Solutions

IMRAN TARIQASSISTANT PROFESSORUNIVERSITY COLLEGE OF PHARMACY,UNIVERSITY OF THE PUNJAB

TYPES OF MIXTURES

HOMOGENOUS e.g. True solutions

HETEROGENOUS e.g. coarse and colloidal dispersions

Homogeneity: something evenly distributedHeterogeneity: something not distributed evenly in space; a clump or cluster (not a single phase as in case of colloidal and coarse dispersion).

Definitions A solution is a homogeneous mixture of two or more

substances OR A solution is a homogenous mixture of solute and

solvent. OR A solution is a homogenous mixture of two

substances but consisting of one phase.

A solute is dissolved in a solvent.– solute is the substance being dissolved– solvent is the liquid in which the solute is dissolved– an aqueous solution has water as solvent

Binary solution: A homogenous mixture consisting of one phase and containing only two components i.e. one solute and one solvent e.g. Solution of Nacl in water.

Dilute Solutions: A solution containing relatively small quantity of solute as compared with the amount of solvent.

Concentrated Solution: A solution containing large amount of solute in the solution than that in dilute solution.

Un-saturated solution: a solution in which more solute can be dissolved at a given temperature is called as an unsaturated solution.

A saturated solution is one where the concentration is at a maximum - no more solute is able to dissolve at a given temperature.– A saturated solution represents an equilibrium: the rate of

dissolving is equal to the rate of crystallization. The salt continues to dissolve, but crystallizes at the same rate or under these condition the number of molecules leaving the solute is equal to the number of molecule returning to the solid phase i.e. solute.

Super Saturated Solution:

A solution that contains relatively larger amount of solute than that required for saturation it is prepared by heating and adding more and more solute.

Types of Solutions: Based on physical states of solute and solvent:

Gas

Gas Solid Smoke in air

Gas

Gasliquid Water vapors in air

Solutions

How does a solid dissolve into a liquid?

What ‘drives’ the dissolution process?

What are the energetics of dissolution?

How Does a Solution Form?1. Solvent molecules attracted to surface ions.2. Each ion is surrounded by solvent molecules.3. Enthalpy (H) changes with each interaction broken or

formed.

Ionic solid dissolving in water

How Does a Solution Form

The ions are solvated (surrounded by solvent).

If the solvent is water, the ions are hydrated.

The intermolecular force here is ion-dipole.

Dissolution vs reaction

Dissolution is a physical change—you can get back the

original solute by evaporating the solvent. If you can’t, the substance didn’t dissolve, it reacted.

Degree of saturation

Saturated solutionSolvent holds as much

solute as is possible at that temperature.

Undissolved solid remains in flask.

Dissolved solute is in dynamic equilibrium with solid solute particles.

Degree of saturation

Unsaturated SolutionLess than the

maximum amount of solute for that temperature is dissolved in the solvent.

No solid remains in flask.

Degree of saturation

SupersaturatedSolvent holds more solute than is normally

possible at that temperature.These solutions are unstable; crystallization can

often be stimulated by adding a “seed crystal” or scratching the side of the flask.

3 Stages of Solution Process Separation of Solute

– must overcome IMF or ion-ion attractions in solute– requires energy, ENDOTHERMIC ( + H)

Separation of Solvent– must overcome IMF of solvent particles– requires energy, ENDOTHERMIC (+ H)

Interaction of Solute & Solvent– attractive bonds form between solute particles and solvent

particles– “Solvation” or “Hydration” (where water = solvent)– releases energy, EXOTHERMIC (- H)

Dissolution at the molecular level? Consider the dissolution of NaOH in H2O

TYPES OF HETEROGENEOUS MIXTURES

SUSPENSIONS A suspension is a mixture in

which particles of a material are dispersed throughout a liquid or a gas but are large enough that they settle out.

The particles in a suspension are large enough to scatter or block light. A suspension can be separated by passing it through a filter.

SUSPENSIONS Different components are in different phase, such as

solids in liquids or liquids in gases It is necessary to shake the substance before using it

COLLOIDS Is a mixture in which the particles are

spread throughout but are not large enough to settle out.

The particles are not as small as those of a solution, however are smaller than those of a suspension

COLLOIDS

Particles in a colloid are large enough to scatter light. A colloid cannot be separated by passing it through a filter.

COLLOIDS Consists of two separated

phases: Disperse phase (or internal phase) and a continuos phase (or dispersion medium).

