SOLUTIONS

In chemistry, a solution is a homogeneous

mixture composed of only one phase. In such a

mixture, a solute is dissolved in another

substance, known as a solvent.

The ability of one compound to dissolve in another compound is called solubility.

The sodium and chlorine units break away from the

crystal surface, get surrounded by H2O molecules,

and become hydrated ions.

NaCl(s) → Na+(aq) + Cl–(aq)

solubility of gases. In physics, Henry's law is one of the gas laws

formulated by William Henry in 1803.

At a constant temperature, the

amount of a given gas that dissolves

in a given type and volume of liquid is

directly proportional to the partial

pressure of that gas in equilibrium

with that liquid. p= KH C

where p is the partial pressure of the

solute in the gas above the solution,

c is the concentration of the solute and

kH (Henry's constant ) is a constant with

the dimensions of pressure divided by

concentration.

The saturated concentration of a gas dissolved

in a liquid depends on its partial pressure of

the solute above the solution.

Henry's law applications.•To increase the solubiity of CO2 in soft drinks and soda water, the bottle is sealed under high pressure. •To minimise the painful effects accompanying the decompression of deep sea divers, oxygen diluted with less soluble helium gas is used as breathing gas.

•In lungs where oxygen is present in air with high partial pressure, haemoglobin combines with oxygen to form oxyhaemoglobin. In tissues where partial pressure of oxygen is low, oxyhaemoglobin releases oxygen for utilization in cellular activities.

Dalton's law of partial pressures (1801 )states that: the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture. pi= ptotal •Xi where ptotal = p1+ p2+ p3… +pn

represent the partial pressure of each component.It is assumed that the gases do not react with each other

where Xi is mole fraction of the i-th component in the total mixture of n components

H2O-is the greatest solvent.

Water appears in nature in all

three common states of matter

and may takes many different

forms on Earth: water vapor

and clouds in the sky; seawater

and icebergs in the polar

oceans; glaciers and rivers in

the mountains; and the liquid in

aquifers in the ground.

Phase map of Water

•melting – is the change of a solid to the liquid statefreesing – liquid - solid (2)

•vaporization – is the change of a solid or a liquid to the vapoursublimation – vapour - solid

•condensation – is the change of a gas to the liquid•boiling – liquid - gas (1).

Colligative properties are properties of

solutions that depend on the number of

molecules (concentration) in a given

volume of solvent and not on the

properties/identity (e.g. size or mass) of

the molecules.

1. Vapor pressure (∆P) of solutions: Raoult's law.

Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed container. All liquids and solids have a tendency to evaporate into a gaseous form, and all gases have a tendency to condense back to their liquid or solid form.

The liquid phase for a binary solution n(solvent) + n(solute) = 1 n(solvent) = 1- n(solute) Substituting the value of n(solute) we get: p = p0 (1- n(solute) ) p0 – p = p0 n(solute) p0 – p/p0 = Χ or ∆ p = p0 ΧThe function p – p0/p0 (∆ p) is known as relative lowering of vapour pressure. Relative lowering of vapor pressure of a given t0 is equal to the mole fraction of solute in a solution of a non-volatile solute and a volatile solvent.

2. Boiling point elevation (ΔTB ). 

The exact relation between the boiling point of the solution and the mole

fraction of the solvent is rather complicated, but for dilute solutions the elevation of the boiling point is directly proportional to the molal

concentration of the solute:

or ΔTb = Kb•CmKb = ebullioscopic constant, which is 0.512°C kg/mol for the boiling point of water.

A solution freezes at a t0 lower than that of the pure solvent. This is due to the lowering of the vapour pressure of a solution as a result of addition of a small amount of non-electrolyte solute. The difference between the freezing point of a pure solvent to the solution is known as the t0 in f.p. of the solution ΔTf the depression (decrease) of f.p. depends on the nature of the solvent K to the concentration of solute Cm. K – called molal depression of f.p.ΔTf = Kf•Cm Kf = cryoscopic constant, which is 1.86°C kg/mol for the freezing point of water.

3. Freezing Point

Depression (ΔTf ).

4. Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides. It may also be used to describe a physical process in which any solvent moves, without input of energy,across a semipermeable membrane (permeable to the solvent, but not the solute) separating two solutions of different concentrations. Although osmosis does not require input of energy, it does use kinetic energy and can be made to do work.

Net movement of solvent is from the less-

concentrated (hypotonic) to the more-

concentrated (hypertonic) solution, which

tends to reduce the difference in

concentrations. This effect can be countered

by increasing the pressure of the hypertonic

solution, with respect to the hypotonic.

The osmotic pressure is defined to be the

pressure required to maintain an equilibrium,

with no net movement of solvent. Osmotic

pressure is a colligative property, meaning that

the osmotic pressure depends on the molar

concentration of the solute but not on its

identity.

Osmotic pressure (π) is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane π= CRT.Hemolysis (or haemolysis)—is the rupturing of erythrocytes (red blood cells) and the release of their contents (hemoglobin) into surrounding fluid (in hypotonic solution). Hemolysis of blood

samples. Red blood cells without (left and middle) and with (right) hemolysis. If as little as 0.5% of the red blood cells are hemolyzed, the released hemoglobin will cause the serum or plasma to appear pale red or cherry red in color.

Plasmolysis is the process in cells where the cytoplasm pulls away from the cell wall due to the loss of water through osmosis (in hypertonic solution). Plasmolysis Hemolysis

The colligative properties of NON- VOLATILE solutions:

4. Osmotic pressure (π) π= CRT

3. The Freezing Point Depression (ΔTf ) ΔTf = K•Cm

2. Boiling point elevation (ΔTB ) ΔTb = K•Cm

1. Vapor pressure (∆P) of solutions ∆p = p0Χ

An electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible. Commonly, electrolytes are solutions of acids, bases or salts.

Electrolyte solutions are normally formed when a salt is placed into a solvent such as water and the individual components dissociate due to the thermodynamic interactions between solvent and solute molecules, in a process called solvation. For example, when table salt, NaCl, is placed in water, the salt (a solid) dissolves into its component ions, according to the dissociation reaction

NaCl(s) → Na+(aq) + Cl−

(aq)

Van't Hoff factor ‘ i ' In order to account for the abnormal behaviour of solutions in which solute undergoes association or dissociation, van't Hoff introduced a correction factor ‘i ' which is called van't Hoff factor and is defined as the ratio of the experimental value of colligative property to the calculated value of property, i.e.,                                  

Since the colligative property is proportional to the number of solute particles in solution, hence :                                  

or we may write :

                                

DEGREE OF DISSOCIATION Consider an electrolyte AxBy which partially dissociate in solution yielding x ions of Ay+ and ‘y' ions of Bx− and α is the degree of dissociation i.e., the fraction of the total number of molecules which dissociates and C be initial concentration of the solute, then the dissociation equilibrium in solution can be represented as :                                

Or i = 1 + α(n − 1) ,

where n-number of ions. For instance, for the following dissociation

KCl → K+ + Cl-

As n = 2, we would have that i = 1 + α

Physical significance of i•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)The value of i is ; i = the actual number of particles in solution after dissociation ÷ the number of formula units initially dissolved in solution. Means the number of particles per formula unit of the solute when a solution is dilute.

The colligative properties of VOLATILE solutions:

1. Vapor pressure (∆P) of solutions ∆p = i p0Χ

2. Boiling point elevation (ΔTB ) ΔTb = i K•Cm

3. The Freezing Point Depression (ΔTf ) ΔTf =i K•Cm

4. Osmotic pressure (π) π= iCRT

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