Aim:

To calculate the van’t Hoff’s factors of different salts using water as the solvent.

Apparatus required:

Electronic balance China dish Beaker stirrer Bunsen burner Stand Clamps Log table Tripod stand Wire gauze Thermometer

Chemicals required:

Distilled water Sodium Chloride (NaCl) Zinc Sulphate(ZnSO4) Ammonium Chloride(NH4Cl)

Van’t Hoff factor:

The van 't Hoff factor i (named after J. H. van 't Hoff) is a measure of the effect of a solute upon colligative properties, such as elevation of boiling point, vapor pressure, osmotic pressure and freezing point depression.

The van 't Hoff factor is the ratio between the actual concentration of particles produced when the substance is dissolved, and the concentration of a substance as calculated from its mass.

For most non-electrolytes dissolved in water, the van' t Hoff factor is essentially 1.

For most ionic compounds dissolved in water, the van 't Hoff factor is equal to the number of discrete ions in a formula unit of the substance. This is true for ideal solutions only. Sometime ion pairing occurs in solution. At a given instant a small percentage of the ions are paired and count as a single particle. Ion pairing occurs to some extent in all electrolyte solutions. This causes deviation from the van’t Hoff factor. The deviation for the van’t Hoff factor tends to be greatest where the ions have multiple charges.

When solute particles associate in solution, i is less than 1. (e.g. ethanoic acid in benzene, benzoic acid in benzene)

When solute particles dissociate in solution, i is greater than 1. (e.g. sodium chloride in water, potassium chloride in water, magnesium chloride in water)

When solute particles neither dissociate nor associate in solution, i equals 1. (e.g. Glucose in water)

Boiling-point elevation:

Boiling-point elevation describes the phenomenon that the boiling point of a liquid (a solvent) will be higher when another compound is added, meaning that a solution has a higher boiling point than a pure solvent. This happens whenever a non-volatile solute, such as a salt, is added to a pure solvent, such as water.

ΔTb = Kb · msolute · i

Non integer i factors result from ion pairs in solution, which lower the effective number of particles in the solution.

Together with the formula above, the boiling-point elevation can in principle be used to measure the degree of dissociation or the molar mass of the solute. This kind of measurement is called ebullioscopy (Greek "boiling-viewing").

Error and correction of thermometer:

The thermometer, like any such measuring devices may have error in its reading.

This may be due to1. The scale may not be graduated

properly.2. The mercury may be impure and

hence may not expand properly.3. There may be other faults also with

it.

That error may be removed if we apply the end correction to thermometer reading. We record thermometer reading of melting point of ice at room temperature.

If the temperature reading comes out then the error is x ºC and correction is –xºC

Procedure:

Measuring the mass of solute and solvent :

Take a clean, dry and empty beaker.

Measure the weight of the beaker with the help of balance

Now pour some distilled water into the beaker and again weigh the beaker by subtracting this mass from initial mass of beaker

We get mass of solvent(water). Now , add some salt to the beaker

and shake the beaker so as to dissolve the salt in water and again weigh the beaker.

This calculated mass when subtracted from the mass of the beaker will give the mass of the salt dissolved.

Determination of boiling point of solvent:

Take some water (solvent) in a beaker and set it on a wire gauze placed on a tripod stand above the burner .

Lower the thermometer into the solution , such that the bulb of the thermometer is completely immersed in the water and clamp it in position.

Light the burner to heat the water . As heating progresses, at some time the

temperature becomes constant. This temperature is the boiling point

solvent.

Determination of boiling point of the solution :

Take the solution in the beaker and set it on a wire gauze placed on the tripod stand above the burner.

Lower a thermometer in the solution such that it’s bulb is completely immersed in the solution and clamp it in position.

Light the burner to heat the solution determination of error in the thermometer:

Take a clean beaker and dry it. Put crushed ice into the beaker. Lower a thermometer into the ice and

clamp it in position. Note the temperature when ice starts

melting. This temperature is equal to the error in

the readings in the thermometer and corresponding correction is negative of it.

