H. A. Neidig and R. T. Yingling Lebanon Valley College Annville, Pennsylvania
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investigation of Interaction in the Pb(NO&-NaCI-Methan~I-Water System
T h e purpose of this investigation is to collect solubility d a b for lead(I1) nitrate-sodium chloridemethanol-water systems from which a microscopic model can be constructed and used to discuss the various types of interaction occurring. The students are asked to use this model to interpret the effect of the methanol on the solubility of lead(I1) chloride. The model should serve to consider the possible interactions between methanol and water molecules, between methanol molecules and lead(I1) ious and chloride ions, and between water molecules and lead(I1) ions and chloride ions. The compatibility of the experimental data with the model, and the anticipated interactions, should be discussed. The solubilit,y of lead(I1) chloride in methanol-water solvents is 'alculated from data obtained by analyzing methanol-water solutions saturated with lead(I1) chloride for lead(1I) ion and for chloride ion. A series of nlethauol-water solvents containing 15y0, 30%, 45%, GO%, 75%, 85%) and 95% methanol by mass are prepared. From each solvent except 85% and 95% methanol two reactant solutions are prepared, a 0.0855 molal solution of lead(I1) nitrate and a 0.1710 molal solution of sodium chloride. The concentrations of the reactant solutions prepared from 85% and 95% methanol are 0.04275 molal lead(I1) nitrate and 0.0855 molal sodium chloride. If molality is used as the unit of concentrat,ion for each of the reactant solutions, the number of moles of each of the components of the reaction mixture can readily be calculated. A 101.00 g portion of the sodium chloride reactant solution and a 102.83 g portion of the lead(I1) nitrate reactant solution are mixed in an Erlemlleyer flask. The flask containing the reaction mixture is stoppered and is placed in a constant temperature bath maintained a t 25'C with a thermostat. The flasks and their contents are shaken periodically for a time period of about one hour. The reaction mixture is filtered through Gooch crucibles with asbestos mats. The mass of solid lead(I1) chloride formed is found by drying the crucible and its cont,ents to constant mass (+0.0004 g). A 100-ml aliquot of the filtrate is mixed with 50 ml of 1 M sulfuric. acid solution to precipitate the lead(I1) ion as solid lead(I1) sulfat,e. The solid lead(I1) sulfate is removed by filtration and dried to constant mass. A 25-rnl aliquot of t,he original filtrate is titrated with a 0.100 Jf solution of silver nitrate using 2,4dichlorofluorescein as an indicator. The densities of each of the filtrates are calculated fronl mass-volume data. The solubility of lead(I1) chloride in each of the methanol-water solvents is calculated in three ways. First, the solubility is calculated from the difference between the number of moles of solid lead(I1) chloride formed in the reaction mixture and the number of moles
of solid lead(I1) chloride that stoichiometrically should have formed. Second, the solubility is calculated from lead(I1) ion in the filtrate, based on the number of moles of solid lead(I1) sulfate obtained from a known mass of filtrate. Third, the solubility is calculated from the volume of 0.100 M silver nitrate solution required to titrate the chloride ion in a known mass of filtrate. Represeutative student data are shown in Figure 1. The solubility data plotted are average solubilities based on nine trials. The limits associated with the average solubilities are the standard deviations. The solubilities obtained by the students based on the silver nitrate titration for chloride ion did not have the same precision as the solubilities based on the other two methods of analysis because of difficultiesin determining the end point of the titration. Consequently, these data were not included in Figure 1. Data obtained from the curve in Figure 1 are used to calculate the dat'a plotted in Figure 2. Figure 2 is a plot of the solubility of lead(I1) chloride. The units are moles of lead(I1) chloride dissolved per mole of water in the solution and the number of moles of methanol per mole of water in the solution. Figure 2 shows the change which occurs in the total number of moles of lead(I1) chloride dissolved in a saturated solution containing one mole of water, as an increasing nuniber of moles of methanol are added to that solution. The students can use Figure 2 to develop their model. An acceptable model is one that explains why the solubility of lead(I1) chloride decreases initially as methanol is added to the saturated solution, even though the total number of moles of solvent (moles of water plus moles
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0.2
0.4
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1.0
Mole frection of methanol in the solvent Figure 1. Solubility of lead(l1) chloride versus the mole froction of methanol in the solvent. 0 represents solubilities cdculoted from h e mars of PbChIsI recovered. represents solubilities calculated from the determination of leod(ll1 ion 0 %PbSO,(rI.
Volume 42, Number 9, Sepfember 1965
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Figure 2. Salvbllity of lead(l1) chloride in a solution containing one mole of water and x moles of rnethmol.
of methanol) is increasing. The model should also explain why the solubility begins to increase after about one mole of methanol has been added to the saturated solution. Finally, the students discuss the various types of interaction in the system as indicated by the model and comment on the applicability of their model to the experimental data. For instance, the data shown in Figure 2 could be explained by assuming that methanol molecules interact with water molecules as the methanol is added to the saturated lead(I1) chloridewater solution. This methanol-water interaction would be expected to reduce the number of water molecules available to solvate the lead(I1) chloride; therefore the solubility of lead(I1) chloride would decrease. The solubility should continue to decrease until enough methanol is added to interact with all of the available
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water molecules. The addition of methanol molecules in excess of those required to interact with the water molecules should serve to increase the solubility of lead(11) chloride since the methanol molecules can solvate the lead(I1) chloride. Furthermore, when sufficient methanol is added so that additional methanol acts only to solvate the lead(I1) chloride, the curve in Figure 2 should be linear with a slope equal to the solubility of lead(I1) chloride in methanol. Other systems that have been used for this type of irwestigatiou make use of other solvents besides methanol, as well as other solutes. A comparison of the, solubility of lead(I1) chloride in the methanol-water, ethylene glycol-water, and dioxane-water systems is a. useful way to discuss interaction. Lead(I1) chloride shows a considerably greater solubility [mole fraction of lead(I1) chloride] in a series of ethylene glycol-water solutions than in the corresponding methanol-water solutions. At low mole fractions of ethylene glycol,, there is no decrease in the solubility as observed with the methanol-water system. At high mole fractions of ethylene glycol, the solubility decreases. The solubility of lead(I1) chloride in a series of dioxane-water. solutions is greater than the solubility in methanolwater solutions hut less than the solubility in ethyleneglycol-water solutions. Other solutes that give interest,ing results are m.ereur,v(II)iodide or the ot,her halides of lead(I1). Data could be collected on the effect of temperature on the solubility of slightly soluble solutes in mixed solvents and could be used to discuss the effect of temperature on the various interactions in the sys-. tems. The effect of ionic strength on t,he solubility can be studied and can be used to calculate the solubility product constants, K,,, for the systems.
