prentice hall © 2003chapter 13 mass percentage, ppm, and ppb definitions: ways of expressing...

Prentice Hall © 2003 Chapter 13

Mass Percentage, ppm, and ppb• Definitions:

Ways of Expressing Ways of Expressing ConcentrationConcentration

100solution of mass total

solutionin component of masscomponent of % mass


Example: How would you prepare 425 g of an aqueous solution containing 2.40% by mass of sodium acetate, NaC2H3O3?

Ans:Mass of NaC2H3O3 = 10.2 gMass of H2O = mass of solution - mass of NaC2H3O3 = 415 g


Exercise:

Concentrated aqueous nitric acid has 69.0% by mass of HNO3 and has a density of 1.41 gcm-3. What volume of this solution contains 14.2 g of HNO3?



solutionin component of masscomponent of ppm

Also mgl-1



solutionin component of masscomponent of ppb

Also μgl-1


Exercise: Seawater contains 0.0064 g of dissolved oxygen, O2, per litre. The density of seawater is 1.03 gcm-3. What is the concentration of oxygen, in ppm?

Ans: 6.2 ppm


solution of moles totalsolutionin component of moles

component offraction Mole

solution of literssolute moles

Molarity

Mole Fraction, Molarity, and Molality


• Converting between molarity (M) and molality (m) requires density.

solvent of kgsolute moles

Molality, m

Exercise: 0.2 mol of ethylene glycol is dissolved in 2000 g of water. Calculate the molality


Example: What is the molality of a solution containing 5.67 g of glucose, C6H12O6 (Mr = 180.2 g), dissolved in 25.2 g of water? (Calc. the mole fractions of the components as well).

Solution: Think about the solute!................glucose (express in moles) Think about the solvent!...............water (express in kilograms)

Ans: 1.25 m


An aqueous solution is 0.907 M Pb(NO3)2. What is the molality of lead nitrate, Pb(NO3)2, in this solution? The density of the solution is 1.252 g/mL. (Molar mass of Pb(NO3)2 = 331.2 g)

Solution: Mass of solution = density x volume Calculate mass of Pb(NO3)2 , ie, moles x Mr Mass of H2O = mass of solution – mass of Pb(NO3)2 Molality = 0.953 m Pb(NO3)2

Example: Converting molarity to molality


• Colligative properties - depend only on the number of particles in solution and not on their identity.

• So NaCl(aq) a +(aq) + Cl-(aq)

• K2SO4(aq) 2K+(aq) + SO42-(aq)

• C12H22O11(aq) C12H22O11(aq)

Colligative PropertiesColligative Properties


• Examining the effect of adding a non-volatile solute to a solvent on:

• 1. vapor pressure• 2. boiling point• 3. freezing point• 4. osmosis


Examples are: anti-freeze in the radiator water in a car prevents freezing in winter and boiling in summer; snow is melted by adding salt on sidewalks and streets


Lowering Vapor Pressure• VP lowering depends on the amount of solute.


Lowering Vapor Pressure• Raoult’s law:

Psoln = XsolventPosolvent

• Recall Dalton’s Law:Ptotal = PA + PB + PC +….PN


• Ideal solution - obeys Raoult’s law

• Raoult’s law is to solutions what the ideal gas law is to gases

• Raoult’s law breaks down when the solvent-solvent and solute-solute intermolecular forces are greater than solute-solvent intermolecular forces

• For liquid-liquid solutions where both components are volatile, a modified form of Raoult’s law applies:

Ptotal = PA + PB = XAPoA + XBPo

B


Example: Predict the vapour pressure of a solution prepared by mixing 35 g solid Na2SO4 (Mr = 142 g/mol) with 175 g water at 25oC. The vapour pressure of pure water at 25oC is 23.76 torr.

Ans: 22.1 torr


Exercise: The hydrocarbon limonene is the major constituent of lemon oil. A solution of limonene in 78.0 g of benzene had a vapour pressure of 90.6 mm Hg at 25oC, and the vapour pressure of pure benzene at 25oC is 95.2 mm Hg. What is its mass and molecular formula?

Ans: C10H16


As with gases, ideal behaviour for solutions is never perfectly achieved

Nearly ideal behaviour is observed if solute-solute, solvent-solvent and solute-solvent interactions are very similar


Boiling-Point Elevation

Goal: interpret the phase diagram for a solution. Non-volatile solute lowers the vapor pressure Therefore the triple point - critical point curve is lowered.


