OBJECTIVE To determine the molar conductivity at infinite dilution (Ʌ0) of sodium chloride, hydrochloric acid, sodium acetate, and acetic acid at 250c.
INTRODUCTION Electrolytes are substances which dissolve in water to produce solutions which conduct electrical current. Such substances produce ions when dissolve in water, and the ions carry the current through solution. Nonelectrolytes are the substances whose aqueous solution do not contain ions and hence do not conduct electrical current. Electrolytes are classified as either strong electrolytes or weak electrolytes. Strong electrolytes when dissolved in water ionize completely to produce ions. For example, when NaCl is dissolved in water: NaCl (s) → Na+ (aq) + Cl- (aq) There are no dissolved NaCl molecule present in the solution. Solutions of strong electrolytes are good conductor of electricity because they contain a relatively high concentration of ions. Strong Electrolytes The electrolytic conductivity (K) (S cm-1) of a solution increases with concentration. However, quantity K is not a suitable quantity for comparing the conductivities of different solutions. If a solution of one electrolyte is much more concentrated than another, it may have a higher conductivity simply because it contains more ions. Instead, molar conductivity (Ʌ) (S cm-1) should be adopted for comparison. It is defined as (K/concentration). Quantity Ʌ decreases as the concentration increases. Onsager showed theoretically for strong electrolytes in dilute solution that the effect of ionic attraction reduces the molar conductivity as in Eq. (1) Ʌ = Ʌ0 - K√c
(1)
Quantities c and Ʌ0 denote concentration of the electrolyte of the molar conductivity at in finite dilution. Below concentrations of about 0.1 M, a plot of Ʌ against √c result in a straight line. The intersection of this line with the ordinate is the Ʌ0. The Ʌ0 values are found to be additive. Kohlrausch assumed that in such a system the molar conductivity at infinite dilution is simply the sum of the independent contribution of the ions.
Consider a strong electrolytes that yields ions A and B solution: ApBq → pAz+ + qBzKohlrausch’s law of independent migration of ions proposed: Ʌ0 = pƛ0+ + qƛ0-
(2)
For a 1-1 electrolyte A+BɅ0 = ƛ0+ + ƛ0-
(3)
Quantity ƛ0 denotes molar ionic conductivity at infinite dilution. (S cm2 mol-1). This equation has been written for infinite dilution since it only under such conditions, when ion-ion interactions are at a minimum that the law strictly holds. It is the applicable to both strong and weak electrolytes.
CHEMICALS 1) 0.1 M sodium chloride, NaCl 2) 0.1 M hydrochloric acid, HCl 3) 0.1 M sodium acetate, NaAc
APPARATUS 1) Digital conductivity meter (1) 2) 100 mL volumetric flask (18) 3) 50 mL burette (3) 4) 100 mL beaker (9) 5) Magnetic stirrer with stirring bar (1) 6) Conductivity probe holder (1)
PROCEDURES 1) A clean burette was filled with 0.1 M NaCl solution.
2) The required volume of 0.1 M NaCl was drained out into each volumetric flask and top up with DI water to prepared the following molarities, 0.05, 0.01, 0.005, 0.001, 0.0005 and 0.0001 M. 3) The digital conductivity meter was calibrated with conductivity standard of 1413 μS cm -1 or 12.88 mS cm-1 at 250c. 4) Stirring bar and conductivity probe was rinsed by using DI water. 5) The beaker was filled with 50 mL deionized water and stirring bar was placed in. 6) The beaker was placed on the magnetic stirrer. 7) The probe was immersed to a depth approximately 5 cm in the solution. Probe was support with probe holder. 8) The magnetic stirrer was switch on and the electrolytic conductivity (K) of the DI water at 250c was recorded. 9) Steps 4-8 were repeated with the diluted NaCl. The NaCl solution was begin with the lowest concentration first. 10) Steps 1-9 by using HCl and NaAc solutions. 11) All electrolytic conductivity measurements was measured at 250c. If the K values of the solutions are comparable to the DI water, quantities of K was corrected as (K-Kwater).
RESULTS AND DISCUSSIONS 1) Tabulate the results.
Refer to the appendix
2) Determine the Ʌ (S cm2 mol-1) for each of the strong electrolyte solutions, then plot Ʌ versus √c (unit of c in mol cm-3). You may include all the three plots (NaCl, HCl, NaAc) 0n the same graph paper.
