chem unit5 ans

7/27/2019 Chem Unit5 Ans

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UNIT-V

PART-A

1. What is atomic spectroscopy?

Ground state of an atom means it is with normal electronic configuration. In

this state the atom remains in its lowest energy state and this is the most stable state

of the atom. Excited state of an atom refers to the electronic configuration availed

by an atom after absorbing certain definite amount of energy. The valence electrons are

promoted to some higher permitted energy level by absorption of energy. In the excited

state, the atom is unstable and the excited electron tends to come back to the original

position i.e., ground state. After about 104

sec. the electron returns to the ground state

by emitting the amount of energy absorbed during excitation. The energy is emitted orabsorbed in the form of electromagnetic waves of definite frequency i.e., of definite

wavelengths.

2. What is atomic absorption spectroscopy?

This is the analytical technique based on the phenomenon of light absorption

(UV or visible). It is applicable both to qualitative and quantitative analyses.

3. What are the parameters for expressing the absorption?

Transmittance is the ratio of the intensity of light transmitted to the intensity of

incident light.

Where

A = Absorbance,

Io = Intensity of incident light,

I = Intensity of transmitted light


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5. Mention the applications of UV.

(a) Qualitative:

Detection of conjugation

Detection of functional groupsDetection of geometrical isomers.

(b)Quantitative: Analysis of various samples (drugs, dyes etc.)

6. What are the different electronic transitions that take place on absorption of UV

light?

When a molecule absorbs UV radiations the electrons are excited to higher energy

levels. In the diagram below the electrons are represented

The following electron transitions take place:

* , n*, n* and *.

These electronic transitions are responsible for UV absorption of a molecule.

7. How does molecular spectrum arise?

It arises due to the interaction of electromagnetic radiation with molecules. This

results in transition between rotational, vibrational and electronic energy levels.

8. State Beer-Lamberts law?

According to this law, when a beam of monochromatic radiation is passed

through a solution of an absorbing substance, the rate of decrease of intensity of radiation

dI with thickness of the absorbing solution dx is proportional to the intensity incident

radiation (I) as well as the concentration of the solution (C). It is mathematically

4. Calculate the concentration of a substance A in an ethanolic solution of which the

absorbance in a 1 cm cell at its A, max 241 nm was found to be 0.890. The is 540 at

241 nm.

A= C l

0.890 = 540 x l x C

C = 0.00165 g/100 ml.


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Diagram

9. Name the important components of colorimeter?

1. Radiation source

2. Filter

3. Slits

4. Cell

5. Detector

6. Meter

10. Write the complexes formed in the colorimetric determination of Fe?

Fe3+

+ NH4CNS blood red colour complex

11. What are the important processes that occur in the flame photometry?

1. It should evaporate the solvent from the sample solution

2. It should decompose the solid into atoms

3. It should excite the atoms and cause them to emit radiant energy

12. What is finger print region? Mention its important uses?

The vibrational spectral region at 1400-1700 cm-1 gives very rice and intense

absorption bands. This region is called as finger print region. It can be used to detect the

presence of functional group and also to identify and characterize the molecule just as a

finger print can be used to identify a person.

represented as

- dI/ dx = k I C


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14. How is an I.R. spectrum recorded?

IR source: A Nernst glower, a rod of an allow of Zirconium, Yttrium

and Erbium oxides. The rod is electrically heated to 1750 K

Rock salt disc or KBr disc is used as glass and quartz absorb I.R.

Sample preparation: Either the sample with KBr is made to pellet or

Nujol mull is used. Nujol is hydrocarbon in nature.

Recording of spectra: The sample is placed in Rock salt cell in the path of

I.R. The change of intensity of light transmitted draws a graph which is

IR spectrum.

15. How will you distinguish CH3COOH from CH3COCH3 with the helpof I.R. spectra?

C = O (str.) peaks will be observed in both the spectra in the region of 1700 cm-1

.

But an absorption bond at 25003000 cm-1

(broad) will be observed in spectrum of

CH3COOH due to dimeric association of CH3COOH molecules through hydrogen

bonding.

16. What happens to a molecule when it is irradiated with (a) UV-Vis., and (b) IRlight.

UV-Vis., light causes electronic transition

IR light causes vibrational and rotational transitions

13. What are the principles of IR?

The atoms in a molecule bond are in a state of constant vibration and

rotation. They may be compared with two balls (atoms) joined by spring (bond). On

absorption of IR the bond may stretch, bend etc., as shown below. So stretching and

bonding of bonds are responsible for IR absorption.


