Analytical chemistry Flashcards
(101 cards)
1
Q
m/z=91?
A
X c6h5ch2
Correct: C6H5CH2+
2
Q
volumetric analysis condition
A
- significant change can easily be detected with the complete consumption of the reagent in the conical flask eg indicator
- reaction must be significantly complete (not eqm
- rate of reaction must be fast enough
3
Q
why I2 need to be freshly produced
A
I2 is volatile
4
Q
why heat is used in a reaction
A
to ensure the reaction go to completion
5
Q
determine octane in petrol
A
gas chromatography-mass spectrometry
6
Q
why blue filter s used in colorimetry for Cr2O7
A
Orange Cr2O72- solution absorb blue light to a large extent
7
Q
MnO4- + Fe2+ to Mn2+ + Fe3+ endpt color change?
A
pale yellow to pale pink
8
Q
how to purify substance after distillation
A
column chromatography
9
Q
how to make up to 250cm3 solution
A
poor to 250cm3 volumetric flask
10
Q
condition of liquid-liquid extraction
A
the organic solvent should be immiscible with water and with lower boiling point
11
Q
how to separate NH4Cl
A
sublimes when heated with inverted funnel
12
Q
why NaOH cant be prepared by dissolving
A
absorb water
reacts with CO2 at atmosphere
13
Q
how to distinguish MgSO4 and MgSO4.7H2O
A
heat in a test tube
place a dry CoCl2 paper on the tube
turn paper from blue to pink
14
Q
process of crystallistion
A
dissolve sample in water
filter off any impurities
heat the filtrate until it becomes saturated
allow solution cool down to crystalline
filter off the crystals
rince the crystals with cold distilled water
dry crystal by filter paper
15
Q
what is the most suitable drying agent for NH3
A
anhydrous calcium oxide (no chemical reaction)
16
Q
How to obtain pure Zinc in (Zn(NH3)4)2+
A
Acidify the solution with acid
Add excess Na2CO3
Heat ZnCO3 strongly with C
17
Q
precaution when using tollens’ reagent
A
wear hand gloves as Ag+ is toxic and turns our skin brown.
18
Q
observation of the measurement of the m.p. of a inpure substance
A
the samples melts gradually over a wide temperature range
19
Q
observation of the analysis of an inpure substance by thin layer chromatography
A
more than one spot will be observed in the chromatographic plate obtained.
20
Q
how to remove NH4Cl from a mixture of NH4Cl and NaCl
A
Heating the sample
Nh4Cl sublimes to NH3 and HCl on heating
21
Q
how to improve the efficiency of separation in fractional distillation
A
use a longer fractionating column
22
Q
why AgNO3 is not suitable for primary standard
A
It is light-sensitive
23
Q
why starch indicator is only added near the end point
A
I2 forms a stable complex with starch
Early addition of starch will affect the accuracy of the end-pt
24
Q
what is a primary standard
A
A primary standard is a reagent used to prepare a solution of known molarity. The solution is prepared by direct weighing a known amount of the reagent and dissolving it in a known volume of solution.
25
how to accurately record the water level
read the upper part of the meniscus level.
26
source of formaldehyde in a newly furnished home
urea-formaldehyde which is used as adhesive or coating material
27
define Rf value
Rf value is defined as the ratio of the distance traveled by the center of a component spot to the distance traveled by the solvent front.
28
why excess dilute HNO3 is added to the AgNO3 before test
To prevent the formation of solid such as Ag2CO3, Ag2SO3 as it can form precipitate with them.