NaCl Concentration = 0.0001 M
Λ (S cm2 mol-1) =
= (
)
[
]
= 509 S cm2 mol-1 Concentration = 0.0005 M
Λ (S cm2 mol-1) =
= (
)
[
]
= 207.8 S cm2 mol-1 Concentration = 0.001 M
Λ (S cm2 mol-1) =
= (
[
= 168.1 S cm2 mol-1 Concentration = 0.005 M
Λ (S cm2 mol-1) =
)
]
= (
)
[
]
= 127 S cm2 mol-1 Concentration = 0.01 M
Λ (S cm2 mol-1) =
= (
)
[
]
= 123.2 S cm2 mol-1
Concentration = 0.05 M
Λ (S cm2 mol-1) =
= (
[
= 114.2 S cm2 mol-1
HCl Concentration = 0.0001 M
)
]
Λ (S cm2 mol-1) =
= (
)
[
]
= 1995 S cm2 mol-1 Concentration = 0.0005 M
Λ (S cm2 mol-1) =
= (
)
[
]
= 430 S cm2 mol-1 Concentration = 0.001 M
Λ (S cm2 mol-1) =
= (
[
= 423 S cm2 mol-1
)
]
Concentration = 0.005 M
Λ (S cm2 mol-1) =
= (
)
[
]
= 418 S cm2 mol-1 Concentration = 0.01 M
Λ (S cm2 mol-1) =
= (
)
[
]
= 412 S cm2 mol-1
Concentration = 0.05 M
Λ (S cm2 mol-1) =
= (
[
= 394.6 S cm2 mol-1
)
]
NaAc Concentration = 0.0001 M
Λ (S cm2 mol-1) =
= (
)
[
]
= 371 S cm2 mol-1 Concentration = 0.0005 M
Λ (S cm2 mol-1) =
= (
)
[
]
= 144.4 S cm2 mol-1 Concentration = 0.001 M
Λ (S cm2 mol-1) =
= (
[
)
]
= 128 S cm2 mol-1 Concentration = 0.005 M
Λ (S cm2 mol-1) =
= (
)
[
]
= 92.2 S cm2 mol-1 Concentration = 0.01 M
Λ (S cm2 mol-1) =
= (
)
[
]
= 85.7 S cm2 mol-1
Concentration = 0.05 M
Λ (S cm2 mol-1) =
= (
[
)
]
= 84.8 S cm2 mol-1
Table of Ʌ versus √c c (mol L-1)
Ʌ (S cm2 mol-1) NaCl
HCl
NaAc
0.0001
3.1623 x 10-4
509
1995
371
0.0005
7.0712 x 10
-4
207.8
430
144.4
1.0000 x 10
-3
168.1
423
128
0.005
2.2361 x 10
-3
127
418
92.2
0.01
3.1623 x 10-3
123.2
412
85.7
0.05
7.0712 x 10-3
114.2
394.6
84.8
0.001
√c (mol cm-3)
Graph of Ʌ versus √c Refer to the appendix
3) From the linear regressions, determine the value of Ʌ0 for each of the solution.
From the graph: Ʌ0 (NaCl) = 230 S cm2 mol-1 Ʌ0 (HCl) = 475 S cm2 mol-1 Ʌ0 (NaAc) = 100 S cm2 mol-1
The value of slope for the graph of Ʌ versus √c are negative. The value of Ʌ 0 for each solution were determined by extended the straight line to the 0.0 M concentration.
4) Report the standard errors in the Ʌ0.
The equivalent conductivity of weak electrolytes increases steeply at very low concentrations (see the above graph) and hence their limiting values (Λo) cannot be determined by extrapolating the Λc to zero concentration.
Molar conductivities of weak electrolytes,Λmo can be easily calculated with the help of Kohlrausch's law.
5) From the Ʌ0 value, determine the value of Ʌ0 for acetic acid (HAc).
Ʌ0 (NaCl) = 230 S cm2 mol-1
Ʌ0 (HCl) = 475 S cm2 mol-1
Ʌ0 (NaAc) = 100 S cm2 mol-1
Ʌ0CH3COOH = λ0CH3COO- + λ0H+ Ʌ0CH3COONa = λ0CH3COO- + λ0Na+ Ʌ0HCl = λ0H+ + λ0ClɅ0NaCl = λ0Na+ + λ0ClTherefore: Ʌ0CH3COOH = Ʌ0CH3COONa + Ʌ0HCl - Ʌ0NaCl
= (100 + 475 – 230) S cm2 mol-1 = 345 S cm2 mol-1
6) Using the table of molar ionic conductivities of electrolyte at in finite dilution in aqueous solution at 250c find the Ʌ0 of NaCl, HCl, NaAc and HAc.
Cations H+ Li+ Na+ K+ Mg2+ Ca2+ Ba2+
λ+0 /S cm2mol-1 349.6 38.69 50.11 73.50 106.12 119.00 127.28
HCl Ʌ0HCl = λ0H+ + λ0Cl= 349.6 + 76.34 = 425.94 S cm2 mol-1
NaCl Ʌ0NaCl = λ0Na+ + λ0Cl= 50.11 + 76.34 = 126.45 S cm2 mol-1
anions OHClBrISO42NO3CH3COO-
λ-0 /S cm2mol-1 199.1 76.34 78.4 76.8 159.6 71.4 40.9
NaAc Ʌ0CH3COONa = λ0CH3COO- + λ0Na+ = 40.9 + 50.11 = 91.01 S cm2 mol-1
HAc Ʌ0CH3COOH = λ0CH3COO- + λ0H+ = 40.9 + 349.6 = 390.5 S cm2 mol-1
7) Compare Ʌ0,exp with the Ʌ0,theory determined from the standard data. Describe possible sources for systematic errors and degree of their importance.
% error (NaCl) =
=
x 100
x 100
= 81.89%
% error (HCl) =
=
x 100
x 100
= 11.51%
% error (NaAc) =
=
x 100
x 100
= 9.88%
Possible factor that affect the errors: Interionic attractions: depends on solute-solute interactions. Solvation of ions: depends on solute-solvent interactions. Viscosity solvent: depends upon solvent-solvent interactions. Concentration of solution: Higher concentration of solution less is conduction. If weak
electrolyte,
ionization
is
less
&
if
strong
electrolyte. Temperature: Electrolytic conduction increase with increase in temp.
CONCLUSION From the experiment, the value of Ʌ0 at 250c for NaCl is 230 S cm2 mol-1, for HCl is 475 S cm2 mol-1, for NaAc is 100 S cm2 mol-1 and for HAc is 345 S cm2 mol-1.
REFERENCES 1) http://www.adichemistry.com/physical/electrochemistry/kohlrausch/kohlrausch-law.html 2) http://en.wikipedia.org/wiki/Conductivity_%28electrolytic%29 3) http://www.jesuitnola.org/upload/clark/labs/PerError.htm