May be solid, liquid or gas

Some are translucent because of the Tyndall Effect (which is the scattering of light)

Solutions Colloids Suspensions

Homogeneous Heterogeneous Heterogeneous

Solution in which dispersoid or consists of small molecules or ions.(Evenly distributed)

Solution in which dispersoid consists of single large molecule or group of small molecule.

Solution in which dispersoid consists of group of large molecules. ( not evenly distributed at all)

Particle size: 0.01-1 nm; can be atoms, ions,

molecules

Particle size: 1-1000 nm, dispersed; can be combined

or large molecules

Particle size: over 1000 nm, suspended; can be large

particles or combined particles

Do not separate on standing Do not separate on standing Particles settle out

Do not scatter light Scatter light (Tyndall effect) May scatter light, but are not transparent

They may pass through ordinary as well as ultra filters.

They may pass through ordinary filter but not usually through ultra filters

Thy cant pass through ordinary as well as ultra filters.

Examples:1)Solution of Nacl in water.2)Solution of glucose in water

Examples:1)Solution of satrch2)Milk3)Solution of gums

Examples:1)Pharmaceutical suspension and emulsions2)Grain of sand in water.

Ultra filtration is a separation process using membranes with a pore sizes in the range of 0.1 to 0.001 micron. Typically, ultrafiltration will remove high molecular weight substances, colloidal materials and organic polymeric molecules. Low molecular weight organics and ions such as sodium, calcium, magnesium and sulfates are not removed.

HETEROGENEOUS MIXTURES

You can distinguish the two or more phases.

Gases in Solution

In general, the solubility of gases in water increases with increasing mass.

Larger molecules have stronger dispersion forces.

Gases in Solution The solubility of liquids

and solids does not change appreciably with pressure.

The solubility of a gas in a liquid is directly proportional to its pressure.

Henry’s LawSg = kPg

where Sg is the solubility of the

gas; k is the Henry’s law

constant for that gas in that solvent;

Pg is the partial pressure of the gas above the liquid.

At a certain temperature, the Henry’s law constant for N2 is 6.0 104 M/atm. If N2 is present at 3.0 atm, what is the solubility of N2?

1. 6.0 104 M2. 1.8 103 M3. 2.0 104 M4. 5.0 105 M

Correct Answer:

Henry’s law, Sg = kPg

Sg = (6.0 104 M/atm)(3.0 atm)

Sg = 1.8 103 M

1. 6.0 104 M2. 1.8 103 M3. 2.0 104 M4. 5.0 105 M

Calculate the concentration of CO2 in a soft drink that is bottled with a partial pressure of CO2 of 4.0 atm over the liquid at 25°C. The Henry’s law constant for CO2 in water at this temperature is 3.1 10–2 mol/L-atm.

Solve:  

Temperature

Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.

Temperature The opposite is true

of gases:Carbonated soft drinks

are more “bubbly” if stored in the refrigerator.

Warm lakes have less O2 dissolved in them than cool lakes.

CONCENTRATION EXPRESSION

5 ways of expressing concentration5 ways of expressing concentration

–Mass percent: (mass solute / mass of solution) * 100

–Molarity(M): moles solute / Liter solution

–Molality* (m) - moles solute / Kg solvent

–Normality (N)- gram equivalent of solute/ liter solution

–Mole Fraction(A) - moles solute / total moles solution

* Note that molality is the only concentration unit in which denominator contains only solvent

information rather than solution.

Mass Percentage or Percentage Expression

Mass % of A =mass of A in solutiontotal mass of solution

100

% expression is an expression of parts of solute per 100 parts Of the solution

Determine the mass percentage of hexane in a solution containing 11 g of butane in 110 g of hexane.

1. 9.0 %2. 10. %3. 90.%4. 91 %

Correct Answer:

Thus,

110 g(110 g + 11 g)

100solution of mass total

solution incomponent of masscomponent of % mass

100 = 91%

1. 9.0 %2. 10. %3. 90.%4. 91 %

% (w/w) =

% (w/v) =

% (v/v) =

% Concentration

100xsolutionmasssolutemass

100xsolutionvolumesolutemass

100xsolutionvolumesolutevolume

% w/w:

It expresses the no. of grams of the solute per 100 gram of the solution.

e.g. a 10 % w/w aqueous glycerine solution means 10 g of glycerine dissolved in sufficient water to make overall 100 gram of the solution.

% v/v:

It expresses the no. of milliliters of the solute per 100 milliliters of the solution.

e.g. a 10 % v/v aqueous ethanolic solution means 10 ml of ethanol dissolved in sufficient water to make overall 100 mls of the solution.

