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Colligative Properties

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Colligative Properties of any solution is the property of the solution that depends on the ratio of the total number of solute particles and the total number of solvent particles. Changing the moles or number of particles of solute or solvent changes the colligative properties of the solution. These colligative properties are not dependent on the chemical nature of the solute or solvent but are rather dependent on the number of solutes and solvent particles in the solution. These colligative properties depend on the number of particles in the solution rather than the nature of the solute and the solvent. These properties can be easily linked with the concentration of the solution, i.e. Molarity, Normality, and Molality.

In this article, we will learn about, various types of Colligative properties of the solution, their examples, and others in detail.

What are Colligative Properties?

Colligative Properties are taken from the Greek word “Colligatus” which means “bound together”. The colligative properties of the solution, are the properties that are bound by the number of solute and solvent particle present in the solution. There are four types of colligative properties exhibited by the solution that are,

  • Elevation of Boiling point
  • Depression Freezing point
  • Lowering of Vapour Pressure
  • Osmotic Pressure

Colligative Properties Definition

The properties of the dilute solution in which a non-volatile solute is added, that depends on the number of the solute particle rather than the chemical nature of the solvent are called the colligative properties of the solution.

Colligative properties of any solution depends on the solute-solvent mass ratio of the solution and is independent of the nature of the solute and the solvent. Colligative properties are inversely proportional to the molar mass of the solute.

Colligative Properties Examples

Various example of the colligative properties can be easily seen in our daily life. Such as when we add some salt in the water the freezing point of the water increases, i.e. it freezes far below its normal freezing point. Also, the boiling point of the water also increases if we add salt or sugar in the water. Similarly adding alcohol to water decreases the freezing point of the water.

As these colligaitive properties are dependent on the concentration of the solution, let’s first learn about the measure of concentration of the solution.

Molarity (M)

Molarity is the concentration of a solution measured as the number of moles of solute per liter of solution. Molarity of any solution is calculated by the formula,

Molarity = Number of Moles of Solute/Volume of Solution in L

Molality (m)

Molality is the amount of a substance dissolved in a certain mass of solvent. It is defined as the number of moles of a solute per kilogram of a solvent. Molality of any solution is calculated by the formula,

Molality = Number of Moles of Solute/Weight of Solvent in Kg

Mole Fraction (x)

Mole fraction of a solute in a solution gives the ratio of the number of moles of the solute present in the solution to the total number of moles of the solute and the solvent present in the solution.There is another definition for mole fraction that is,

“Mole fraction of a compound is the ratio of the number of moles of the compound to the total moles of compounds in the mixture.” Mole fraction of any solution is calculated by the formula,

Mole Fraction = (Number of Moles of Solute)/(Total Number of Moles of Solute and Solvent)

Types of Colligative Properties

There are in general four types of the collugative properties of any solution that are mentioned below

  • Lowering Of Vapour Pressure
  • Elevation in Boiling Point (Boiling Point Elevation)
  • Depression in Freezing Point (Freezing Point Depression)
  • Osmotic Pressure.

Now let’s learn about them in detail.

Lowering Of Vapour Pressure

Lowering of vapour pressure is the ratio of vapour pressure of the solution to the vapour pressure of the pure solvent. If we add any non-volatile solute to the solvent we can easily lower the vapour pressure of the solution. the image added below shows the relative lowering of the vapour pressure of any solution.

Lowering-of-Vapour-Pressure

Suppose if Po is the vapour of the pure solvent and Ps is the vapour pressure of the solvent then relative lower of the vapour pressure is calculated by taking the ratio of (Po – Ps) and Po. i.e.

Relative Lowering of Vapour Pressure = (Po – Ps)/Po

The relative lowering of the vapour pressure of a solution having non-volatile solutes is equal to the mole fraction of the solute. If n is the number of moles of the solute and N is the number of moles of the solvent. Then we can say that,

(Po – Ps)/Po = n/(n + N)

Elevation in Boiling Point (ΔTb)

Boiling point of the liquid is the temperature at which the vapour pressure of the liquid is equal to the atmospheric pressure. If we add any solute to the solvent the boiling point of the solution is greater than the boling point of the solvent.

