Exp2

CHM170L Organic Chemistry 1 Laboratory 4th Quarter SY 2016-2017

Measurement of Density and Determination of Partial Volume of Ethanol-Water System Premarion, Matthew M.1, Rivera, Hazel Anne T.1, Villaflor, Shekinah Mae J.1 1

Student(s), CHM170L /B40, School of Chemical Engineering, Chemistry and Biotechnology, Mapua Institute of Technology

Abstract This experiment is primarily focused on the measurement of the density and the determination of the partial molar volume of ethanol-water system. A partial molar volume is a thermodynamic quantity defined as a change in volume per mole of substance added to the mixture at constant temperature and pressure, indicating that molar volumes are non-additive. Solutions of ethanol and water had been prepared with varying concentrations and a pycnometer was calibrated and used by weighing it along with the liquid samples one at a time, for the accurate measurement of the density of each mixture. The latter was measured to calculate for the excess molar volume and partial molar volume for both components. The data were then used for calculating other physical quantities such as molar volume, molar fraction, and molecular weight. For the determination of the partial molar volume of each component, tangent lines to the curve were drawn from the graph relating the negative excess molar mass of the concentrated mixture, and the mole fraction of water. From these lines, the y-intercepts at certain points had been identified for the determination of the partial molar volume of ethanol and water which were used for correlation. Results showed that the partial molar volume of a substance increases with its molar fraction, exhibiting direct proportionality. Hence, in this experiment, the partial molar volume of ethanol water-mixtures is to be determined using density and specific gravity measurement. Keywords: density, partial molar volume, pycnometer, mole fraction, excess molar volume

Introduction Volume, by definition, is the amount of space occupied by a substance measured in cubic units. Molar volume, on the other hand is the volume occupied by one mole of a solid, liquid, or gas. This experiment focuses on determining the partial molar volume of ethanol-water system. Partial molar volume is the “contribution that a component of a mixture makes to the total volume of a sample” [1]. In other words, the partial molar volume of a substance in a mixture is the change in volume per mole of the substance added to the mixture. Moreover, it is the easiest extensive property to visualize. Generally, for solutions with different components, the volumes of the components are not additive. Partly because the molecules in each component undergo different intermolecular forces than in pure substances. For instance, a water molecule, together with an ethanol molecule, observe a different interaction than that of two water molecules alone, or two ethanol molecules alone [2]. Also, the water and ethanol molecules have various shapes and sizes. Hence, the molecules of a solution of ethanol and water fit quite differently than that of pure water and pure ethanol [3].

Given a system with two components X and Y, the partial molar volume of the first component X can be determined mathematically by the equation: (1) where V is the total volume, NA is the number of molecules of X, NY is the number of molecules of Y, T is the temperature, and P is the pressure. As can be seen, the partial volume of X is the change in volume per mole of X added, if the temperature and pressure are constant. This experiment will try to use the concept of partial molar volume using the materials and methods provided. The objectives of this experiment are, first, to be familiarized with the use of pycnometer and the chain balance for measuring density and specific gravity, respectively, and second, to determine the partial molar volume of ethanol-water system at different concentration using density and specific gravity measurement.

Methods Selection and Preparation of Sample

Experiment #2│ Group No. 6 │19th of May 2017

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Pycnometers was mostly known in determination of alcohol concentration. Alcohol and water solution is a homogenous liquid. Homogenous liquids are simple systems because it is a single-phased system. This is ideal to have the temperature constant throughout the experiment. Ethanol was used as this experiment’s sample. The ethanol used was a mother liquor, which is pure. Information about the sample used is shown in Table 1.

Compound Ethanol

Boiling point 78.37˚C

Density

Classification

0.789g/mL

Alcohol

Table 1. Boiling point, Density and Classification of the sample used.

For the molar volumes of ethanol-water mixture at different concentrations, different volumes of water and ethanol was pipetted in the pycnometer. In the first pycnometer, there was 0-mL of water with 30mL ethanol. The volume of the water was increased by multiples of 3-mL, while the volume of the ethanol was decreased by the same multiple, making up a solution with a total volume of 30-mL. First, the water and ethanol was mixed in the beaker. Then the solution was transferred into a pycnometer, allowing it to overflow, the cap and thermometer was placed simultaneously to make sure the pycnometer was filled with the solution (see Figure 2). The temperature was then taken as soon as the thermometer was placed on top, then it was weighed (see Figure 3). The same procedures were repeated for different molar fractions of 0.1 differences.

