Internal Resistance Of a Given Primary Cell Using a Potentiometer

OBJECTIVE

To determine the internal resistance of a given primary cell using a potentiometer.

APPARATUS

A potentiometer, a battery (or battery eliminator), two one-way keys, an id resistance, a galvanometer, a high resistance box, a fractional resistance box, an ammeter,  voltmeter, a cell (say Leclanche cell), a jockey, a set square, connecting wires and sandpaper.

CIRCUIT DIAGRAM

CIRCUIT DIAGRAM

THEORY

When Key K2 is open and K1 is closed,
Let null point be obtained at a distance l1 from A
Therefore E = Kl1 –> (1)
When Key K2 is closed and K1 is open,
Let null point be obtained at a distance l2 from A
Therefore V = Kl2 –> (2)
(1) divided by (2)
E/V = l1/l2
(S+r)/S  = l1/l2
r = S*(l1-l2)/l2

Where S : the shunt resistance in parallel with given cell.
l1 and l2 : Balancing lengths without and with shunt respectively.
r : internal resistance of the cell.

PROCEDURE

  1. Make the connections as shown in circuit diagram.
  2. Clean the ends of the connecting wires with sandpaper and make tight connections. Tighten the plugs of the resistance box.
  3. Check the E.M.F. of the battery and cell and see that E.M.F. of the battery is more than that of the given cell otherwise null or balance point won’t be obtained. (E’ > E)
  4. Take maximum current from the battery, making rheostat resistance small.
  5. Insert the plug in Key K1 and adjust the rheostat so that a null point is obtained or the fourth wire of potentiometer.
  6. Insert the 2000 Ohm plug in its position in resistance box and obtain a null point by slightly adjusting the jockey.
  7. Measure the balancing length L1.
  8. Take out the 2000 Ohm plug from the resistance box. Introduce the plugs in Key K1, as well as in Key K2. Take act a small resistance from the resistance box R connected in parallel with the cell. Slide the jockey along the potentiometer wire and obtain a null point. Insert the 2000 Ohm plug back in its position in R.B. and make a further adjustment for a sharp null point.
  9. Insert the 2000 Ohm plug back in its position in R.B. and make a further adjustment for a sharp null point.
  10. Measure the balancing length l2 from end P.
  11. Remove the plugs keys K1 and K2. Wait for some time and respect the activity for the same current.
  12. Record your observations.

OBSERVATIONS

  • Range of voltmeter
  •  Least count of voltmeter
  •  E.M.F. of battery (or battery eliminator)
  •  E.M.F. of cell

 

CALCULATIONS

 

RESULT

The internal resistance of the given cell is…………… ohm.

 

PRECAUTIONS

  1. The e.m.f. of the battery should be greater than that of the cell.
  2. For one set of observation, the ammeter reading should remain constant.
  3. Current should be passed for a short time only while finding the null point.

 

 

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Viva Voce – Questions on Protein, Fat, Carbohydrate Test

  • What are proteins? Name their basic unit.

Proteins are complex nitrogenous organic compounds of high molecular masses. Amino acids are the basic unit of proteins.

  • What is the ring due to in Molisch’s test?

Concentrated sulphuric acid (H2SO4) produces furfural or its derivative on reacting with carbohydrates.  This furfural forms a violet coloured compound on coupling with naphthol.

  • What is the chemistry of Acrolein Test?

Oil or fat gets hydrolyzed to free glycol which forms pungent smelling acrolein on further reaction.

Chemical Equation of Acrolein Test
Chemical Equation of Acrolein Test

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Viva Voce – Questions on Preparation of Sols

  • What is a sol?

Sol is a colloidal suspension of a solid in a liquid.

  • What is the criteria for classifying sols? What type of sols are known?

Sols are classified based on their affinity for the dispersed phase for the dispersion medium. There are types of sols – lyophobic and lyophilic.

  • What is the difference between lyophilic and lyophobic sol?

A sol in which colloidal particles have an affinity for the dispersion medium is called lyophilic sol whereas, in a lyophobic sol, colloidal particles have no or very little affinity for the dispersion medium.