Elevation of the boiling point is the difference between the boling point of the solution and the boling point of the solvent. If Tb is the bolinig point of pure solvent and TIb is the boling point of the pure solvent. Then elevation of boling point is given as,

ΔTb = TIb – T

Experimentally it is proven that, ΔTb is proportional to morality of the solute, then

ΔTb ∝ m

ΔTb = kbm

Where, kb is molal elevation constant.

ΔTb = 1000kb(m2)/(M2m1)

where,

  • m2 is the mass of solvent in g
  • m1 is the mass of solvent in kg
  • M2 is the molar mass of solute

Depression in Freezing Point (ΔTf)

Freezing point of the liquid is the temperature at which the vapour pressure of the substance in its liquid phase is equal to its vapour pressure in the solid phase. If we add any solute to the solvent the freezing point of the solution is lower than the freezing point of the solvent.

Depression of frezzing point is the difference between the frezzing point of the solution and the freezing point of the solvent. If Tf is the frezzing point of pure solvent and TIf is the frezzing point of the pure solvent. Then deprezzion in freezing point is given as,

ΔTf = TIf – T

Experimentally it is proven that, ΔTf is proportional to morality of the solute, then

ΔTf ∝ m

ΔTf = kfm

Where, kf is molal elevation constant.

ΔTf = 1000kb(m2)/(M2m1)

where,

  • m2 is the mass of solvent in g
  • m1 is the mass of solvent in kg
  • M2 is the molar mass of solute

Osmotic Pressure (π)

Osmotic Pressure is the colligative property of the solution and is defined as the difference needed to stop the flow of solvent across the semipermeable membrane.

We can also define the Osmotic Pressure as the minimum pressure that needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a solution to take in a pure solvent by osmosis. 

Osmotic-Pressure

Osmotic Pressure is calculated by the formula,

 π = CRT

Where,

  • C is Concentration (or) Molarity
  • R is Universal Gas Constant
  • T is temperature

Types of Solutions

Different Types of the solution are,

  • Isotonic Solution
  • Hypotonic Solution
  • Hypertonic Solution

Isotonic Solution

Isotonic Solutions are the solutions that have same osmotic pressure at any given temperature. No osmosis happens in case of Isotonic Solution.

Hypotonic Solution

If a solution have lower osmotic pressure then the surrounding then it is called Hypotonic Solution.

Hypertonic Solution

If a solution have higher osmotic pressure then the surrounding then it is called Hypertonic Solution.

Van’t Hoff Factor

For a solute that dissociates in the solution its properties changes by a factor and this factor is called the Von’t Hoff Factor. Von’t Hoff Factor is denoted using(i). It is calculated as,

  • i = (Observed Collugative Properties)/(Theorotical Colligative Properties)
  • i = (No. of Paticles after Dissociation Or Association)/(No. of Particles is case of no Dissociation Or Association)

Raoult’s Law

Raoult’s law states that, “Vapour pressure of a pure component (liquid or solid) multiplied by its mole fraction in the mixture results in the partial pressure of that component in a perfect mixture of liquids. As a result, the mole fraction of solute in the solution equals the relative decrease in vapour pressure of a diluted solution of a non-volatile solute.

Raoult’s Law states that,

Psolution = (Xsolvent)×(P° solvent)

where

  • Psolution is Vapour Pressure of a Solution 
  • Xsolvent is Mole Fraction of solvent in a Solution
  • solvent is Vapour Pressure of a Solvent

Read More,

Solved Problems on Colligative Properties

1. Find the relative lowering of vapour pressure, if 18 g of glucose is dissolved in 90 g of water.

Solution:

Given,

  • Mass of glucose = 18 g
  • Mass of water = 90 g

Molar Mass of Glucose = 180 g/mol

Number of moles of glucose(nB) = 18/180 = 0.1 

Molar Mass of Glucose = 18 g/mol

Number of moles of water(nA) = 90/18 = 5 

Relative lowering ΔP/PA° is equal to XB

XB = nB/(nA + nB)

XB = 0.1/0.1 + 5 = 1/51

Relative Lowering of Vapour Pressure(ΔP/PA°) = 1/51

2. Boiling point elevation of a solution containing sucrose and water is 0.256 °C. The molal elevation constant of water is 0.512 °C/m and molar mass of sucrose is 342 g/mol. What is molality?