% w/v

It expresses the no. of grams of the solute per 100 mls of the solution.

e.g. a 10 % w/v aqueous Nacl solution means 10 g of Nacl dissolved in sufficient water to make overall 100 mls of the solution.

% v/w

It expresses the no. of mls of the solute per 100 gram of the solution.

e.g. a 10 % v/w aqueous glycerine solution means 10 ml of glycerine dissolved in sufficient water to make overall 100 gram of the solution.

This is another way to expressing concentration, particularly those of very dilute solutions. e.g. to express the impurities of substances in water.

ppm denotes the amount of given substance in a total amount of 1,000,000 of solution e.g. one milligram per kilogram. 1 part in 10 6

ppb denotes the amount of given substance in a total amount of 1,000,000,000 of solution e.g. 0.001 milligram per kilogram. 1 part in 10 9

Parts per Million andParts per Billion

Parts per Million andParts per Billion

ppm = mass of A in solutiontotal mass of solution 106

Parts per Million (ppm)

Parts per Billion (ppb)

ppb =mass of A in solutiontotal mass of solution 109

If 3.6 mg of Na+ is detected in a 200. g sample of water from Lake Erie, what is its concentration in ppm?

1. 7.2 ppm2. 1.8 ppm3. 18 ppm4. 72 ppm

Correct Answer:

610solution of mass total

solution incomponent of masscomponent of ppm

ppm 1810g 200.

g 0.0036g 200.

mg 3.6 6

1. 7.2 ppm2. 1.8 ppm3. 18 ppm4. 72 ppm

mole of soluteL of solutionM =

Molarity (M)

molarity= Given weight / molecular weight substance x 1/ volume of solution in liters

e.g.1 mole of Nacl= 58.5 gm of Nacl No. of moles= Given weight / molecular weight substance Because volume is temperature dependent, molarity can change with

temperature.

It expresses the no. of moles of solute dissolved per liter of the solution.

Concentration: Molarity ExampleIf 0.435 g of KMnO4 is dissolved in enough water to give 250. mL of solution, what is the molarity of KMnO4?

Now that the number of moles of substance is known, this can be combined with the volume of solution — which must be in liters — to give the molarity. Because 250. mL is equivalent to 0.250 L .

As is almost always the case, the first step is to convert

the mass of material to moles.

0.435 g KMnO4• 1 mol KMnO4 = 0.00275 mol KMnO4 158.0 g KMnO4

Molarity KMnO4 = 0.00275 mol KMnO4 = 0.0110 M

0.250 L solution

moles of solutekg of solventm =

Molality (m)

Because both moles and mass do not change with temperature, molality (unlike molarity) is not temperature dependent.

It expresses the no. of moles of solute dissolved per kg of the solvent.

Changing Molarity to Molality

If we know the density of the solution, we can calculate the molality from the molarity, and vice versa.

A solution is made by dissolving 4.35 g glucose (C6H12O6) in 25.0 mL of water at 25°C. Calculate the molality of glucose in the solution.

Solution molar mass of glucose, 180.2 g/mol

water has a density of 1.00 g/mL, so the mass of the solvent is

Normality (N)

It expresses the no. of gram equivalent of solute dissolved per liter of the solution.

Normality= Given weight / equivalent weight x 1/ volume of solution in liters

No. gram equivalent of soluteL of solution

N =

EQUIVALENT WEIGHT

The no. of parts by weight of that substance that will combine with one part by weight of replaceable H+ or OH- or number of positive charge on elements

moles of Atotal moles in solutionXA =

Mole Fraction (X)

The sum of the mole fractions of all the components is always equal to unity i.e. 1

The ratio of the number of the moles of that component to the total number of moles of all the components of the solution

Colligative Properties

Colligative Properties are properties of a liquid that change when a solute is added.

The magnitude of the change depends on the numbernumber of solute particles in the solution, NOT on the nature nature and size and size of the solute particles.

The colligative properties arise from the attractive forces that are exerted by the solute on the solvent. e.g. such attractive forces reduces the tendency of the solvent molecules to escape from the liquids as vapours and vapours pressure of the solvent is therefore reduced by the presence of the solvent.

Following are the colligative properties of the solution.

