DETERMINATION OF MOLECULAR WEIGHTSREPORTcompiled to fulfill the practicum report of Physical Chemistry subject

By:Rifaldi Hadiansyah (131411046)Risma Regiyanti (131411047)

Class:1B

STUDY PROGRAM DIPLOMA III CHEMICAL ENGINEERING DEPARTMENT OF CHEMICAL ENGINEERINGSTATE POLYTECHNIC OF BANDUNG2014

ABSTRACTWe have done an experiment titled Determination of Molecular Weights which aims to determine the molecular weight of a volatile and non-volatile substance. The principles of this experiment are colligative properties and ideal gas law. To determine the molecular weight of the volatile substance, we used the depression freezing point method and we used the determination of the density of the gas method. We did the experiment, collect the data from literature and compared the results of the experiment with the data from literature. Experimental results contained the molecular weight of the non-volatile substance is equal to 127,84 gr/mole and the molecular weight if the volatile substance is equal to 51,13 gr/mole. The experimental results are slightly to the literatures data which is caused by environments condition and the properties of the substances.Keywords: Colligative properties, Freezing-point depression, Ideal gas law, Mole fraction, Raoults law, Vapor pressure

INTRODUCTION

A. BACKGROUNDBased on its form, substance can be divided into three kinds of substances that are solids, liquids and gases. Each substance is composed of small particles that can be atoms, molecules, or ions. Changes in circumstances often found in chemical reactions. Substances which initially produced in a gaseous state can quickly condense in liquid form. Energy change accompanying a chemical reaction depends on the state of the reactants and reaction products. For example, the combustion of methane as the main constituent of natural gas to produce carbon dioxide and water. The amount of energy released in the form of steam and liquid.The molecular weight of a substance is the sum of the atomic masses of its constituent elements. The molecular weight can be calculated by adding the relative atomic mass of the elements making up the molecule. The molecular weight can be measured in many ways.. For example, measurement of volatile substances can be done by using the ideal gas equation to determine the density, pressure, and temperature of the substance. As for determining the molecular weight of non-volatile substances can be calculated by the method of freezing-point depression. Based on those statements, the experimental determination of molecular weight carried by the gas density and freezing-point depression.B. OBJECTIVES1. Determine the molecular weight of a non-volatile substance by using the freezing- point depression method.2. Determine the molecular weight of a volatile substance by using the determination of density of the gas.C. BASIC THEORY1. Freezing-Point DepressionIf a non-volatile substance is dissolved in a solvent, vapor pressure loss will occur and in a certain temperature, the solvents vapor pressure in the solution will be lower than its normal. The value of vapor pressure is determined (Yahya, 1979)

Po

P

Po

Picture 1. Pure solvent Picture 2. Solution Po = vapor pressure of pure solventP = vapor pressure of solutionBased on Raoults law :

..(1)Which is : X1 = mole fraction of solution Po = vapor pressure of pure solvent P = vapor pressure of solution

If the equation (1) being integrated, then:

.(2)

Based on Clausius-Clapeyron law:

.(3)

Explanation: : freezing point of solventT : freezing point of solution

By the equation (2) and (3), then:

.(4)Explanation:For aqueous solution, ln(1-X_2 )= -X_2G1 weight of solventG2 weight of dissolved solutionM1 molecular weight of solventM2 molecular weight of dissolved solutionHf fusion heat of solventKf: molal freezing-point depression constant (when 1 mole of dissolved substance is being solved in 1000 grams of solvent)

2. Determination of the Density of GasTo determine the molecular weight by the method of determining the density of the gas can be done by using a Victor Meyer. The equation used is the ideal gas equation.

..(5)Explanation:Mr = molecular weight of dissolved substance (gr/mole)P = pressure (atm)R = ideal gas constant (0,082 L atm/mole K)T = temperature (K)d = = molecular weight of volatile substance

D. TOOLS AND MATERIALS1. ToolsToolsSpecificationNumber

Freezing-point depression tools1

Pignometer1

Beaker glass50 mL1

Measuring pipette10 mL1

Balancer1

Kaca arloji1

Stopwatch1

Chemical glass500 mL1

Erlenmeyers flask100 mL2

Hotplate1

Thermometer1

Desiccator1

Bottle500 mL1

Spatula1

Table 1. Tools

2. MaterialsName of MaterialsSpecificationNumber

Glacial Acetic acid (solvent)

Naphthalene (dissolved substance)

X Substance (sample)