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18. Name two fuel gases used in flame photometry?

1. Acetylene

2. Propane

19. What is the main application of flame photometry?

Analysis of alkali metals, particularly in biological fluids and tissues.

20. How can you identify an unknown element from emission spectrum?

By comparing the unknown spectrum with the spectrum of a known

element.

21. Find out the absorbance of a solution if the transmittance of a solution is

18.5%.

Solution:

% Transmittance = 18.5 %

T = 0.185

Absorbance, A = - log T

A = - log 0.185

= 0.733

17. How to calculate the number of vibrational modes for a different types of

compounds?

i) For a non-linear molecule containing n atoms, the number of vibrational

modes (3n-6)

ii) For a linear molecule containing n atoms, the number of vibrational modes

(3n-5)


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1. Explain flame photometry with a neat diagram?

Components

The various components of the flame photometer are described as follows:

Burner

The flame must possess the following functions:

(i) it should evaporate the solvent from the sample solution

(ii) it should decompose the solid into atoms

(iii) it should excite the atoms and cause them to emit radiant energy.

Mirror

The radiation from the flame is emitted in all directions in space. In order

to increase the amount of radiation reaching the detector, a concave mirror is used

which is set behind the burner.

Slits

Entrance slits: It is kept between the flame and monochromator. It permits only

the radiation coming from the flame & mirror.

Exist slits: It is kept between the monochromator and detector. It prevents the

entry of interfering lines.

Monochromator

It allows the light of the required wavelength to pass through but absorbs

the light of other wavelength.

PART B


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Amplifier & Recorder

The current coming out from the detector is weak, so it is amplified and

recorded.

2. Expalin the estimation of sodium by flame photometry.

The instrument is switched on. Air supply and gas supply are regulated.

First distilled water is sent and ignition is started. After the instrument is warmed

up for 10 min, the instrument is adjusted for zero reading in the disply. Since

sodium produces a characteristic yellow emission at 589 nm, the instrument is set

at max= 589 nm and the readings are noted.

A series of standard NaCl solution (1, 2, 3, 4, 5, ppm) is

prepared and is sent one by one and the readings are noted. The calibration graph

is drawn between the concentration vs intensity of the emitted light. A straight

line is obtained.

Now the unknown sodium solution is sent and the reading is noted. Then

the concentration of sodium in the water sample is determined from the

calibration curve.

Detector

The radiation coming from the filter is allowed to fall on the detector,

which measures the intensity of the radiation falling on it. Photo cell or photo

multiplier is used as detector, which converts the radiation into an electrical

current.


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Components

The various components of a visible UV spectrometer are as follows:

Radiation source

In visible UV spectrometers, the most commonly used radiation sources are

hydrogen or deuterium lamps

Monochromators

Monochromator is used to disperse the radiation according to the wavelength. The

essential elements of a monochromator are an entrance slit, a dispersing element and an

exit slit. The dispersing element may be a prism or filter.

Cell

The cells, containing samples or reference for analysis, should fillfil the following

conditions:

i) They must be uniform in construction

ii) the material of construction should be inert to solvents

iii) they must transmit the light of the wave length used.

Detectors

There are three common types of detectors used in UV-Vis., spectrometers. They

are barrier layer cell, photomultiflier tube, photo cell.

Recording system

The signal from the detector is finally received by the recording system. The

reading is done by recorder pen.

3. Draw a neat diagram of UV-Visible soectrophotometer and explain the each

component ? Explain the working of UV-Visible spectrophotometer.


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The radiation from the source is allowed to pass through the monochromator unit.

The monochromator unit allows a narrow range of wavelength to pass through an exit

slit. The beam of radiation coming out of the monochromator is split into two equalbeams. One half of the beam is directed to pass through cell containing a solution of the

compound to be analysed. The another half is directed to pass through an identical cell

that contains only the solvent. The instrument is designed in such a way that it can

compare the intensities of the two beams.

If the compound absorbs light at a particular wavelength, then intensity of the

sample beam (I) will be less than that of the reference beam (I 0). The instrument gives

output graph, which is a plot of wavelength vs absorbance of the light. This graph is

known as an absorption spectrum.

Working

2. State Beer Lamberts law and explain its applications.

According to this law, when a beam of monochromatic radiation is passed through a

solution of an absorbing substance, the rate of decrease of intensity of radiation dI

with thickness of the absorbing solution dx is proportional to the intensity incident

radiation (I) as well as the concentration of the solution (C). It is mathematically

represented as

- dI/ dx = k I C

- dI/ I = kC dx

Integrating the above equation between limits.