  • What is meant by ‘dialysis’?

The phenomena of purification of sols by passing through a semipermeable membrane is known as dialysis.

  • What is the special characteristic of the paper used in dialysis?

It only permits the electrolyte to pass through it and doesn’t allow the passage of colloidal particles.

  • What is the nature of charge on (i) ferric hydroxide and (ii) arsenious sulphide sols?

Ferric hydroxide sol is positively charged whereas arsenious sulphide is negatively charged.

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Viva Voce – Questions on Chromatography

  • What is chromatography?

It is a rapid and efficient technique for separating various components of a mixture.

  • What is a chromatogram?

The paper strip after running of the solvent and drying is called chromatogram.

  • What are the moving and stationary phases in paper chromatography?

Both phases are liquid. The solvent is the mobile on moving phase while the water absorbed in the pores of the paper acts as a stationary phase.

  • Why should the strip not touch the walls of the jar?

That is necessary to avoid the uneven flow of the developing solvent.

  • What do you understand by ‘Rf’?

Rf (Retention Factor) is the ratio of the distance travelled by the component from the origin line and distance travelled by solvent from origin line.

  • Why is the rate of movement of mobile phase less in ascending paper chromatography?

This is because the mobile phase moves against gravity.

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Viva Voce – Questions on Salt Analysis Practicals

  • What is qualitative analysis?

The type of analysis that deals with the methods which are used to determine the constituents of a compound.

  • What is a radical?

A radical may be defined as an atom or group of atoms which carry charge & behaves like a single unit in chemical reactions.

  • What are preliminary tests?

The tests which are done for getting an indication of the radicals are called preliminary tests.

  • Name a few preliminary tests.

Physical examination of salt, dry heating, the action of diluted and concentrated sulphuric acid (H2SO4).

  • Which radicals are absent in a white salt?

Copper – Cu2+, Iron(II)-  Fe2+,  Iron(III) – Fe3+,  Cobalt – Co2+, and Nickel – Ni2+

  • Can sulphuric acid (H2SO4) be used for preparing the original solution?

No, because sulphuric acid (H2SO4) will produce sulphates of the cations and some sulphates like lead sulphate (PbSO4) are insoluble. So, we may not get the required clear original solution.

  • Explain how Aluminium (III) [Al3+] can be confirmed using cobalt nitrate.

Take a salt solution and add cobalt nitrate to it. Burn a filter paper which has been dipped into this solution. If you detect blue colour on black ash of filter paper, Aluminium (III) [Al3+] confirmed.

  • Why is ammonium chloride [NH4Cl] in III group? Can we add ammonium hydroxide (NH4OH) first?

NH4Cl is added to prevent precipitation of radicals other than III group as hydroxides by common ion effect. If we add NH4OH first, radical of group IV, V & VI will also precipitate and accordingly we cannot do it.

  • Why is it essential to boil off hydrogen sulphide [H2S] gas before precipitating III group?

If H2S isn’t boiled off group IV cations will also get precipitated.

  • How does the addition of ammonium hydroxide (NH4OH) help in precipitation of cations as sulphides?

Solubility product of Group IV sulphides is greater than those of group IV radicals. Thus, a higher concentration of S2- ions are required to precipitate Group IV radicals as sulphides. OH ions of ammonium hydroxide (NH4OH) with H+ ions of H2S, shift the equilibrium towards right or increasing the concentration of S2- ions.

  • Black precipitates in Group IV indicate which cations?

Presence of either Ni2+ or Co2+.

  • Why can sodium hydroxide [NaOH] not be used instead of ammonium chloride [NH4Cl] and ammonium hydroxide (NH4OH)?

NaOH is a very strong electrolyte and its ionization cannot be supressed. Moreover, it dissolves precipitates of Al3+, and Cr3+ as a result of formation of AlO2-, CrO42- complexes.

  • Can Barium Nitrate [Ba(NO3)2]solution be used instead of Barium Chloride [BaCl2]for testing sulphate ions [SO42-]?