Lowering of the vapour pressure Elevation of the boiling point Depression of the freezing point Osmotic pressure

Lowering of the vapour pressure

If any non-volatile and non- electrolyte solute is dissolved in the solvent, the escaping tendency of the solvent molecule is reduced and in turn vapour pressure is also reduced. This is because solvent molecules are hindered by the solute molecules and so escaping tendency of the solvent molecules is reduced. Therefore a less no. of solvent molecules will be able to escape and so less no. of the vapours will be formed and ultimately the vapour pressure of the solvent is reduced. Greater the no. of the solute particles, greater will be the hindrance thus lower is the vapour pressure.

In case, when any non-volatile and electrolyte is dissolved in the solvent, the lowering in the vapour pressure will be abnormally high than that of the non- electrolyte. This is because, the electrolyte dissociate into ions in the solution, so no. of the particle will increased and as a result, lowering in vapour pressure will also high.

Elevation in boiling point

Greater the vapour pressure of a liquid, lower will be its boiling point. Similarly, lower the vapour pressure of the liquid, higher will be its boiling point. So If any non-volatile and non- electrolyte solute is dissolved in the solvent, the escaping tendency of the solvent molecule is reduced and in turn vapour pressure is also reduced and ultimately its boiling point will be elevated.

In case, when any non-volatile and electrolyte is dissolved in the solvent, the lowering in the vapour pressure will be abnormally high than that of the non- electrolyte. This is because, the electrolyte dissociate into ions in the solution, so no. of the particle will increased and as a result, lowering in vapour pressure will also high. There fore elevation in boiling point is also greater than that in case of non electrolyte.

Normal Boiling ProcessExtension of vapor pressure concept:Normal Boiling Point: BP of Substance @ 1atmWhen solute is added, BP > Normal BPBoiling point is elevated when solute inhibits solvent from escaping.

Elevation of B. pt.

Express by Boiling Express by Boiling point Elevation point Elevation equationequation

Depression in freezing point

The normal freezing point of solvent (pure liquid) is defined as the temperature at which solid and liquids forms of the solvent co- exist in equilibrium at a fixed external pressure commonly 1 atm. ( i.e. 760 mm Hg)

Pure water has freezing point at 0˚C.

“At this equilibrium state the solid and liquid forms of the solvent must have the same vapour pressure. If it is not so, the form having the higher vapour pressure would change into that form having lower vapour pressure.”

( lowering in vapour pressure of the liquid after addition of solute has been discussed earlier)

So the freezing point of the solution is the temperature at which solid forms of the pure solvent co- exist in equilibrium with the solution at a fixed external pressure commonly 1 atm. ( i.e. 760 mm Hg)

As the vapour pressure of the solution is lower than that of pure solvent so it is obvious that the solid and solution cant be co exist in equilibrium ( this is because at equilibrium both states must have same vapour pressure). They can now co – exist a t some lower temperature ( new freezing point), where the solid and solution have the same vapour pressure. That’s why freezing point of the solvent will be lowered upon addition of solute.

Greater no. of particles= lower of V.P= Depression in F.P

Osmotic pressure

Osmotic pressure Diffusion: Is the movement of particles along the concentration gradient i.e the movement of particles from higher solute concentration to lower solute concentration. Osmosis is the spontaneous movement of water across a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration

orosmosis is the movement of solvent molecule from dilute solution to concentrated solution through semi permeable membrane.

( semi permeable membrane is a type of membrane which permits only solvent molecules to pass through it)

Osmotic Pressure - The hydro static pressure that builds up on the semi permeable membrane which just stops the osmosis of pure solvent into the solution through semi permeable membrane.

Demonstration

Tie a cellophane paper (semi permeable) at the end of the thistle funnel and immersed it in the bath of pure water. Then add sugar solution in the funnel up to the mark “X” and kept the apparatus for few hours. After the level of the sugar solution will rise from mark “X” to mark “Y”. This is due to the osmosis of water from the water bath to the sugar solution in the thistle funnel due to the movement of solvent molecules. After reaching at mark “Y”, the movement of the molecules will be stopped further that was due

to the hydrostatic pressure exerted by the column of the solution on the semipermeable membrane which prevents the entry of the pure solvent to the solution. So, if the concentration of the solution will high, more solvent molecules will move the solution and the level of the column of the solution will rise higher and greater will be the pressure on the membrane and ultimately greater will be the osmotic pressure and vice versa.

Osmotic pressure α concentration of the solution

Osmosis and Blood Cells(a) A cell placed in an isotonic solution. The net

movement of water in and out of the cell is zero because the concentration of solutes inside and outside the cell is the same.

