Expression of concentration 8-7.ppt

7/27/2019 Expression of concentration 8-7.ppt

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Expression of concentration

ByProf. Dr/ Iman El-shabrawy


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ObjectivesAt the end of this lecture you should be able

to:-1- Describe various expressions of concentrations

used.2- Explain dilution, concentrated, saturated and

supersaturated solution.3- Explain biological solution concentration i.e

hypertonic, hypotonic and isotonic.4- Differentiate molarity, molality & normality of

solutions.5- List units of concentration used in medicine(g%, m.mol, mg/dl, etc.)


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Concentrations Concentration of a solution representsamount of solute dissolved in a givenamount of solvent or solution.

Concentration =Amount of solute

Amount of solution


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Ways of expressing concentration

1-QuantitativelyDilute solution : a solution thatcontains a small proportion of solute

relative to solvent.Concentrated solution: a solution thatcontains a largeproportion of solute

relative to solvent.To concentrate a solution, one must add moresolute, or reduce amount of solvent (e.g., byevaporation).


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These glasses containing red dye demonstratequantitative changes in concentration.

Dilution

By contrast, to dilute a solution, one mustadd more solvent, or reduce amount ofsolute.
http://en.wikipedia.org/wiki/Image:Dilution-concentration_simple_example.jpg


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2-Semi-Quantitatively

Unsaturated solution : a solution in whichmore solute will dissolve.Saturated solution : a solution in which nomore solute will dissolve.

Supersaturation If additional solute is added to a saturated

solution, it will not dissolve (supersaturation

occur). So, a solution containing more solute over

saturated solutions is called supersaturated.


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Biological solution concentration

Hypertonic, isotonic & hypotonic Solutions Words hypertonic, isotonic and hypotonic

describe water conc. of 2 solutionsseparatedby a selectively permeable membrane such as

cell membrane. Hypertonic = solution with lower water conc.

(higher solute concentration)

Hypotonic = solution with higher water conc.(lower solute concentration)

Isotonic = 2 solutions with same water conc.

(equal in their solute concentrations)


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Solute will move from hypertonic

solution into hypotonic solution.


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3- Qualitatively Many ways, it is sometimes preferred to

measure volume rather than mass of a solution.

Some qualitative expressions for conc. aretemperature-dependent(i.e,conc of solutionchanges as temp. changes), whereas others are

not. Important for experiments in which temperature

does not remain constant.


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Concentrationexpression

Measurementsrequired

Temperaturedependence

Percent composition

(by mass)mass of solute

& mass ofsolution

No(mass does notchange with temp.)

Percent composition

(by volume) volume of solute& volume of

solution

yes(volume changes with

temp.)

percent composition

( mass/volume)mass of solute& volume of

solution


temp.)

Qualitative expression of concentration


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molarity moles of solute&volume ofsolution


temp.)

molality Moles of solute

&mass of solvent

no(neither mass normoles changes with

temp.)

mole fraction moles of solute

&moles of solvent

no

(moles do not changewith temp.)

Concentrationexpression

Measurementsrequired

Temperaturedependence

continuous


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Mole Mole(from molecule), with symbol = mol is

used to measure amount of a substance.

1 mole is amount of a substance which contains as

manyelementary unitsas there are atoms in0.012 Kg (12 gram).of C12 (Avogadro constant).

This measure tells us number of reactingparticles.
http://en.wikipedia.org/wiki/Avogadro_constanthttp://en.wikipedia.org/wiki/Avogadro_constant


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Molarity (M) , of a solution is calculated by

taking moles of solute & dividing by liters ofsolution, hence

Molarity = moles / litre

In other words

Therefore


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Molar solution

Is a solution, which contains themolecular weight of solute in gramsdissolved in one liter of the solution.

Usually concentration is expressed as afraction of mole e.g.:-

Millimole (m M) = 1/1000 mole

Micromole ( M) = 1/1000,000 moleNanomole (nM) = 1/1000,ooo,ooo


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Remember!!!

Measurement in moles is a measurementof amount of a substance.Measurement in molarity is ameasurement of concentration of asubstance [amount (moles) / unit volume

(liters)].
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Dilution of stock solutions Dilution of stock solutions is frequently used

to make solutions of any desired molarity.What does it mean to dilute a solution? Means to add more solvent without adding

more solute. The fact that solute amount stays constant

allows calculations to be made:


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Continuous

moles before dilution = moles after dilution (X)

From definition of molarity (above):

moles of solute = molarity * volume

So we can substitute MV (molarity * volume) intoequation (X) above:

Molarity 1 * Volume 1 = Molarity 2 * Volume 2

In short, M1V1 = M2V2

1 refers to the situation before dilutionand 2 refers to it after dilution.


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Example 1 :

You have 60 ml of a 1.5 M solution of

NaCl, but 0.6 M solution is needed. Howmany ml of 0.6 M can you make?

