3- Solutions & It's Colligative Properties(Physical Pharmacy)

Khalid T MaaroofMSc. Pharmaceutical sciences

School of pharmacy – Pharmaceutics department

1Online access: bit.ly/physicalpharmacy

Solutions

Physical Pharmacy

10/31/2015

Solutions

Types of solutes

Na+

Cl-

• Electrolytes (Conductive): Dissociation in to ions in solution Eg: NaCl

• Nonelectrolytes (no conductivity): no dissociation Eg: suger

Concentration expressions for solutions Molarity? Normality? Molarity? Mole fracction? Mole percent? Percent

Percent by weight % w/w Percent by volume %v/v Percent weight in volume % w/v

Concentration expressed as percentage

– Percent weight-in-weight (w/w) is the grams of solute in 100 grams of the solution.

– Percent weight-in-volume (w/v) is the grams of solute in 100ml of the solution.

– Percent volume-in-volume (v/v) is the milliliters of solute in 100ml of the solution.

A 20 % w/w solution contains 20g solute how many grams the solvent is?

Molarity, Normality, and Molality

Molarity and normality both depend on the volume of the solvent, so their values are affected by change of volume caused by factors such as change in temperature.

Molality doesn’t has this disadvantage.

Mole fraction In a solution containing 0.01 mole of solute

and 0.04 mole of solvent, the mole fraction of the solute is 0.2 and for solvent it is 0.8

Mole percent = mole fraction X 100

An aqueous solution of ferrous sulfate was prepared by adding 41.50 g of FeSO4 to enough water to make 1000 mL of solution. The density of the solution is 1.0375 and the molecular weight of FeSO4 is 151.9.

Calculate (a) molarity (b) molality (c) mole fraction of FeSO4, mole fraction of

water, and the mole percent of the two constituents

(d) % w/w of FeSO4.

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Ideal and real solutions Ideal solution is defined as a solution in which

there is no change in the properties of the components other than dilution when they are mixed to form the solution

Molecules exhibit complete freedom of motion and randomness of distribution in the solution.

Ideality in solutions means complete uniformity of attractive forces

Ideal Solutions and

P = pA + pB

pA = pA◦ XA pB = pB◦ XB

What is the partial vapor pressure of benzene and of ethylene chloride in a solution at a mole fraction of benzene of 0.6? The vapor pressure of pure benzene at 50◦C is 268 mm, and the corresponding pA ◦ for ethylene chloride is 236 mm.

Raoult's Law States that, in an ideal solution, the partial vapor pressure of each volatile constituent is equal to the vapor pressure of the pure constituent multiplied by its mole fraction in the solution. Thus, for two constituents A and B,

Vapor pressure composition curve (for previous example)

Real Solutions

Ideality in solutions presupposes complete uniformity of attractive forces.

Many examples of solution pairs are known, however, in which the “cohesive” attraction of A for A exceeds the “adhesive” attraction existing between A and B.

Similarly, the attractive forces between A and B may be greater than those between A and A or B and B.

Such mixtures are real or nonideal; that is, they do not adhere to Raoult’s law

Two types of deviation from Raoult’s law are recognized, negative deviation and positive deviation.

Cohesion and Attraction ??

When the “adhesive” attractions between molecules of different species exceed the “cohesive” attractions between like molecules, the vapor pressure of the solution is less than that expected from Raoult’s ideal solution law, and negative deviation occurs.

Negative deviation

Adhesion > Cohesion

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When the “adhesive” attractions between molecules of different species are weaker than “cohesive” attractions between like molecules, the vapor pressure of the solution is more than that expected from Raoult’s ideal solution law, and positive deviation occurs.

Positive deviation

Adhesion < Cohesion

Questions !

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Colligative properties

Colligative properties Colligative properties of solutions are those that

affected (changed) by the presence of solute and depend solely on the number (amount of solute in the solutions) rather than nature of constituents.

Examples of colligative properties are:

Vapor pressureBoiling pointFreezing pointOsmotic pressure

Colligative vs Non-colligative

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Compare 1.0 M aqueous sugar solution to a 0.5 M solution of salt (NaCl) in water.

both solutions have the same number of dissolved particles

any difference in the properties of those two solutions is due to a non-colligative property.

Both have the same freezing point, boiling point, vapor pressure, and osmotic pressure

Non-Colligative Properties

Sugar solution is sweet and salt solution is salty.

Therefore, the taste of the solution is not a colligative property.

Another non-colligative property is the color of a solution.

Other non-colligative properties include viscosity, surface tension, and solubility.

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Lowering of vapor pressure

Vapor pressure: Pure solvent > solutions

Lowering of vapor pressure According to raoult’s law Psolvent = Pºsolvent Xsolvent

But if the solute used in non volatile only pressure from solvent can be considered.

So:

On the other hand

Psolute = Pºsolute Xsolute

Psolution = Pºsolvent Xsolvent

X1 = mole fraction of solvent X2 = mole fraction of solute

∆p = p1◦ − p is the lowering of the vapor pressure and ∆p/p1◦ is the relative vapor pressure lowering.

The relative vapor pressure lowering depends only on the mole fraction of the solute, X2, that is, on the number of solute particles in a definite volume of solution. Therefore, the relative vapor pressure lowering is a colligative property.

Calculate the relative vapor pressure lowering at 20◦C for a solution containing 171.2 g of sucrose (w2) in 100 g (w1) of water. The molecular weight of sucrose (M2) is 342.3 and the molecular weight of water (M1) is 18.02 g/mole.

Boiling point elevation

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Boiling point elevation is a colligative property related to vapor pressure lowering.

The boiling point is defined as the temperature at which the vapor pressure of a liquid equals the atmospheric pressure.

Due to vapor pressure lowering, a solution will require a higher temperature to reach its boiling point than the pure solvent.

Elevation of the Boiling Point The boiling point of a solution of a nonvolatile

solute is higher than that of the pure solvent owing to the fact that the solute lowers the vapor pressure of the solvent.

ΔTb = K X2

ΔTb = Kbm

boiling point is a colligative property

In dilute solutions:ΔTb = K X2 ΔTb = Kbm

Tb: is known as the boiling point elevation Kb: is called the molal elevation constant.m: is molality of solvent

Freezing Point

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Every liquid has a freezing point - the temperature at which a liquid undergoes a phase change from liquid to solid.

When solutes are added to a liquid, forming a solution, the solute molecules disrupt the formation of crystals of the solvent.

That disruption in the freezing process results in a depression of the freezing point for the solution relative to the pure solvent.

Depression of the Freezing Point

∆T f = Tº f – T f

Kf is the molal epression constant

What happens to the triple point?

Osmotic Pressure

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When a solution is separated from a volume of pure solvent by a semi-permeable membrane that allows only the passage of solvent molecules, the height of the solution begins to rise.

The value of the height difference between the two compartments reflects a property called the osmotic pressure of a solution.

Osmotic Pressure

Whereπ is the osmotic pressure .V is the volume of the solution in liters.n is the number of moles of solute.R is the gas constant, equal to 0.082 liter atm/mole deg.T is the absolute temperature.

Van't Hoff and Morse Equations for Osmotic Pressure:

MOLECULAR WEIGHT DETERMINATION The four colligative properties can be used to

calculate the molecular weights of nonelectrolytes present as solutes.Using vapor pressure lowering

Using boining point elevation

Using Freezing point depression

M2 =

Using Osmotic pressure

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Questions !

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