solutions and 1 for the storage of ISA

81
Q

Use and number of 100mL graduated cylinders in the water analysis lab

A

2, measuring volume of NA and NB

82
Q

Use and number of 10 mL pipets in the water analysis lab

A

1, for measuring ISA and 1 for making standard solution

83
Q

Use and number of 150mL beaker in the water analysis lab

A

1, holding stock solution that is poured from storage bottle so you don’t contaminate your stock solution

84
Q

Use and number of 400mL beakers in the water analysis lab

A

2, collecting NA and NB from the storage bench

85
Q

Use and number of 250mL sample bottle in the water analysis lab

A

collecting environmental samples between weeks 2 and 3

86
Q

How to figure out how much stock solution is needed to make the standard solutions in the water analysis lab

A

M1V1=M2V2

87
Q

Suggestion concentrations of standard solution concentrations in the water analysis lab

A

1, 2, 4, 6, and 10 mg/L

88
Q

what is used to transfer the stock solution to make the standard solutions in the water analysis lab?

A

a 10 mL Mohr pipet

89
Q

What is ISA transferred to and from for storage in the water analysis lab and why?

A

it is transferred to a 125 mL Erlenmeyer flash since the volumetric flask it is prepared in is used by other lab sections

90
Q

How is ISA mixed with the standard solutions after it is added in the water analysis lab?

A

inverting the mixture 10 times

91
Q

How is the volume of standard solution being poured in to the test tubes estimated in the water analysis lab?

A

they are filled halfway, an estimate of 40 mL

92
Q

What is done in between measurements with the ISE?

A

it is dipped in deionized water and blotted dry

93
Q

What is added to the phosphate buffer and what is added to the unknown buffer?

A

phosphate: HCl

Unknown: NaOH

94
Q

Purpose of complex ions in qualitative analysis lab

A

add reagents to dissolve precipitates by form complexes with the metal portion and identifying metal cations

95
Q

Why is the flame testing only actually useful in detecting Na?

A

the flame color from Na will mask out other alkali metal ions that have their own distinctive color in flame

96
Q

Purpose of glass stirring rod in water analysis experiment

A

(week 3 only) mix environmental samples after addition of ISA

97
Q

purpose of fluted filter paper

A

in week 3 of water analysis experiment, use gravity filtration on environmental samples

98
Q

voltaic cell

A

electrochemical cell in which oxidation-redox reactions reactions occur spontaneously and transfer of electrons occurs through an external pathway.

99
Q

electrodes

A

metal strips(solids) in electrochemical cells

100
Q

Which strips go in which solutions?

A

the metal strip goes in the solution with the matching metal ions in it

101
Q

At which electrodes to oxidation and reduction take place?

A

Anode: oxidation
Cathode: reduction

102
Q

salt bridge

A

porous material that provides an electrical connection through which ions can migrate from one compartment to another

103
Q

characteristics of voltaic cell reactions

A

spontaneous, easily measured electrical potential that can be packaged into batteries, forward reactions, not equilibrium reactions

104
Q

why are voltaic cell reaction forward reactions and not equilibrium reactions?

A

they are spontaneous, electrical potential is what makes batteries work

105
Q

Standard cell potential

A

E*cell, depends on standard reduction potentials of cathode and anode

106
Q

standard reduction potential

A

standard electrode potentials tabulated for reduction reactions

107
Q

How to determine cathode from standard reduction potentials

A

the electrode with the more positive value is more easily reduced, and therefore the cathode

108
Q

What is true of E*cell in spontaneous reactions?

A

it is greater than 0

109
Q

what does a negative 🔼G* value indicate?

A

the reaction is spontaneous

110
Q

Relationship between 🔼G* and E*cell?

A

inverse relationship, if one is negative, the other is positive and vice versa

111
Q

what R value do you use in the formula related free energery change and standard free energy change?

A

R=8.314

112
Q

difference between E*cell and Ecell

A

E*cell refers to the standard state of 1M for all dissolved substances, Ecell refers to different concentrations

113
Q

Under what conditions can you used the Ecell=E*cell-(0.0592/n)logQ formula?

A

room temperature(298K)

114
Q

Role of KNO3 in the voltaic cell lab

A

to soak filter paper to form salt bridge

115
Q

Which electrode was connected to the red lead and the black lead?

A

The cathode is supposed to be connected to the red lead and the anode to the black lead

116
Q

How do you know if you properly matched the cathode/anode to the red/black lead?

A

the measured potential will be positive

117
Q

purpose of part 2 of the voltaic cell experiment

A

see the dependence of the potential of voltaic cells ont he salt solution concentrations

118
Q

purpose of part 3 of the voltaic cell experiment

A

see the dependence of cell potentials upon addition of EDTA

119
Q

Why cant deionized water be used as a standard solution in the water analysis lab?

A

it has an unpredictable electrode response

120
Q

what can affect the ISE accuracy once the probe in put into the solution?

A

The probe can not be touching the bottom, have air bubbles, and solution must be up to the white dot

121
Q

What does a precipitate consist of

A

an insoluble ionic solidp

122
Q

purpose of ferrocyanide test

A

for colorless metal cations

123
Q

What is the ISA solution transferred with?

A

a 10 mL mohr pipet

124
Q

What is the ISA in the water analysis lab?

A

ionic strength adjusting solution, (NH4)2SO4

125
Q

Use of 400 ml beaker in week 3 of water analysis lab

A

hold filtered environmental samples

126
Q

How many test tubes and needed in week 3 of the water analysis lab and why?

A

14: 5 for standard and 3 for each of the 3 environmental samples

127
Q

What is used to transfer standard solutions into test tubes in water analysis lab?

A

a mohr pipet that is separate from then one you used for ISA solutions

128
Q

How much ISA is added to the environmental water samples?

A

2 mL for every 100 mL of environmental sample

129
Q

process for cleaning metal strips in the voltaic cell lab

A

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
Q

Precautions to be taken with filter paper in voltaic cell experiment

A

Make sure it reaches both solutions, does not dry out and does not touch leads or salt bridge

131
Q

How do you find the buffer capacity graphically?

A

the moles at the point at which the pH graph starts to change rapidly

132
Q

purpose of KNO3 in the water analysis lab

A

it is used to make the stock solution

133
Q

Why must the slope and y intercept of the ISE be measured experimentally?

A

they differ between each individual ISE

CHM 145 Prof. Jessa (2 decks)

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