Determination of error in the thermometer:

o Take Clean beaker and dry ito Put crushed ice into beakero Lower a thermometer into the ice

and clamp it in positiono Note the temperature when ice

starts meltingo This temperature is equal to the

error in the readings in the thermometer and corresponding correction is negative of it

Observation:

Solvent used=water

End error and correction in the thermometer

o Observed melting point of ice =3 ºC

o Real melting point of ice =0 ºC

o Error in reading = -3 ºCo Correction e = 3 ºC

Solute used=Sodium chloride(NaCl)

a) Mass of solute and solventb) Mass of beaker x

= 95 gmc) Mass of beaker+solvent y =135

gmd) Mass of beaker+solvent+solute z= 140

gme) Mass of solvent Wa =y-x= 40

gmf) Mass of solute Wb=z-y=5 gm

1. Molecular mass of solute Mb= 58.5

2. Kb for solvent = 0.52 kg mol-1 3. Elevation of boiling point

a) Observed boiling point of solvent = 99ºC

b) Corrected boiling point of solvent =102ºC

c) Boiling point of solution =100ºC

d) Elevation in boiling point ∆Tb =2

4. Van’t Hoff factor

i = ∆Tb*Mb*Wa/Kb*Wb*1000

i = 2 X 58.5X 40 5 X 1000 X 0.52

i = 1.8

Van’t Hoff factor for NaCl is 1.8

Solute used=ZINC SULPHATE(ZnSO4)

g) Mass of solute and solventh) Mass of beaker x

= 91 gmi) Mass of beaker+solvent y =130

gmj) Mass of beaker+solvent+solute z= 138

gmk) Mass of solvent Wa =y-x= 39

gml) Mass of solute Wb=z-y=8 gm

5. Molecular mass of solute Mb=161.5

6. Kb for solvent = 0.52 kg mol-1 7. Elevation of boiling point

a) Observed boiling point of solvent = 98ºC

b) Corrected boiling point of solvent =101ºC

c) Boiling point of solution =100ºC

d) Elevation in boiling point ∆Tb =1

8. Van’t Hoff factor

i = ∆Tb*Mb*Wa/Kb*Wb*1000

i = 1 X 161.5 X 39 8 X 1000 X 0.52

i = 1.5

Van’t Hoff factor for ZnSO4 is 1.5

Solute used= AMMONIUM CHLORIDE(NH 4Cl)

m)Mass of solute and solventn) Mass of beaker x

= 95.2 gmo) Mass of beaker+solvent y

=142.3 gmp) Mass of beaker+solvent+solute z= 145.3

gmq) Mass of solvent Wa =y-x=

47.1 gmr) Mass of solute Wb=z-y= 3 gm

9. Molecular mass of solute Mb= 53.5

10. Kb for solvent = 0.52 kg mol-1

11. Elevation of boiling point a) Observed boiling point of solvent

= 98ºC b) Corrected boiling point of solvent

=101ºC

c) Boiling point of solution =100ºC

d) Elevation in boiling point ∆Tb =1

12. Van’t Hoff factor

i = ∆Tb*Mb*Wa/Kb*Wb*1000

i = 1 X 53.5X 47.1 3 X 1000 X 0.52

i = 1.65

Van’t Hoff factor for NH4Cl is 1.65

Inference:

o It was experimentally proved that the boiling point of water increases on addition of a non volatile solute.

o The value of van’t Hoff factor for1. NaCl = 1.82. ZnSO4 =

1.503. NH4Cl = 1.65

o The Vant’s Hoff’s factor for these salts was found to be greater than 1 (One).

o Hence, the compounds dissociates into ions when dissolved in water.

RESULT :

S.No.

Salt Used Solvent Used

Van’t Hoff’s Factor

1. NaCl Water 1.8

2. ZNSO4 Water 1.5

3. NH4Cl Water 1.65

Precautions:

The Pans of the balance should be clean dry and dust free as the mass of dust may be more than sensitivity

A clean and dry beaker must be used The beaker should never be heated

directly on the burner The error in the thermometer should be

determined and apply the corresponding correction

The Glass case of beam balance must be closed while reading is being taken to prevent disturbances due to air droughts

Only distilled water must be used and the salt used must be pure

BIBLOGRAPHY:

o LAB Manual – Saraswathi Publications

o Chemistry Text Book for XIIth NCERT

Publications

o Wikipedia , the free encyclopedia.

o Encarta reference