I

- dI/ I = kC dx

I

-ln I/ I = kCx

Where k = Molar absorption coefficient

ln I/I =kCx

2.303 log I/I = kCx

log I/I = k/2.303 Cx

Where E = k/2.303 (Molar extinction coefficient )


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Beer Lamberts law can be used to determine the concentration of unknown solution.For that first we measure the absorbance of standard solution. The absorbance will be

As = ECsx --------(1)

Now measure the absorbance of unknown solution. The absorbance will be

Au = ECux ----------(2)

From 1 and 2

Au / As = Cu/ Cs

Cu = Au / As Cs

From the values of Au , As and Cs we can calculate the concentration of unknown

slolution.

5. How is nickel estimated by AAS?

The instrument is switched on. Air supply and gas supply are regulated.Now prepare

stock solution of nickel by dissolving one g of Nickel nitrate in dilute nitric

acid.When a blank solution is aspirated into the flame, the meter of the AAS

assembly is adjusted to zero absorbance.From the ststandard solution of Nickel, a

series of standard solutions are prepared by appropriate dilutions.Now these standard

solutions are aspirated one by one into the flame and absorbance are measured at 232

nm.Now a calibrated graph is plotted between absorbance and concentration of

nickel in ppm. The nickel solution of uknown concentration is now aspirated into the

flame and absorbance is measured under the same conditions as above. From the

absorbance, the concentration of the unknown nickel ion sample is determined.

A = ECx

Where A = log I/I

Application:


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Block diagram:

Components

Radiation source

The main sources of IR radiation

(a) Nichrome wire

(b) Nernst glower

When they are heated electrically at 1200 to 2000C, they glow and produce IR

radiation.

Monochromator

It allows the light of the requires wave length to pass through, but absorbs thelight of other wavelength.

Sample cell

The cell, holding the test sample must be transparent to IR radiation.

Detector

IR detector convert thermal radiant energy into electrical energy. There are photo

PART C

1. Explain the principle, working of IR spectroscopy with a block diagram?

Principle

The atoms in a molecule bond are in a state of constant vibration and

rotation. They may be compared with two balls (atoms) joined by spring (bond). On

absorption of IR the bond may stretch, bend etc., as shown below. So stretching and

bonding of bonds are responsible for IR absorption.


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Working of IR Spectorphotometer

The radiation emitted by the source the source is split into two identical beams

having equal intensity. One of the beams passes through the sample and the other

through the reference sample.

When the sample cell contains the sample, the half-beam travelling through it

becomes less intense. When the two half beams (one coming from the reference and the

other from the sample) recombine, they produce and oscillating signal, which is measured

by the detector. The signal from the detector is passed to the recording unit and

recorded.

conductivity cell, thermocouple, Pyroelectric detector.

Recorder

The recorder records the signal coming out form the detector.


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The excited atoms which are unstable, quickly emit photons of different wave

lengths and return to the lower energy state. Then the emitted radiation is passed through

the filter, which permits the characteristic wave length of the metal under examination. It

is then passed into the detector, and finally into the recorder.

Block diagram

Components

The various components of the flame photometer are described as follows:

Burner

The flame must possess the following functions:

i) it should evaporate the solvent from the sample solution

(ii) it should decompose the solid into atoms

(iii) it should excite the atoms and cause them to emit radiant energy.

Mirror

The radiation from the flame is emitted in all directions in space. In order to

increase the amount of radiation reaching the detector, a concave mirror is used which is

set behind the burner.

2. a) Draw a neat diagram and explain the principle, working of flame photometry?

Principle

When a metallic salt solution is introduced into a flame, the following processes

will occur.

1. It should evaporate the solvent from the sample solution2. It should decompose the solid into atoms

3. It should excite the atoms and cause them to emit radiant energy


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Slits

Entrance slits: It is kept between the flame and monochromator. It permits only the

radiation coming from the flame & mirror.

Exist slits: It is kept between the monochromator and detector. It prevents the entry of

interfering lines.Monochromator

It allows the light of the required wavelength to pass through but absorbs the light

of other wavelength.

Detector

The radiation coming from the filter is allowed to fall on the detector, which

measures the intensity of the radiation falling on it. Photo cell or photo multiplier is used

as detector, which converts the radiation into an electrical current.