Solution:

Given:

  • Boiling point elevation (ΔTb) = 0.256 °C
  • Molal elevation constant (Kb) = 0.512 

(ΔTb) = Kb × m

0.256 = 0.512 × m

m = 0.5 mole/kg

3. Calculate the freezing point depression and the freezing point after adding 100.0 g of table salt to 400.0 g of water. (Kf of water = 1.86) 

Solution:

Moles of NaCl = mass/molar mass

Moles of NaCl = 100.0/58.443 = 1.71107 mol

Mass of water = 400.0 g = 0.400 kg 

Molalilty(m) = (moles of NaCl)/(mass of water in kg)

m = 1.71107/0.400 = 4.2777

NaCl ⇢  Na+ + Cl

Van’t Hoff factor(i) = Numberof mole after dissociation/number of mole before dissociation 

i = 2

Freezing point depression constant for water Kf = 1.86

Freezing point depression = i × Kf × m = 2 × 1.86 × 4.2777 = 15.9 °C

Freezing point of solution = (freezing point of water – freezing point depression) = 0.0 – 15.9

Freezing point of solution = -15.9 °C

4. If 6.8% w/v of cane sugar is isotonic with 1.52% w/v with Thiocarbamide if the molecular weight of cane sugar is 342 find the molecular weight of Thiocarbamide?

Solution:

In Isotonic solution,

 π1 = π2  = i2C2RT  

Now, i1C1 = i2C

For canesugar and thiocarbamide are non electrolytes,

So i = 1

Thus, C1 = C2 

% (W/V) percent  is the number of grams of solute in 100 mL of solution

C = (Number of moles of solute)/(Volume of solution in L)

C1 = 6.8 × 1000/342 × 100

C2 = 1.52 × 1000/x × 100

 As C1 = C2  

6.8 × 1000/342 × 100 = 1.52 × 1000/x × 100

x = 76

Thus, weight of Thiocarbamide is 76 g

5. Osmotic pressure, of a solution of glucose, is 117.4 atm. Find the molarity of the solution at 298 K.

Solution:

π = iCRT    

(Glucose is a Non Electrolyte, i = 1)

117.4 = C × 0.0821 × 298

C = 4.8 mole/l

Thus, the concentration of glucose is 4.8 moles/litre

6. x grams of solute is dissolved in 500 gram solvent if the sum of elevation of boiling point and depression in freezing point for sucrose in water is 5 find its molality. (if Kb = 0.52 and Kf = 1.86) find x?

Solution:

ΔTf + ΔTb = 5

We now that,

  • ΔTf = i × kf × m
  • ΔTb = i × kb × m

Here, i = 1 (for nonelectrolyte like sucrose)

kf × m + kb × m = 5 

2.38 × m = 5

m = 2.1 mole/kg 

Molality = Number of Moles of Solute/Weight of Solvent in Kg

(x × 1000)/(342 × 500) = 2.1 

x = 359 gram

Thus, the solute dissolved is 359 grams.

Colligative Properties-FAQs

1. What are Colligative Properties Class 12?

The colligative properties of the solution are the properties that are dependent on the number of solute added in the solution.

2. Why are Colligative Properties Important?

Colligative Properties of any solution are important for studying various natural phenomenon and designing various appratus.

3. Are Colligative Properties Physical or Chemical?

The physical changes that occurs when the solute particle is added to any solvent are called the colligative properties of the solution. These properties only depend of the number of mole of the solute but not on the nature of the solute particles.

4. What are the 4 Colligative Properties?

The Four colligative properties of the solution are,

  • Elevation of Boiling point (Boling Point Elevation)
  • Depression Freezing point (Freezing Point Depression)
  • Lowering of Vapour Pressure
  • Osmotic Pressure

5. On which factor Colligative Properties Depends?

Colligative Properties Depends on the number of solute particle present in the solution



Last Updated : 24 Nov, 2023
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