Materials and Apparatuses Utilized The materials and apparatus used in this experiment were pycnometer, 25-mL graduated cylinder, 250 mL beaker and analytical balance, while the reagents used were ethanol and distilled water. Calibration of Pycnometer For the calibration of the pycnometer, the weight of an empty pycnometer with a thermometer was measured. 30mL of water was then poured into it, and its temperature and weight were altogether to determine the corresponding density of water inside. (see Figure 1).

Figure 1. Transferring water into the pycnometer for calibration


Figure 2. Pycnometer filled with ethanolwater solution with cap and thermometer

Figure 3. Weighing the pycnometer with the solution

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Prior to use for the subsequent test runs, the calorimeter components must first be cleaned properly to avoid the possible deviation from result brought about by impurities from the previous sample tested. Shown in Table 1 are the specified and distinct volume of water and ethanol respectively added per sample solution to be evaluated. Table 2. Volume of Each Component of Mixture Volume of water, cm3 Volume of Ethanol, cm3 0 30 3 27 6 24 9 21 12 18 15 15 18 12 21 9 24 6 27 3 30 0

Determination of Molar Volume Prior to the actual utilization of the flask with a close-fitting ground glass stopper with fine hole through it, the pycnometer was first calibrated by means of getting the required measurements. It was initially weighed dry and then after filling it up with water, the mass reading was taken note of. The filled-in weight reading was subtracted to that of the empty reading to determine the weight of the water alone.

molecular weights of water and of ethanol. The density of the mixture was measured with the mass in grams of the sample and the volume it occupies. Having the pycnometer weighed with a full amount, the empty pycnometer weight was simply subtracted and yielded the mass of the mixture. Subdividing the average molecular weight with that of its corresponding density gave the molar volume. The change in molar volume due to mixing was calculated by differentiating the computed molar volume with the appropriate molar volume. This value tells the increment in volume with that of a pure and of a mixture. As for the determination of the partial molar volumes of ethanol and water accordingly, the excess molar volume of the mixture vs. the mole fraction of water was plotted. From the graph, tangent lines were drawn with respect to the curve and specified points from which the equations of these tangent lines were known. By getting the y-intercepts at x=0 for the solute and x=1 for the solvent, the partial molar volume of the ethanol-water system could be determined.

Treatment of Results The obtained data was used to compute for the average molar mass of each solution Eqn. (2), experimental weight of the solution using Eqn. (3), density using Eqn. (4), specific gravity of the solution using Eqn. (5), molar change in volume using Eqn. (6) and partial molar volume of ethanol Eqn. (7). (2)

Knowing that the density of water at a temperature of 28˚C as 0.996237 g/cm3*, the corresponding volume of the pycnometer could be ascertained substantially.

(3)

*Literal value of water at 28 °C [5]

(4)

Calculations underwent through quite a few reckonings for the molar volume count. These included densities, molecular weights, and mole counts of each differently concentrated solution.

(5)

First, the mole count of water and of the ethanol was measured with the corresponding volume that both occupied in a solution and then their mole fractions were obtained varying to the quantity they occupy. The molar volume of the mixture was computed with the density and the average molecular weight which was then identified with the mole fractions and Experiment #2│ Group No. 6 │19th of May 2017

(6)

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(7)

Results and Discussions V1 (water) 0 3 6 9 12 15 18 21 24 27 30

V2 (ethanol) 30 27 24 21 18 15 12 9 6 3 0

M1 0 0.166 0.332 0.498 0.664 0.831 0.995 1.161 1.327 1.493 1.659

M2 0.511 0.460 0.409 0.358 0.306 0.256 0.204 0.153 0.102 0.051 0

X1 0 0.219 0.448 0.582 0.684 0.765 0.830 0.884 0.929 0.967 1

Mavg, g/mol 46.068 39.925 33.489 29.738 26.871 24.613 22.788 21.282 20.018 18.942 18.015