29
why chromatography is used to seperate a mixture
Because different substance have different adsorptivity to the stationary phase and different solubility in mobile phase
30
how to describe the peak in a mass spectra
significant peak at m/z=
31
why direct titration between ascorbic acid and iodine solution is not preferred
iodine solution is not stable in air
| reaction rate between ascorbic acid and iodine is slow
32
why a redox titration is better than acid alkali solution in determining the amount of ascorbic acid in the orange juice sample
orange juice contains other acids and so the result of acid alkali titration shows the total amount of acids in the sample(not only ascorbic acid)
33
why the glass bottle should be capped in TLC
to ensure that the glass bottle is saturated with the solvent vapour
34
what is qualitative analysis
determination of the identity of a chemical species
35
what is quantitaitve analysis
determination of how much a chemical species is present
36
is Ba(OH)2 soluble in water
yes
| it is a strong alkali
37
Ca2+ react with NaOH to form Ca(OH)2 but cant react with NH3 to form Ca(OH)2
its becuase Ca(OH)2 is slightly soluble
while NH3 is a weak alkali
so conc of OH- is not enough to form Ca(OH)2
38
suggest a common use of compounds of group 1 and 2 metals
fireworks
39
what will be observed when a flame test is conducted with Mg2+ ion
no characteristic flame!!
only burning Mg ribbon cause bright white flame
ions cant
40
what is the smell of H2S
bad egg smell
41
CuSO4.nH20
| n usually =?
5
42
what is the bleaching action of SO3 2-
SO3 2- + dye --> SO4 2- + (dye-O)
43
is AgCl soluble in conc NH3
yes
Br2 --> slightly soluble only
I2 --> insoluble
44
test for SO4 2-
add acidified BaCl2
| white precipitate formed
45
how to distinguish Na2CO3 and NaHCO3
add MgCl2
CO3 2- will form white preicipate
while HCO3 - wont
46
how to distinguish S2O3 2- ion
```
add acid (HCl /H2SO4)
turns cloudy with the formation of yellow solid (sulphur)
```
47
what is the general precaution dealing with explosive , flammable and oxidizing substances
keep away from heat and naked flame
48
is Tollens' reagent OA or RA
OA
it reduces to Ag (silver mirror)
and oxidize aldehyde to carboxylates (COO-)
49
how to distinguish amide
boil with NaOH
| NH3 form which turns moist red litmus paper blue
50
how to write the observation when aldehyde is tested by Tollens' reagent
silver mirror is formed on the wall of test tube
51
what is the solid-liquid mixture called
suspension
52
why crystals form when colling a hot saturated solution
the solubility of most solids decrease with decreasing temperature
53
why rapid filtration is needed when removing insoluble impurities during recrystallization
and how to do so?
it is because it is necessary to avoid recrystallization during filtration in this step
use suction filtration by Buchner funnel to increase the speed of filtration
54
why the filter paper should be fluted
to increase surface area for filtration
55
why stemless funnel should be used during recrystallisation
to avoid recrystallization inside the stem
56
should we extract a chemical with one portion of solvent or with several smaller portions in liquid-liquid extraction
it is more efficient to extract a solute with several smaller portions of solvent than with one portion of solvent, although the total volume of solvent used is the same
57
what is the use of the fractionating column
it provides a large surface area for repeated condensation and vapourization of the mixture to occur
58
what is the reverse of sublimation
deposition
59
name in the paper chromatography
starting line
sample spot
solvent front
cover
60
how to observe spot with amino acid on a chromatogram
spray ninhydrin
| purple spots can be seen
61
how to observe spot with organic colorless compound on a chromatogram
standing the chromatogram in a container saturated with iodine vapor
the iodine vapor will combine with the organic compound which results in a brown spot
62
what is Rf value
it is the ratio of distance moved by the spot to the distance moved by the solvent front from the baseline
63
describe the principle of paper chromatography
paper chromatography consists of a stationary phase and a mobile phase which is the solvent
different components in a mixture have different degrees of adsorption to the stationary phase and different solubilities in the mobile phase
the component which is more soluble in the solvent or adsorb less to the stationary phase will move more rapidly with the solvent
so, the components can be seperated
64
what does the mobile phase called in column chromatography
eluent
| the process is called elution
65
how to test for the purity of a subtance
if a substance is pure, it melts or boils at a precisely defined temperature
it has a very sharp melting and boiling point
66
what will the melting point and boiling point be if a substance is impure
impure substance usually has a lower melting point but a higher boiling point
67
why some people spread sodium chloride on highway after a heavy snow
add salt to ice can lower the melting point of ice, so ice melt below its normal melting point and help remove ice on the road surface
68
what is the requirement of primary standard
highly pure
stable in air
should not absorb water an CO2 in air
should be soluble in water
primary standard is a substance that can be used to prepare a standard solution without standardization
69
why conc H2SO4 and NaOH is unsuitable for preparing standard solution
they absorb water
70
why NH3 is unsuitable for preparing standard solution
it is very volatile
71
which methods is more accurate?
