Review of Basic Concepts in Analytical Chemistry

7/28/2019 Review of Basic Concepts in Analytical Chemistry

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Review of Basic Concepts inAnalytical Chemistry

Percent Concentrations


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Weight percent, (w/w)= mass solute x 100%

mass solution

temperature invariant

same units for numerator and denominator

Volume percent, (v/v)

= volume solute x 100%

volume solution

same units for numerator and denominator


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Weight/volume percent, (w/v)

= mass solute, g x 100%

volume solution, mL

Note:

units have to be specified


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parts per million for dilute solutions= mass solute x 106

mass solution

= mg/L

parts per billion

= mass solute x 109

mass solution

= g/mL


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Example: 4 -21 Skoog, 8th ed.

a. C KCl.MgCl2 = 1.037 x 10-2 M

b. [Mg2+] = 1.037 x 10-2 M

c. [Cl-] = 3.11 x 10-2 M

d. %w/v of KClMgCl26H2O = 0.288 % (w/v)

e. mmol Cl-

= 0.778 mmolf. ppm K+ = 405 ppm

g. pMg = 1.984

h. pCl = 1.507


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Review of Basic Concepts inAnalytical Chemistry

Ionic Equilibria Calculations

]


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Activity,a

x

effective concentration

arises from transient electrostaticinteractions between ions in solution

attraction between unlike charges brings ions

closer together and behave as neutralmolecules

Important in equilibrium calculations

explains behavior of strong electrolytes


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Relationship between activity and

concentration

ax = x[x]

where

x = activity coefficient[x] = concentration


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Activity coefficient

depends on concentration of all ionsin solution

x unity as [x] zero since theprobability of interactions decreaseswith decreasing concentration of ions


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Properties of Activity Coefficients

explains non-ideal behavior of electrolytes

1. Ionic strength is minimal in very dilute

solutions

(x 1)

x decreases with increasing ionicstrength

at moderate ionic strength: x< 1

at infinite dilution, x 1, ax [x]


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2. Ionic strength determines the value of xNOT the nature of the electrolyte

3. x

is dependent on the magnitude of the

electrical charge on the ion, NOT on thesign of the charge. The higher the charge

of an ion, the farter x is from unity.


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4. The size of the hydrated ions affects xSmaller ions show greater departure for

ideal behavior vs. larger ions at the sameionic strength.

5. Ions have less effect on the activity of

neutral molecules than upon anotherelectrolyte.

6. x describes behavior of ion x in all equilibriain which it participates


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Ionic strength,

property of a solution that depends on thetotal concentration of ions in solution, and

on the charge carried by each of these ions = [MiZi

2]

where

Mi= molar concentration of ion iZi = charge on ion I

Note: for very dilute solutions: m 0

x 1

ax Cx


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Calculate the ionic strengths of the

following solutions:

1. 0.10 M NaCl

2. 0.10 M Al2(SO4)33. 0.15 M HCl + 0.15 M NiCl2


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Ionic strengths:

1. 0.10 M NaCl = 0.10 M

2. 0.10 M Al2(SO4)3 = 1.50 M

3. 0.15 M HCl + 0.15 M NiCl2 = 0.60 M


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Debye-Hckel Equation-log x = 0.51Zx

2

1 + 3.3 ax

where it the effective diameter of hydrated ion x innanometers, 10-9 m

Debye-Hckel Limiting Law

-log x

= 0.51Zx

2

for low ionic strength solutions ( < 0.01 M)


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Calculate Fe2+using the DHL

given that = 0.75 and aFe2+ = 0.9

Ans. = 0.20


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Review of Basic Concepts in

Analytical Chemistry

Chemical Equilibrium


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Types of Equilibrium Constants:Thermodynamic (K)

mathematical description of equilibriumcondition

Consider the reaction

mA + nB pC + qDK = (aC)

p(aD)q

(aA

)m(aB

)n

at 25oC, depends on T, solvent


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Types of Equilibrium Constants:Concentration based (K)Note that ax = x[x]

If solutes have small difference betweenactivity and equilibrium concentration

for the reactionmA + nB pC + qD

K = (caC)p(DaD)

q

(AaA)m(BaB)

n


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Types of Equilibrium Constants:Concentration based (K)

K = (ca

C)p

(Da

D)q

(AaA)m(BaB)

n

K = [C]p[D]q . [c]p[D]q[A]m[]n [A]

m[B]n

K = K . [c]p[D]q

[A]m[B]

n


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Types of Equilibrium Constants:

Note:

at infinite dilution, K K

most calculations use K instead of K;

molarities as approximations of activitiessince x difficult to calculate due tonumerous unknown parameters

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Review of Basic Concepts in Analytical Chemistry