We know that, M1V1 = M2V2

1.5 M * 60 ml = 0.6 M * (V 2) V2= 150 ml


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Mole Fraction (X):- X, of a component is ratio of no.of moles of

that component to total no of moles of allcomponents in solution.To calculate mole fraction, we need to know:Number of moles of each component present in

the solution. if we need to know, mole fraction of A[XA] in

a solution consisting of A, B, C, ... iscalculated using the equation


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continuous

To calculate mole fraction of B, XB,

then


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The molality (m)Molality (m) , molality (m) molality (m)

(yes this is the right spelling) . Concentration measured as

moles of solute /kilogram of solvent(molality = moles / kilogram).

Molalities are preferred in experimentsthat involve temperature changes e.g.

calorimetry & freezing point depression.


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continuous

Therefore:-

A 1 m (not 1 M) NaCl solution contains 1 moleof NaCl /kilogram of water.


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Normality (N) Is number of equivalents of solute/ liter of

solution.

Equivalent weight equals molecular weight in

grams divided by valence. Normal solution is a solution which contains

equivalent weight of solute in grams dissolved in

1 liter of solution.


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continuous

For example, hydrogen has atomic weight 1.008& valence 1, has an equivalent weight of 1.008.

Oxygen has an atomic weight of 15.9994& avalence of 2, so it has an equivalent weight of7.9997.

Elements may assume different equivalentweights in different compounds.

For example, iron (atomic weight 55.845)assumes equivalent weight 27.9225 ,if it isvalence is 2 in the compound (ferrous Fe+2); or18.615 if it is valence 3 (ferric fe+3).

N B:


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N.B:

- Weight of a molecule is the sum of weights ofatoms of which it is made.

- Unit of weight is dalton (da) which equals 1/12of weight of an atom of C12.

- Thus (MW) of water is 18 daltons & glucose is

180 daltons.- Mole is quantity of a substance whose weight in

grams is equal to MW of substance.

- Thus 1 mole of glucose weight 180 g.

- Furthermore , if you dissolve 1 mole of asubstance in 1 liter (L) of water to make asolution , you have made a 1 molar (1M)

solution.


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continuous

- Molarity

M= number of moles (mol.w) of solution/Lof solution.

- Normality

N= number of equivalents of solution/ Lof solution.

- Molality

m= number of moles of solute/1000 g ofsolvent.


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Parts / million ( p p m) A unit of conc. often used when

measuring levels of pollutants in air,water, body fluids etc.

1p p m (1 part in 1000,000) is equal to

the common unit mg/liter.


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Some units of concentration used in medicine

milligrams / liter (mg/L), SI unit of conc.Parts / million is an older expression of mg/L,(since 1liter of water under standard conditions weights 1kilogram or 1 million milligrams).mg/Lis often used in medicine & prescriptions.Similarly milligrams / deciliter (mg/dl) ,m.mol &also g% are common medical unit.

Unit/ liter (U/L)Amount of enzyme that hydrolyse 1 m.mol ofsubstrate under optimal conditions.


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How to change from m.mol/L into mg/dl:

mg/dl= m. Mol / L X molecular weight10

How to change from mg/dl into m. mol:

m.mol/L= mg/dl X 10Molecular weight

Molecular weight= total atomic weight


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QuestionsMCQMeasurement in moles is a measurement of :-A- amount (moles) / unit volume (liters).B-number of equivalents of solution/ L of

solution.C-number of moles of solute/1000 g of solvent.D- amount of a substance.E- the substance weight in grams equal to its

molecular weight.D & E


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OBA

The molecular weight of glucose is 180 daltons.If you dissolve 180 grams of glucose in water,which of the following is the best descriptionfor the result of this process?

A. a solution of glucose in water.B. a homogenous mixture.

C. a 1 molar solution if water is 1 liter.

D. a hypertonic solution.E. a concentrated solution.

C


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Relational

Volume/volume % expression of concentrationis not used in temperature dependentexperiments.

BECAUSE

Volume/volume % is often used whenmeasuring levels of pollutants in air.

C


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How to prepare 1 molarity solution?

No of mol. wt of substance 1 L

How to prepare 1 normality solution?

No of equivalent wt. 1L

equivalent wt. =Mol .wtvalence

Q i


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QuestionYou need 225 ml of 0.8 M NaOH solution and

you have a 2.5 M stock solution. How would

you make up the solution?2.5 M * (V1) = 0.8 M * 225 mlV1 = 72 mlQuestion :

using M1V1 = M2V2 relationship, determinethe following;60 ml of a solution are diluted to a volume of120 ml. The concentration of the diluted

solution is 2 M. What was the concentrationof the original solution?

M1 = (2 * 0.12)/0.06 = 4 M120ml 60ml


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Expression of concentration 8-7.ppt

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