CHM171L Physical Chemistry 2 Laboratory 4th Quarter AY 2015-2016
Conductimetry: Determination of the Electrical Properties of Solutions Ricky Jay C. Gomez1 1Students,
Mapúa Institute of Technology, School of Chemical Engineering and Chemistry
ABSTRACT The study focused on conductivity, wherein it gave importance on determining the relative mobility of the monovalent ions and the dissociation constant of a weak electrolyte. In this experiment we determined the electrolytic conductance and ion mobility of different solutions, specifically on a concentration of 0.1 M NH 4Cl, HCl, NaCl, NaC 2H3O2, NaOH. In determining the electrolytic conductance, it was easier to differentiate the conductance of an acid, base and salt. On the other hand, the d etermination of dissociation constant of a weak electrolyte, specifically KOH mixed with H2O, a sample of 1 M NH 4OH, and lastly a sample of 0.01 M NH 4OH. This then proved that law of independent migration of ions, wherein it states that ions of opposite charges move independently from each other in the case of infinite dilution. Keywords: mobility, dissociation dissociation constant, conductance, electrolyte.
INTRODUCTION Conductimetry is the chemistry of measuring the conductivity of solutions. Conductivity is the measure of an ability to conduct electricity. The electrolytes are the ones conducting the solutions wherein they produce ions when dissolved. Therefore, the solutions are then conducting the current or electric current from the shift of the ions from the field of electricity. By the strength of the current, it can be distinguished whether it is a strong or weak electrolyte. A strong electrolyte is a solute that completely, or almost completely, ionizes or dissociates in a solution. These ions are good conductors of electric current in the solution. Originally, a strong electrolyte was defined as a chemical that, when in aqueous solution, is a good conductor of electricity. Conductance is known as the degree on which an object conducts electricity, and calculated as the ratio of the current that flows to the potential difference present, it is the reciprocal of the resistance, giving us the equation: 1 =
=
R is the resistance, K is the conductivity, A is the cross sectional area of the electrodes, while l is the distance between electrodes. EXPERIMENT 02 | GROUP 05 | MAY 31, 2016
On the other hand, Molar conductivity is defined as the conductivity the conductivity of an electrolyte an electrolyte solution solution divided by the molar concentration of the electrolyte, and so so measures the efficiency with which a given electrolyte conducts electricity in solution. Giving us the equation: Λ=
Λ is the molar conductivity; k is the specific conductance while M is the molar concentration of the solution.
For weak electrolytes the fraction dissociated, , is not unity but is given by the ratio of equivalent conductance to the conductance at infinite dilution, assuming that ionic mobility is independent of concentration. Giving the equation: equation: α=
For weak electrolytes with a small value of the dissociation constant , it follows that α increases as the concentration decreases until α = 1 in the case of infinite dilution. Even weak electrolytes dissociate completely when infinitely dilute because recombination does not take place due to the infinite distances between cations and anions. 1
MATERIALS AND METHOD The materials and equipment used in this experiment were 50-mL beaker, 10-mL graduated cylinder and conductivity meter. The reagents used were deionized water, 0.1 M HCl, 0.1 M NH 4Cl, 0.1 M NaCl, 0.1 M NaC2H3O2 and 0.1 M NaOH. This experiment consists of two parts. The first part is about electrolytic conductance and Ionic Mobility. The second part is the determination of the dissociation constant of a weak electrolyte. In the first part of the experiment, the conductance meter has been set up for the measurements. 10-mL of different solutions with molarity of 0.1 have been prepared. By immersing the electrode in the solution, the conductance of each of the electrolyte was measured. After the measurement procedures, the electrode was washed with deionized water and dried with tissues paper before immersing again to another solution. For the treatment of the results, the equations for the dissociation of each of the electrolyte used were wrote. The conductance of the solutions containing chloride ions were tabulated in an increasing manner. From these, results, the order for the relative mobility of the cations in the solution was inferred. Same thing was done with the anion, but this time, the increasing conductance of solutions containing sodium ions was considered. In the second part of the experiment, Different mixtures of water and 0.01 M KOH were prepared in 50-mL beaker, which give a varying concentrations of the KOH in the prepared solutions. The conductance of the mixtures was then measured. Each 10-mL of 1.0M and 0.1 M of ammonia solution was separated in two test tubes then the conductance was measured for each solution. For the treatment of the results, the concentration of potassium hydroxide in each mixture prepared was calculated. The concentration and the conductance of the potassium hydroxide solutions were tabulated. Also, the plot of the concentration versus conductance was generated. The chemical equation for the dissociation of ammonia into ions was drawn. From the curve fitting equation of the plot for the KOH, the concentration of the ammonia for each solution was calculated since this equation gives the same conductance for ammonia as that of the KOH.