| titration or measuring conductivity
titrimetric method is more accurate
as the end-point of acid base titration is sharp and easily observable
besides experimental errors, there are also errors in plotting and extrapolating graphs in conductometric method
72
why I2 cannot prepare a standard solution
I2 is volatile
| its difficult to weigh accurately a sample of I2
73
why starch should be add at the later stage of the titration
starch irreversibly combines with iodine at a high conc. of I2 so that I2 will not be released from starch at the end point
74
when should starch solution be added in the I2 S2O3 2- titration
when the color change from brown to pale yellow
75
why KMnO4 solution cannot be prepared by dissolving a known mass of KMnO4 in water
it is because KMnO4 cannot be obtained in highly pure form as it decomposes to give MnO2 easily in air
the aqueous solution of KMnO4 is unstable
it should be stored in a brown bottle
76
what is the end-pt color change of the reaction between permanganate and iron(ii)
pale yellow to pink
77
for better quality of water
| the permanganate index should be smaller or larger?
smaller
| less POTASSIUM permanganate required to oxidize all reducing agent in the water
78
what is the indicator in Mohr's method
Na2CrO4
| reddish brown precipitate formed (formation of Ag2CrO4 after AgCl)
79
what is color intensity directly proportional to?
concentration of the solution
80
what are the limitations on using colorimetry
the sample must be in solution form
the sample should be colored
the sample should not be a mixture of substance with similar colors
81
explain whether volumetric analysis or colorimetry is more appropriate in determining the concentration of a very dilute solution
colorimetry is more appropriate as the low color intensity of the solution can still be determined by colorimetry
82
what is infrared spectroscopy
it is the method using the infrared spectrometer to analyze the covalent bonds present in a covalent compound
83
what is an infrared spectrum
it is a graph of transmittance against wavenumber
84
when will the frequency of vibration increase
mass of atom decrease
| bond strength increase
85
why the presence of intermolecular H-bonds in the sample produces a broad band
the vibration is complicated by H-bond formed between the molecules
86
limitation of IR spectroscopy
peaks always overlap
intermolecular interactions between molecules can result in complicated infrared spectra
not all vibration give rise to strong absorption peak
87
uses of mass spectrometry
determine relative molecular mass
determine composition of a mixture
determine the structures of organic compound
88
what are the procedures of mass spectrometer
```
vaporization
ionization
acceleration
deflection
detection
```
89
why sample need to be ionized
to form positive ions (molecular ions)
90
what is a nass spectrum
a graph of relative intensity against m/e ratio
91
how a positive ion is formed in mass spectrometer
In the ionization chamber, AB3 molecules were bombarded by fast moving electrons.
This leads to knocking out of an electron from a AB3 molecule to give AB3+
92
what does base peak means
it is the most intense peak which represents the most stable or abundant ion
93
what does molecular ion peak means
the peak with the relative molecular mass
94
sources of CO
incomplete combustions of fossil fuels in car engines
95
negative impact of CO
it decrease the oxygen carrying capacity f haemoglobin, causing dizziness or even death
96
what are the source of formaldehyde
furniture made of wood products with formaldehyde-based adhesive
97
hazard of formaldehyde
toxic and carcinogenic
| it causes burning and watery eyes or even trigger asthma symnptoms
98
what can be done to identify fingerprints
iodine sublimation
99
how to prevent fading of the brown fingerprint
spray starch solution
100
why bromine liquid is not suitable to be a primary standard
Br liquid is volatile
101
propanone is...
flammable
