CHEM 142

Physical Chemistry I

Experiment 1

Liquid Density

September 07, 2012

Abstract

The purpose of this activity was to determine the densities of different salt solutions using

the pycnometer and hydrometer as a tool of determination. Appropriate statistical tools were

utilized to treat the data obtained in the laboratory. All statistical treatments were done at 90%

confidence level and the condition upon which the densities are obtained are at 300C. In the

analysis, it has been found out that the concentration of sea water is 3.4% m/v in terms of salt.

With a pooled standard deviation of ±0.004%m/v, the reliability of the result is acceptable. With

these, the experiment is a success.

Introduction

Density, ρ, is defined as the ratio of mass m of a substance to its volume, v. Because

volume expands with increasing temperature, density is thus greatly influenced by temperature.

In a liquid system, the number of particles present in the solution also affects density. When the

concentration is increased density is also increased.

Consequent to the above concept, we can solve for the concentration of a solution with

known analyte but unknown concentration using linear regression. In this paper, sea water salt

concentration will be analyzed. This can be done by obtaining the density of salt solutions with

known concentration for the linear regression plot, then plugging in the density of unknown salt

solution which is the seawater, then calculate for its density using appropriate variables.

Thus, this paper seeks to determine the concentration of unknown concentration of salt in

sea water by employing hydrometric and pycnometric determination method and provide

practice for chemistry major students in dealing with this type of analysis.

Methodology

A. Solution preparation

Salt solutions with concentrations of 0, 2, 4, 6, and 8 percent (g/mL) were prepared by dissolving

2 g NaCl(S) in 98 mL water for 2 percent salt solution, continuing using proportionality of NaCl-

water solution up to 8%.

B. Pycnometer determination method

Pre-boiled distilled water was used throughout the density determination of solutions. The

determination was carried out at 300C and at constant pressure condition.

a. Calibration of Pycnometer

Three 50 mL pycnometers were washed, dried, and cooled to room temperature. The

pycnometers were weighed to constant weight and then the weight was recorded as the weight of

pycnometer. . The pycnometer was then filled with distilled water and then the mass of water in

air was calculated from the difference of the mass of water in pycnometer and mass of

pycnometer. The actual volume of pycnometer was calculated from the mass of water in air. The

determination was done in triplicates. The actual volumes of individual pycnometers were

obtained using appropriate statistics.

b. Determination of Density of Salt Solutions

Calibrated pycnometers 1, 2, and 3 were filled with the prepared salt solutions and then

weighed. The weights were noted and the densities of individual solutions were calcultaed as a

function of their concentrations. A correction to vacuo was applied in the calculation for the

densities of solutions. The densities were calculated using appropriate statistical tools.

Unknown salt solution as sea water was analyzed for its density and then the density with

correction to vacuo and appropriate statistics was calculated.

C. Hydrometric Determination of Density of salt solutions.

A 100 mL graduated cylinder was filled with salt solution. A hydrometer was dipped in

the solution and the density was noted. The density was obtained and noted as a function of the

concentration of salt solution.

D. Treatment of Data

The mean densities of all salt solutions in pycnometric determination were compared

with the mean densities of the salt solutions in hydrometric concentration at 90% confidence

limit. The grand mean was obtained with the corresponding pooled standard deviations of salt

solutions. A calibration curve was constructed using the data obtained from standard salt

solutions, and then the concentration of salt in sea water was obtained by linear regression

equation using the obtained density of the salt solution.

Results and Discussion

Density determination was employed to calculate for the concentration of salt solution

with unknown concentration. Pycnometric and hydrometric determination of density was also

employed.

In pycnometric determination, three 50mL pycnometers were calibrated. Table 1 presents

the pertinent data in the calibration of pycnometers.