ρmix, g/cm3 0.998 1.031 1.065 1.088 1.126 1.156 1.180 1.191 1.219 1.231 1.243

V,Ṿ cm /mol 46.160 37.472 31.456 27.336 23.872 21.311 19.332 17.858 16.436 15.387 14.493 3

∆Vmix, Ṿ 3 cm /mol -12.548 -13.451 -9.030 -7.711 -6.992 -6.288 -5.626 -4.938 -4.528 -4.018 -3.568

Table 3. Molar Volumes of Ethanol-Water mixture at different concentrations

Ethanol-water solutions of different concentrations had been prepared and each of them had been placed in the pycnometer and then weighed afterwards. A pycnometer had been used for a given volume and density to be accurately obtained by reference to an appropriate working fluid, using an analytical balance.

molecular weight and molar volume of the mixture at different concentration can be seen. As per Table 2, the average molecular weight of the mixture decreases as you increase the concentration of water in the mixture, because the water has a lower molecular weight than the water (see Figure 2.1).

At the start of the experiment, you would need to calibrate the pycnometer to reduce and minimize errors in computations of the different values. You could see on table 1 the result in calibration of the pycnometer. Table 4. Calibration of Pycnometer

Weight of empty pycnometer Weight of empty pycnometer + water Temperature of water, T Density of water at T

28 °C 0.996237 g/cm3*

Volume of pycnometer

25.00 cm3

20.077g 46.511g

*Literal value of water at 28 °C [5]

After the calibration of the pycnometer, we performed the measuring of masses of different concentration of ethanol-water solution. In Table 2, the data such as the masses, number of moles, mole fraction, average Experiment #2│ Group No. 6 │19th of May 2017

Figure 2.1. Average molecular weight/ Molar volume of the mixture VS mole fraction of water.

In Table 2, the density of the solution increases since the density of ethanol is lower than water (see Figure 2.2). The molar volume of the mixture decreases because of the presence of hydrogen bond, these makes the distance 4 of 7


between molecules of ethanol and water smaller (see Figure 2.1). From the data gathered, it could be observed that there is a trend between concentration of solution and the density of the mixture. This relationship could further be elaborated by having the illustrated graph as follows:

1

0

0

0

Table 4.1. Determination of Partial Volume of Ethanol. where V1– V1* is obtained from the y-intercept of the

tangent lines, V1 stands for the volume of ethanol, obtained by dividing the molar masses of ethanol over its density, and V1* for the partial molar volume of ethanol. To clearly understand the relationship between the partial molar volume of ethanol and the mole fraction of water, an illustration showing how these two are related is shown:

Figure 2.2. Density of mixture VS mole fraction of water graph.

In addition, the ∆Vmix column in table has a distinct noticeable data that we can concur. Both the pure ethanol and pure water has a positive data, while the mixed solutions have negative. This leads to the conclusion that the intermolecular molecules affect the mixture of the solution. Since we know that both the ethanol and water are polar protic solvents, they are miscible with each other, thus interacting between the two. The pure water has higher value than that of the pure ethanol; this also leads to the fact that the water has stronger intermolecular force that exhibits hydrogen bonding. Also, the data from the mixture also supports the conclusion because the more the volume of water increases, the higher the value of the ∆V mix. For Ethanol, the table below shows a summary of its partial molar volume for each concentrated solution: X1, ml 0 0.265 0.448 0.582 0.684 0.764 0.829 0.884 0.928 0.967

V1 – V1* 0.59415 -1.22042 -2.46212 -3.37657 -4.07131 -4.61854 -5.06135 -5.42547 -5.73177 -5.99123

V 1* 19.73 19.73 19.73 19.73 19.73 19.73 19.73 19.73 19.73 19.73


V1 59.7795897 59.040299 58.498829 57.778079 56.711289 56.333329 53.733329 52.007419 50.2193790 48.4198690

Figure 3.2. Partial Molar Volume of Ethanol VS mole of Water

The graph portrays that as the mole fraction of water increases, the partial molar volume of ethanol decreases. In addition, it could be observed that the more dilute the solution is, the lower is the partial molar volume of ethanol. In the case of water, the table below shows the summary of the data obtained for each mole fraction. X1, ml 0 0.265 0.448 0.582 0.684 0.764 0.829 0.884 0.928 0.967 1