• Molal boiling-point-elevation constant, Kb, expresses how much Tb changes with molality, m:

mKT bb


Freezing Point Depression

mKT ff


Freezing Point Depression

Colligative PropertiesColligative Properties


Example: How many grams of ethanol, C2H5OH, must be added to 37.8 g of water to give a freezing point of -0.15oC?

Solution:

Water is the solvent and ethanol the solute

From table 13.4, Tf = 0.15oC; Kf for water is 1.86oC/m

mKT ff


Colligative properties of ionic solutions

Tf = iKfm

where i is the no. of ions resulting from each formula unit


Example: Estimate the freezing point of a 0.010 m aqueous solution of aluminium sulphate, Al2(SO4)3. Assume the value of i based on the formula of the compound.

Ans: -0.093oC


Osmosis

• Semipermeable membrane: permits passage of some components of a solution. Example: cell membranes and cellophane.

• Osmosis: the movement of a solvent from low solute concentration to high solute concentration.


Osmosis• Eventually the pressure difference between the arms

stops osmosis.


Osmosis• Osmotic pressure, , is the pressure required to stop

osmosis:

• Isotonic solutions are solutions….?

MRT

RTVn

nRTV


• Hypotonic solutions are solutions….?• Hypertonic solutions are solutions…?• Osmosis is spontaneous.• Red blood cells are surrounded by semipermeable

membranes.


Example: The formula for low-molecular weight starch is (C6H10O5)n, where n averages 2x102. When 0.798 g of starch is dissolved in 100 mL of water solution, what is the osmotic pressure at 25oC?

= 0.006 atm


Exercise: Fish blood has an osmotic pressure equal to that of seawater. If seawater freezes at -2.3oC, what is the osmotic pressure of the blood at 25oC?

Ans: 30 atm


• Crenation:– red blood cells placed in hypertonic solution

(relative to intracellular solution); – The cell shrivels or swells up?


• Hemolysis:

there is a higher solute concentration in the cell;

What happens to the cell?


Osmosis


Hypertonic solution

Hypotonic solution


To prevent crenation or hemolysis, IV (intravenous) solutions must be isotonic.


– Cucumber placed in NaCl solution loses water to shrivel up and become a pickle.

– Limp carrot placed in water becomes firm because water enters via osmosis.

– Salty food causes retention of water and swelling of tissues (edema).

– Water moves into plants through osmosis.

– Salt added to meat or sugar to fruit prevents bacterial infection (a bacterium placed on the salt will lose water through osmosis and die).


• Active transport is the movement of nutrients and waste material through a biological system.

• Active transport is not spontaneous.


Colloids

A colloid is a dispersion of particles of one substance (the dispersed phase) throughout another substance or solution (the continuous phase)

Examples….??


The particle sizes range from ~1 x 103 pm to 2 x 105 pm in size


Although a colloid appears to be homogeneous because the dispersed particles are quite small, it can be distinguished from a true solution by its ability to scatter light

This is called the……effect?


Left: vessel containing colloid; Right: true solution


Aerosols – liquid droplets or solid particles dispersed in a gas e.g. fog and smoke

Emulsion – liquid droplets dispersed throughout another liquid e.g. butterfat in milk

Sol – solid particles dispersed in a liquid e.g. AgCl(s) in H2O


Colloids in which the continuous phase in water can be hydrophilic (e.g. protein molecules) or hydrophobic colloids (Au particles in water).


How does soap stabilise oil in water?

And how do we digest fats in our digestive systems?


Removal of colloidal particles

• Because of their small size, colloidal particles tend to be difficult to remove by processes such as filtration.

• Thus enlargement of particles by coagulation is required

• Heating or adding an electrolyte can cause coagulation


• Heating increases collisions and hence particle size

• Electrolytes neutralise surface charges and reduce repulsions e.g. Alum in water purification, clay deposits in deltas

• Semi-permeable membranes can also be used to separate ions from colloidal particles (dialysis e.g. waste removal from blood by kidneys)

prentice hall © 2003chapter 13 mass percentage, ppm, and ppb definitions: ways of expressing...

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