Yes, since it is the Ba2+ ions which interact with SO42- and form the white precipitate of Barium sulphate [Ba SO4].

  • Sometimes a white precipitate is obtained in Group VI even in the absence of magnesium cation [Mg2+]. How?

This happens if the fifth group cations have not been precipitated there completely.

  • In the test for bromide ion [Br], why does only organic layer acquire brown colour?

Because Br2 formed is soluble in organic solvents.

  • Ammonium molybdate test applied for phosphate ions [PO43-] on a solution is positive but it doesn’t contain phosphate ions [PO43-] . How?

If the solution contains arsenate salt of arsenic, then this happens.

  • Why does Bromide not give chromyl chloride test?

Because chromyl chloride isn’t stable.

  • What is the chemical name of the yellow precipitate formed in Ammonium molybdate test of phosphate ion [PO43-] ?

Ammonium phosphomolybdate

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Determining resistance per cm of given wire by plotting a graph of potential difference versus current

AIM To determine the resistance per cm of a given wire by plotting a graph of potential difference versus current.

APPARATUS REQUIRED
Battery, key, rheostat, voltmeter, ammeter, resistance wire (unknown resistance), connecting wires, meter scale, sandpaper.

PRINCIPLE
This Experiment is based on OHM’S LAW
Ohm’s Law states that the electric current passing through a conductor is directly proportional to the potential difference applied across its ends.
Mathematically, V=IR
The resistance R of the wire depends on the material of the wire and its dimensions.

CIRCUIT DIAGRAM

internal resistance
Circuit Diagram – Determining Unknown Resistance

PROCEDURE

  1. Draw the circuit diagram as shown in figure above
  2. Arrange the apparatus as per the circuit diagram
  3. Clean the ends of the connecting wires with sandpaper and make them shiny.
  4. Make the connections as per circuit diagram. All connections must be neat and tight. Take care to connect the ammeter and voltmeter with their correct polarity. (+ve to +ve and -ve to -ve)
  5. Determine the zero error and least count of the ammeter and voltmeter and record them.
  6. Adjust the rheostat to pass a low current.
  7. Insert the key K and slide the rheostat contact to see whether the ammeter and voltmeter are showing deflections properly.
  8. Adjust the rheostat to get a small deflection in ammeter and voltmeter.
  9. Record the readings of the ammeter and voltmeter
  10. Take at least six sets of readings by adjusting the rheostat gradually
  11. Plot a graph with V along X axis and I along axis.
  12. The graph will be a straight line which verifies Ohm’s law
  13. Determine the slope of the V-I graph. The reciprocal of the slope gives the resistance of the wire.

OBSERVATIONS

  1. Range
    Range of the given Ammeter =  0-500m A
    Range of the given voltmeter =  0-5V
  2. Least Count
    Least Count of the given Ammeter = 10mA
    Least Count of the given voltmeter = 0.1V
  3. Zero Error
    Zero Error of the given Ammeter = 0A
    Zero Error of the given voltmeter = 0V
  4. Zero Correction
    Zero Correction of the given Ammeter = 0A
    Zero Correction of the given voltmeter = 0V
  5. Observation Table for Ammeter and Voltmeter Readings
S No Ammeter Observed (A) Ammeter Corrected (A) Voltmeter Observed (V) Voltmeter Corrected (V) Ratio (V/I) = R (ohm)
1 0.3 0.3 0.1 0.1 0.33
2 0.7 0.7 0.2 0.2 0.28
3 1.1 1.1 0.3 0.3 0.27
4 1.5 1.5 0.4 0.4 0.26
5 1.9 1.9 0.5 0.5 0.26

Graph

Voltage v/s Current Graph
Voltage v/s Current Graph

CALCULATIONS
Mean Value of V/I from observations, R = 0.28Ω
Length of resistance wire = 40.2cm
Value of slope of VI graph =0.27 Ω
Resistance per unit length = 0.675 Ωm-1

RESULT

  1. Ohm’s Law is verified as the I vs V graph is a straight line
  2. The resistance of the given wire = 0.28Ω
  3. The resistance per cm of given wire is 0.675 Ω m-1.