RESULTS AND DISCUSSIONS
Here is the data for the electrolytic conductance and ionic mobility of the different solutions used in the experiment: EXPERIMENT 02 | GROUP 05 | MAY 31, 2016
Solution Conductance 12.51 mS 0.1 M NH4Cl 51.9 mS 0.1 M HCl 10.36 mS 0.1 M NaCl 7.24 mS 0.1 M NaC 2H3O2 19.3 mS 0.1 M NaOH TABLE 1: Conductance for different solutions used. The chemical equations for the dissociation of the different reagents used in the experiment are tabulated below: Equation NH4Cl HCl NaCl NaC2H3O2 NaOH
NH4+ + ClH+ + ClNa+ + ClNa+ + C2H3O2Na+ + OH-
TABLE 2: Dissociation equations. From the data of the first part of the experiment tabulated in TABLE 1, comparing the conductance of the chloride salts, the sodium chloride has the lowest value observed. This might be the consequence of the electrolyte present. Cation with less mass have greater magnitude of conductance. This is due to the mobility of lighter cations as opposed to the heavier ones. The molar mass of ions present in the solution is inversely proportional to the electrical conductance of the solution. This means that an ion with higher conductance has higher mobility. This also means that the ion has a low molar mass, which makes it more mobile. The data for determining the dissociation constant of ammonium hydroxide is tabulated below:
Mixture
Volume Water mL
Conductance mS
0 10 20 30 40 50
2 1.564 1.365 0.976 0.461 0.0802
Conc.
Volume 0.1 M KOH mL
0.01 50 0.008 40 0.006 30 0.004 20 0.002 10 0.0004 2 Dissociation Sample Conductance Constant 1.325 4.03x10-5 1 M NH4OH 1.408 4.59x10-4 0.01 M NH 4OH TABLE 3: Data for the dissociation of NH 4OH. 1 2 3 4 5 6
2
From the data above, as the concentration of KOHwater mixture decreases, the conductance also decreases. The trend of the data suggests a direct proportionality between the concentration and conductance of the mixture. Since the data shows a linear nature, rough estimation of the concentration of the NH 4OH when dissociated with water can be done by linear regression with the use of the gathered conductance for 1 M and 0.01 M NH4OH.
increases, the conductance also increases. This effect is due to the increase of the solute or the solid particles in the solution in which eventually will dissociate with the solvent used. Solids are more conductive that liquids. By increasing the amount of solute in the solution, the ions will also increase after the dissociation, which then give same effect to the conductance. Dissociation constant is calculated from the product of the concentration of the dissociated ions to the concentration of the solution at equilibrium. REFERENCES
Concentration vs Conductance
[1] Atkins. (n.d.). Physical Chemsitry. Oxford University.
2500 S o r 2000 c i m , e 1500 c n a t 1000 c u d 500 n o C 0
[2] Levine. (2009). Physical Chemistry. New York: McGraw Hill.
0
0.002
0.004
0.006
0.008
0.01
0.012
Concentration, M
FIGURE 1: Plot for concentration vs conductance curve during the dissociation of NH 4OH in water. y = 195050x + 86.111 The equation for the estimation of the concentration of NH4OH at the measured conductance could be formulated from the regression data. At equilibrium, dissociation constant can be calculated. CONCLUSION Conductance is a property of a substance that determines its susceptibility to electricity flow. This is the degree to which an object conducts electricity, which is calculated as the ratio of the current that flows to the potential difference. There are two types of conductors, the electronic conductor which allows the flow of electricity through electron transfer and the another one is the electrolytes which conduct electricity through the migration of ions. Conductance is also measured to determine the concentration of a mixture at equilibrium after the dissociation reaction. The trend of the data gathered suggests that conductance is directly proportional to the concentration of the solution. As the concentration EXPERIMENT 02 | GROUP 05 | MAY 31, 2016
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