TABLE 1 Calibration of Pycnometer at 300C

Pycnometer Trial

WtH2O (g)

Density(g/mL)

Wt Corrected(g)

Volume Pycnometer(mL)

Meanvolume STDEV

1

1 50.6505 0.99565 50.59669 50.81775

51.09844 0.2430852 51.0699 0.99565 51.01564 51.238533 51.0704 0.99565 51.01614 51.23903

2

1 52.3564 0.99565 52.30078 52.52928

52.54503 0.0136442 52.3802 0.99565 52.32455 52.553163 52.3797 0.99565 52.32405 52.55266

3

1 51.5408 0.99565 51.48604 51.71099

51.69286 0.0156982 51.5137 0.99565 51.45897 51.68383 51.5137 0.99565 51.45897 51.6838

The actual volumes for the pycnometer were presented in table 1. As manifested, the

volume of pycnometer is not exactly 50mL. As such, calibration is really critical in this acitivity.

After the calibration, the densities of salt solutions were determined using the calibrated

pycnometers. Table 2 shows the densities of salt solution and sea water using pycnometric

determination.

TABLE 2 Densities of NaCl solutions and unknown as sea water

Results and Discussion

Table 2 presents the densities of salt solutions. From the densities of salt solutions with known concentration the concentration of NaCl in seawater can be estimated. The concentration of sea water as a salt solution will be calculated later on.

Hydrometric determination was also employed in the determination of densities. Table 3 shows the densities of salt solutions at 300C.

TABLE 3 Densities of Salt Solutions using Hydrometeric DeterminationDensity (g/mL)

%m/v salt trial 1 trial 2 trial 3 mean stdev0 0.995 0.995 0.995 0.995 N/A2 1.015 1.015 1.015 1.015 N/A4 1.02 1.02 1.02 1.02 N/A6 1.0418 1.045 1.04175 1.04285 0.0018628 1.04175 1.0583 1.05 1.050017 0.008275

sea water 1.0195 1.0233 1.021 1.021267 0.001914

Hydrometric and pycnometric determination of densities of substances show a slight deviation to each other due to apparatus limitations.

Appropriate statistical tools were utilized and it was found that at 90% confidence limit, both methods show no difference in the results. Thus, table two shows the calculation of grand mean for the densities of the solutions and the concentration of sea water in terms of NaCl was presented with linear regression equation y=0.0.996x + 0.0066.

TABLE 4 summarized densities of salt solutions and concentration of sea water% m/v pycno hydro Grand

meanSpooled

0 0.9945087

0.995 0.994754

0.003887

2 1.0087015

1.015 1.011851

0.002228

4 1.0211178

1.02 1.020559

0.003072

6 1.0361772

1.04285

1.039514

0.002228

8 1.0440617

1.050017

1.047039

0.006822

Concentration[seawater]

seawater

1.0159845

1.021267

1.018626

0.004384

3.4 % (m/v) NaCl

Table 4 can be better understood using the calibration plot illustrated by figure 1.

0 1 2 3 4 5 6 7 8 90.96

0.97

0.98

0.99

1

1.01

1.02

1.03

1.04

1.05

1.06

f(x) = 0.0066116241186107 x + 0.996296859493765R² = 0.983248634483759

Calibration plotLinear (Calibration plot)sea water

Concentration (%m/v)

Dens

ity (g

/mL)

Fig. 1 Calibration of salt solutions and the determination of concentration of unknown

Figure 1 illustrates the significance of linear regression to the determination of unknown with the construction of a calibration curve of known salt solutions. With 10% uncertainty, it can be concluded that sea water is 3.4% m/v salt.

Conclusion

In this paper, the concentration of sea water was analyzed in terms of %m/v NaCl. With

appropriate statistical tools, most reliable density from two methods was obtained, supplemented

by their pooled standard deviations.

At 90% confidence limit, the two methods show no difference in their results.

The density of the sea water being analyzed is not purely contributed by NaCl. Other

dissolved gases such as NH3 and O2 can also account for the density of sea water. Other

substances may still be present, since sea water is a real sample.

Even with these, the activity was very successful in providing the chemistry major

student to provide practice in the utilization of density of known salt concentrations to the

concentration of unknown salt solution with the use of Linear Regression method and/or

calibration construction method.

With these at hand, it can be concluded that the determination is a total success.