V2– V2* 0.59415 -1.22042 -2.46212 -3.37657 -4.07131 -4.61854 -5.06135 -5.42547 -5.73177 -5.99123 -6.21615

V2* 18.01899 18.01899 18.01899 18.05416 18.05416 18.05416 18.06503 18.05416 18.05416 18.05416 18.05416

V2 18.18182 15.1406 16.0936 16.7333 17.3195 18.8190 18.2078 18.4842 18.6617 18.7566 18.7850

Table 4.2. Determination of Partial Volumes of Water

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where V2 - V2* represents the y-intercept of the tangent line at x=1, V2 stands for the volume of water, obtained by dividing the molar mass of water over its density, and V2* for the partial molar volume of water. Plotting the points and data in a graph would produce an outcome of:

Figure 3.2. Partial Molar Volume of Water VS Mole of Water

The graph suggests that when the mole of water increases the partial molar volume of water increases as well. Water and ethanol will always have a negative excess volume when mixed; indicating the partial molar volume of each component is less when mixed than its molar volume when pure.

molar volume of mixture vs. the mole fraction of water, density of mixture vs mole fraction of water, ∆Vmix vs mole fraction of H2O and the like. Furthermore, the tangent lines of the curve were determined to calculate the partial volume of ethanol and water at the given mole fraction of ethanol and water. In the experiment, certain relationships in the ethanolwater mixture ware observed. It was observed that the average molecular weight of the mixture decreases as you increase the concentration of water in the mixture. Also, as the volume of water increases and the volume of ethanol decreases, the weight of the mixture increases. Per the results, the pure water has higher value than that of the pure ethanol, leading to the fact that the water has stronger intermolecular force that exhibits hydrogen bonding. Moreover, water and ethanol will always have a negative excess volume when mixed which indicates the partial molar volume of each component is greater when both components are pure. In the matter of the deviation in the total volume of the ethanol-water system, the intermolecular forces of attraction between the molecules of ethanol and water were responsible for it. Thus, as the amount of both ethanol and water approaches to a 50:50 ratio the greater the deviation will be. Also, the deviation will get smaller if the other liquids would be more compared.

The partial molar volume of the components of a mixture will always vary by its composition because when the two is mixed the environment of the two is changed, thus the interaction between the molecules also changed. Or we can also suggest that the composition of the mixture wherein the larger is its contribution to the mixture, the larger is the partial molar volume.

The potential sources of errors in this experiment are the improper use of the pipette in transferring the liquids into the container, presence of air space in the pycnometer, presence of liquid outside of the pycnometer before the initial weighing and the improper use of the pycnometer.

Conclusion and Recommendation

[1] Caparanga A., Baluyut J.Y.G, and Soriano A. (2006) Physical Chemistry Laboratory Manual Part 1

In the experiment, different concentration of ethanolwater system was prepared. The calibrated pycnometer was used to obtained masses of the different concentrations of ethanol-water mixture needed for the computations of the necessary data required to determine the partial-molar volume of the system. The necessary data includes the number of moles, mole fraction, average molecular weight and molar volume of the mixture at different concentration. To show the some of the relationships of the data obtained certain graphs were made. These graphs include the relationship of the average molecular weight and Experiment #2│ Group No. 6 │19th of May 2017

References

[2] Levine I (2009) Physical Chemistry, Fifth Edition [3] Atkins, P. W., & de Paula, J. (2006). Atkins’ physical chemistry (8th ed). Oxford: Oxford UniversityPress. [4] Imai, T. (2007). Molecular Theory of Partial Molar Volume and Its Application to Bimolcular Systems. Department of Bioscience and Bioinformatics, Ritsumeikan University. P.525 [5] Engel, T. (2006). “Physical Chemistry”, p.210 6 of 7


Appendices I.

Sample Computations (Part B)



m1 – moles of distilled water



m2 – moles of ethanol



x1 – mole fraction of water



Mavg – average molecular weight of mixture ) + (1-x1) (MMe)



ρmix – density of the mixture



V – molar volume



ΔVmix – molar change in volume


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Exp2

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