 PRECAUTIONS

  1. All the electrical connections must be neat and tight.
  2. Voltmeter and Ammeter must of proper range
  3. The key should be inserted only while taking readings.

VIVA VOCE

  • Give the mathematical form of Ohm’s law.
    V = RI,  is the mathematical representation of Ohm’s law.
  • State Ohm’s Law
    Ohm’s law states that the electric current ‘I’ flowing through a conductor is directly proportional to the potential difference V (voltage) across its ends (provided that the physical conditions, temperature, pressure, and dimensions of the conductor remain same).
  • What is the material chosen for rheostat wire and why?
    It is constantan alloy. It is chosen to make rheostat as its resistivity is very high and temperature coefficient of resistance is quite small.
  • What is the material used for making connection wires?
    Here, copper has been used.
  • What are the factors affecting resistance?
    The resistance depends upon length, the area of cross-section, nature of material and temperature of conductors.
  • What is electrical current? Define its S.I. unit?
    The flow of electric charge per unit time through a conductor is called the electric current. It’s S.I. unit is ampere (A). 1 Ampere is the amount of current flowing in a conductor which offers resistance 1 Ohm when 1 V potential difference is maintained across the conductor.
  • Define S.I. Unit of electric potential.
    S.I. unit of electric potential is Volt. 1 Volt is said to be the potential difference between two points if 1 joule of work is done in bringing 1 coulomb of charge from one point to the other.
  • Why is ammeter connected in series?
    To measure the current without any change in magnitude.
  • Why is voltmeter connected in parallel?
    A voltmeter is connected in parallel so that it can measure the potential drop without any change in its magnitude.

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Protein Test

AIM: To test the presence of protein in the given food sample.

PROCEDURE:

S. No. Experiment Observation  Inference
1

BIURET TEST

Food sample + few drops of NaOH + CuSO4 solution.

A violet colouration is obtained. Protein present.
OR
2

XANTHOPROTEIC TEST

Food sample + few drops of concentrated HNO3. Heat.

A yellow precipitate is obtained. Protein present.
OR
3

NINHYDRIN TEST

Food sample + few drops of 0.15 ninhydrin solution. Boil the contents.

A blue colour is obtained. Protein present.

RESULT:  (ON RULED SIDE )  The food sample has been tested for proteins.

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Preparing Potash Alum Crystal (Volumetric Analysis)

AIM: To prepare crystals of Potash alum.

THEORY:

Potash alum, a double salt, commonly known as ‘fitkari’ has the formula K2SO4.Al2(SO4)3.24H2O. It can be prepared by making an equimolar solution of potassium sulphate and aluminium sulphate in the minimum amount of water. A few ml of diluted H2SO4 is added to prevent the hydrolysis of Al2(SO4)3.18H2O. Cooling of the hot saturated solution yields colourless crystals of Potash alum.

K2SO4   +     Al2(SO4)3.18H2O   + 6H2O   →   K2SO4.Al2(SO4)3.24H2O

RESULT

Colour of the crystals: Colourless

Shape of the crystals: Octahedral.

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Preparing Mohr’s Salt Crystals (Volumetric Analysis)

AIM: To prepare crystals of Mohr’s salt.

THEORY:

Mohr’s salt i.e. ferrous ammonium sulphate [FeSO4.(NH4)2SO4.6H2O] is a double salt. It can be prepared by making an equimolar solution of hydrated ferrous sulphate and ammonium sulphate in a minimum amount of water. A few ml of diluted H2SO4 is added to prevent the hydrolysis of FeSO4.7H2O. Cooling of the hot saturated solution yields lights green crystals of Mohr’s salt.

FeSO4.7H2O     +    (NH4)2 SO4   →  FeSO4.(NH4)2SO4.6H2O  + H2O

RESULT

Colour of the crystals: Light green

Shape of the crystals: Monoclinic.

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