(b) In a hypertonic solution, the concentration of solutes outside the cell is greater than that inside. There is a net flow of water out of the cell, causing the cell to dehydrate, shrink, and perhaps die.

(c) In a hypotonic solution, the concentration of solutes outside of the cell is less than that inside. There is a net flow of water into the cell, causing the cell to swell and perhaps to burst.

IDEAL SOLUTIONS

Cohesive forces: forces that exist between the two similar types of the molecules i.e. between solute-solute or between solvent-solvent.

Adhesive forces: forces that exist between different types of molecules i.e. between solute and solvent.

“ Solutions in which the adhesive and cohesive forces are same (equal) are known as ideal solutions”

Or“Solutions that obey Raoult's law”

Raoult`s Law:

At a definite temperature, the partial pressure (PA) of component (A) in a liquid mixture is equal to the vapour pressure of that component in the pure state (P°A ) multiplied by the mole fraction (A) of that component in the solution.

PA = A P°A

Mixtures of Volatile LiquidsBoth liquids evaporate & contribute to the vapor pressure

Raoult’s Law: Mixing Two Volatile Liquids Since BOTH liquids are volatile and contribute to the

vapour, the total vapor pressure can be represented using Dalton’s Law:

PT = PA + PB

The vapor pressure from each component follows Raoult’s Law:

PT = AP°A + BP°B

Also, A + B = 1 (since there are 2 components)

Benzene and Toluene

Consider a two solvent (volatile) system– The vapor pressure from each component follows

Raoult's Law.– Benzene - Toluene mixture:

• Recall that with only two components, Bz + Tol = 1

• Benzene: when PBz = P°Bz = 384 torr & Bz = 0.5

• Toluene: when PTol = P°Tol = 133 torr & Tol = 0.5

PT = Bz P°BZ + Tol P°TOL

384 torr

133 torr

X BenzeneX Toluene

0 11 0

P (Total)P (Benzene)

P (Toluene)

133 torr

384 torr

Characteristics of an ideal solution: Ideal behavior is expected to be exhibited by the systems

which comprises of the chemical similar compounds, because it is only in such systems that the conditions of equal intermolecular forces between components are likely to be satisfied.

Examples: solutions of ethyl alcohol- methyl alcohol, chloroform-bromoform, benzene- toluene.

Ideal solutions have zero enthalpy change i.e. heat is neither absorbed nor evolved during solution formation.

The volume of the solution is exactly equal to the sum of the individual volumes of the components.

Non- Ideal or Real Solutions

Solutions in which cohesive and adhesive forces are not equal are known as non-ideal or real solutions

Solutions in which solute-solute, solvent-solvent and solute-solvent attractive forces are not equal.

Solutions which don’t obey Raoults`s law

Deviations from Raoult`s Law

Negative deviation from Raoult`s Law:

Real solution which showed negative deviation from Raoult`s Law are those solution in which adhesive forces ( i.e. solute-solvent) are stronger than the cohesive forces (i.e. solute-solute or solvent-solvent) and vapour pressure of the solution is less than expected from Raoult`s law.

i.e. A-B > A-A, B-B

Reasoning of lowering of vapour pressure

It attractive forces between solute and solvent (A-B) are stronger than those exerted between solute – solute (A-A) and solvent- solvent (B-B) molecules, then this strong mutual affinity between solute and solvent molecules results in the formation of complex or compound which results in strong holding of solvent molecules and results in lowering of escaping tendency of solvent molecules and ultimately lowering of vapour pressure. When this occur, there may be decrease in solution volume occur than the sum of volume of the components.

E.g. Chloroform- Ethanol, Benzene - Ethanol

Positive deviation from Raoult`s Law:

Real solution which showed positive deviation from Raoult`s Law are those solution in which cohesive forces (i.e. solute-solute or solvent-solvent) are stronger than the adhesive forces ( i.e. solute-solvent)and vapour pressure of the solution is greater than expected from Raoult`s law.

i.e. A-B < A-A, B-B

Reasoning of elevation of vapour pressure

It attractive forces between solute and solvent (A-B) are less than those exerted between solute – solute (A-A) and solvent- solvent (B-B) molecules, then the presence of “A” reduces the (B-B) attraction and similarly presence of “B” molecules reduces (A-A) attraction. This results in greater escaping tendency of A and B and ultimately partial vapour pressure of the components are greater than expected from Raoult`s law showed positive deviation. When this occur, there may be increase in solution volume occur than the sum of volume of the components.

E.g. Chloroform- Acetone, Pyridine- Acetic Acid