Amplifier & Recorder

The current coming out from the detector is weak, so it is amplified and

recorded.

Working of the Flame photometer:

Air, at a given pressure, is passed into an atomiser. The suction so-produced

draws some solution of the sample into the atomiser. Air+sample solution is then mixed

with fuel gas in the mixing chamber. The Air+sample solution +fuel gas mixture is then

burnt in the burner. The radiation, emitted by the burner flame, is passed successively

through the lens, filter, detector, amplifier and finally into a recorder.

The above experiment is first carried out using a series of standard solutions and

the reading for each solution is noted. The graph is drawn between the concentration

against intensity of emitted light. The test solution is taken and similar experiment is

carried out. From the graph the concentration of the unknown sample can be determined.

b) The percentage transmittance of 0.0005 M solution of disodium fumarate in a

1 cm cell is 19.2 %. Caluculate (i) the absorbance (ii) the molar extinction coefficient

().

Solution:

Given

% T = 19.2 %


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4. Explain the principle, working of colorimeter with a block diagram. How

will you estimate iron using colorimetry

Principle

This colorimteric method is convenient for the coloured substances. The intensity

of colour can be easily measured by using a photoelectric colorimeter, from which the

concentration of coloured solution can be obtained using Beer-Lamberts law.

If the substance is colourless, then a suitable complexing agent is added to the solution so

that a coloured complex is obtained which can absorb the light.

Working

In a colorimeter, a narrow beam of light is passed from radiation source through

the test solution towards a sensitive detector. Usually, colorimeter is provided with the

arrangement of filter and slits, which select the light of required wavelength. The detector

generates the current, which is proportional to the amount of light transmitted by the

solution.

The amount of light transmitted depends on the depth of colour of the testsolution. Thus the current from the photo cell will be more when the light transmitted is

more. This will be possible only if the coloured solution is most diluted.

Current light transmitted 1 / Concentration

T = 0.192

C = 0.0005 M

l = 1 cm

(i) Absorbance

A = - log T

= - log 0.192

= 0.717

(ii) Molar extinction coefficient

= A / C . l

= 0.717 / (0.0005 1)

= 1.434 103mol dm

-3


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Components:

Radiationsources :

The wavelength range of visible light les between 400 750 nm. In this region, atungsten-filament lamp is most widely used.

Filter or monochromator :

It is a instrument, which allows the light of the required wavelength to pass

through,but absorbs the light of the other wavelengths.

Slits:

(a) Entrance slit: It provide a narrow source of the light.

(b) Exit slit: It select a narrow band of dispersed spectrum for observation by the

detector.

Cell:

The cell, holding the test sample (usually a solution), should be transparent. For -

visible region the cell is made of colour-corrected fused glass.

Detector :

It is used for measuring the radiant energy transmitted through the sample.

Photosensitive devices are used to detect radiations. These detectors produce current,

which is directly proportional to the intensity of the incident radiation.

Meter:

It is used to measure directly the fraction of light absorbed.

The transmitted light is now a days are allowed to send through a meter, which is

calibrated to show not the fraction of light transmitted but the fraction of light absorbed.

The light absorbed is proportional to the concentration of the test solution.

Block diagram


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Reagents required

Standard iron solution

0.865 gms of FAS is dissolved in distilled water 5-10 ml of conc. HCl is added

and the solution is diluted to 1 litre. 1 ml of this solution contains 0.1 mg of Fe.

KCNS solution

20 gms of KCNS is dissolved in 100 ml of water

1:1 HCl

50 ml of conc.HCl is added to 50 ml of distilled water

Procedure

A series of standard solution of Fe3+ are prepared by adding KCNS with small

amount of 1:1 con.HCl. then the colorimeter is set at zero absorbance using a blank

solution , with a proper filter. Now absorbance of each standard solution is then measured

using the same filter. A graph is plotted between absorbance vs concentration. This plot is

called as calibration curve and will be the straight line passing through origin. This is

according to Beer-Lamberts law:

A = C l

WhereA is absorbance

C is concentration

l is path length

is molar extinction coefficient

Estimation of iron

Principle

Fe3+

is a colourless but Fe3+

forms blood red colour complex with KCNS or

NH4CNS.

Fe3+

+ KCNS [Fe(CNS)6]3-

+ 6K+


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Similarly, the absorbance of test solution is measured using the same colorimeter. From

the calibration curve, the concentration of the unknown ferric iron solution can be

evaluated.


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chem unit5 ans

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