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CH271—Analytical Chemistry > Exam 2 > Flashcards

Exam 2 Flashcards

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1
Q

advantages of gravimetric

A
  • not many measurements
  • mass is easy to measure
  • fairly accurate
  • inexpensive
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2
Q

disadvantages of gravimetric

A
  • prone to error
  • meticulous
  • time consuming
  • not suited to many trials
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3
Q

3 types of gravimetric

A

precipitation
combustion analysis
thermogravimetric analysis

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4
Q

TGA curve

A

percent original mass as a function of temperature (gravimetric)

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5
Q

methods of precipitating non-water-soluble analytes for gravimetric

A
  • in concentrated acids
  • wet ashing
  • dry ashing
  • fusion
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6
Q

aqua regia

A

HCl + HNO3

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7
Q

wet ashing

A

heat + acid

metals converted to metallic cations

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8
Q

dry ashing

A

heating to extremely high temps in air

metals converted to oxides soluble in acid

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9
Q

fusion

A

used for rocks and minerals

mix with Na2CO3 and heat to high temp

Na2SiO3 is formed, soluble in water

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10
Q

_________ agent is added in excess in gravimetric precipitation

A

precipitating

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11
Q

characteristics of good precipitates for gravimetric

A
  • stable at high temp
  • not colloidal - settle out
  • filterable - large particles
  • purity
  • very insoluble
  • known composition
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12
Q

explain primary adsorbed layer

A

outer layer of precipitate particle

usually made of ion in excess in solution (precipitating agent)

gives a surface charge, and opposite charged spectators are attracted

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13
Q

explain counter-ion layer

A

layer of spectator ions attracted to primary adsorbed layer of precipitate

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14
Q

______ held counter-ion layer produces a colloidal suspension

A

loosely

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15
Q

explain saturation during precipitation

A

requires supersaturation, but not too much, because supersaturation favors nucleation but not crystal growth

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16
Q

2 steps of precipitate formation

A
  1. nucleation - formation of small nuclei of prec.
  2. crystal growth - combination of nuclei upon collision
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17
Q

how to achieve correct supersaturation in precipitate formation

A
  • high temp
  • don’t use overexcess of prec agent
  • stir with addition of prec agent
  • use large volumes of faily dilute solutions
  • homogenous precipitation
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18
Q

explain homogenous precipitation

A

precipitating agent is formed by a slow reaction in situ, and released slowly

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19
Q

explain digestion (Ostwald ripening)

A

product is allowed to sit in mother liquor for long period at high temp

increases purity and crystal size

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20
Q

2 methods of precipitate filtration

A

filter paper

porous crucibles

21
Q

washing precipitate with a volatile electrolyte solution (HCl or HNO3) does what?

A

prevents peptization (conversion to colloid)

electrolytes replace primary adsorbed layer, and heat then drives them off

22
Q

main problem with gravimetric analysis

A

coprecipitation

23
Q

3 types of coprecipitation

A
  • occlusion
  • inclusion
  • adsorption
24
Q

explain occlusion

type of error

A

mechanical entrapment of impurities within crystal

(+) error

25
Q

explain inclusion

type of error

A

contaminant ions of similar size/charge as desired ions are incorporated into crystal

(+) or (-) error

26
Q

explain adsorption (coprecipitation)

type of error

A

contaminants are attracted to surface of crystal

(+) error

27
Q

most difficult type of coprecipitation to correct for

A

inclusion

must redissolve and try again

28
Q

characteristics of rxn suitable for titration

A
  • instantaneous
  • equilibrium lies far toward products (high Keq)
  • need indicator of completion
29
Q

titration error

A

difference between endpoint and eq point

30
Q

how is titration error estimated?

A

with a blank titration

31
Q

2 types of indicators

A

visual

graphical/instrumental

32
Q

First derivative curve

A

𝚫pH/𝚫vol as a function of Vavg

33
Q

Gran plot

A

V10^-pH as a function of V titrant

34
Q

the end point on a gran plot is where…

A

slope changes

35
Q

good qualities of a primary standard

A

pure
stable
low cost
soluble
large MW (smaller error)

36
Q

explain back titration

A

analyte + excess reagent –> product + unreacted reagent

unreacted reagent + titrant –> product

37
Q

moles reacted reagent in backtitration =

A

moles total reagent - moles unreacted reagent

38
Q

general category of precipitation titrations

A

argentometric methods

39
Q

3 argentometric methods

A

Volhard
Mohr
Fajan’s

40
Q

Volhard method

_____ is indicator, _____ is titrant, ______ is analyte

A

Fe3+ is indicator (blood red)
SCN- is titrant
cation is analyte

41
Q

Mohr method

_____ is indicator, ______ is titrant, _______ is analyte

A

CrO4 2- (yellowish) is indicator
Ag+ is titrant
bromide, chloride and cyanide are analytes

42
Q

Fajan’s method

_______ is indicator, _______ is titrant, ______ is analyte

A

anionic organic dye (adsorptive) is indicator
Ag+ is titrant
anions are analytes

43
Q

in Fajan’s method we want colloidal particles because…

A

more surface area for dye means more vibrant endpoint

44
Q

acid-base reaction with no H2O involved

A

neutralization

45
Q

acid-base reaction related to Arrhenius concept, making water and salt

A

dissociation

46
Q

strong acids (6)

A

HCl
H2SO4
HI
HBr
HNO3
HClO4 (sort of)

47
Q

strong bases (2)

A

hydroxides of Group 1 & 2 metals
oxides of group 1 & 2 metals

48
Q

Kw =
(all)

A

1.0 x 10^-14 @25°C

[H3O][OH]

KaKb

49
Q

when is x negligible in ICE table?

A

when 500Ka is < the concentration of HA

when 500Kb is < the concentration of B-

CH271—Analytical Chemistry (3 decks)

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