Garam krosok

Ice cube

Acetone50 mL

Aquades

Table 2. Materials

METHODOLOGYA. COLLECTING DATAThese are the methods that being used in this report:1. Observation MethodObservation method is a method of data collection that done by direct observation in the field, that is by doing laboratory experiments.2. Literature MethodLiterature method is a method of data collection is done by taking the data that is required from literatures related.B. DATA ANALYSISHaving obtained the necessary data from the observation, the data is being aggregated and disaggregated for further analysis to calculate the value of the molecular weight of the substance being analyzed.C. COMPARING DATAAfter receiving the results of the data analysis, the results were compared with data obtained from the literature.PROCEDUREI. Freezing-point Depression Method1. Write down the molecular weight of acetat acid glacial, take a look on the bottle2. Take 50 mL of solvent, add it into the tool3. Add some ice and salt into the tool4. Stir it until the solvent is frozen and then write down the temperature5. Remove the solvent that has been frozen out of the tool, prepare a new 50 mL of solvent 6. Add 3 grams of naphthalene in 50 mL of solvent, insert it into the tool 7. Stir until the solution is frozen and write the temperature8. Remove the solution that has been frozen out of the tool, prepare a new 50 mL of solvent9. Add 3 grams of sample in 50 mL of solvent, insert it into the tool 10. Stir until the solution is frozen and write the temperature

II. Gas Density Determination Method1. Prepare the Erlenmeyer flask2. Cover it with Alumunium foil3. Take 5 mL of liquid volatiles, put it into the flask 4. Give a small hole in the lid 5. Place the Erlenmeyer flask in a water bath 6. Write the temperature of the bath when the liquid evaporates 7. Lift and place the flask in a desiccator 8. Weigh the cooled flask 9. Determine the volume of the flask 10. Measure the atmospheric pressure

RESULTS AND FINDINGSOBSERVATIONAL DATAWeight of Naphtalene: 3 grWeight of sample: 3 grVolume of solvent: 50 mLDensity of sovent: 1,049 gr/mlWeight of solvent: 52,45 grFreezing point of solvent (T1): 8 oCFreezing point of naphtalene (T2): 10 oCFreezing point of sample (T2): 6 oC

Table 1. Observational data of freezing-point depression methodWeight of empty flask (+tube): 149,55 grWeight of flask + acetone (cold): 150,03 grWeight of flask + water: 458,58 grDensity of water when T = 25 oC: 0,99686 gr/mLTemperature of water bath: 91 oCPressure: 687 mmHg = 0,9039 atm

Table 2. Observational data of gas density determination method

Molecular Weight of Substances based on the Experiment = 127,84 gr/mol

DISCUSSION1. Freezing-point Depression MethodDepression of freezing point is a way to determine the molecular weight of substance by using solvent which is freezing easily and a substance which its molecular weight is known. In this experiment, we used glacial acetic acid as a solvent and naphthalene as a substance which its molecular weight is known (Mr=128 gr/mole). Experiment performed 3 times, those are:a. determination of the freezing point of the solvent ( glacial acetic acid ) b. determination of the freezing point of the solvent ( glacial acetic acid ) + naphthalenec. determination of the freezing point of the solvent ( glacial acetic acid ) + sample substanceFrom the experiments above, we found the freezing point of each substances and we used it to calculate the molal freezing-point depression constant of solvent (Kf). It is found that Kf of solvent is 4,47 oC while the value of Kf of solvent listed in the literature is 3,90 oC. It is happened because of the atmospheric pressure and temperature in the laboratory is not in standard condition so its freezing-point doesnt match to what is listed in the literature (the literature showed data in the standard condition). And from the calculation, its found that the molecular weight of x substance is 127,84 gr/mole.

2. Gas Density Determination MethodIn this experiment, we used acetone because acetone is a volatile substance. We were looking for the temperature when acetone being evaporated and weight of acetone after its evaporated. After that, we calculated the molecular weight of acetone by using the data obtained from the experiment. Based on the calculation, the amount of the molecular weight of acetone is 51,8 gr/mole which is slightly to the value listed in the literature, that is equal to 58 g / mol. This is due to the properties of the acetone gas which is evaporating easily so that when gas weighted, there are some of gas missing and being unweight.

CONCLUSIONAfter doing the observation and calculation, we can conclude that:1. Molecular weight of x substance as a non-volatile substance is 127,84 gr/mole2. Molecular weight of acetone as a volatile substance is 51,8 gr/mole

ATTACHMENTDATA ANALYSIS1. Freezing-point Depression

= 127,84 gr/mole

2. Determination of the Density of GasWeight of empty flask (+tube): 149,55 grWeight of flask+acetone (cold): 150,03 grThus, weight of acetone: 150,03-149,55 = 0,48 gr

Weight of flask+water: 458,58 grThus, weight of water: 458,58 149,55 = 309,03 gr

Density of water at T = 25 oC: 0,99686 gr/mLTemperature of water bath: 91oCPressure: 687 mmHg = 0,9039 atm

Volume of water is: water= volume of water= = = 310,003 mL= 0,310 LCalculate the molecular weight of gas

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