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General Chemistry Chem. 105 Laboratory Manual Prepared by: Dr.Mahmoud Alqudah and Tariq Batianeh. 2020

Experiment 7 Molecular weight of a volatile liquid Objectives: • To measure the physical properties of pressure, volume, and temperature for a gaseous substance • To determine the molar mass (molecular weight) of a volatile liquid Introduction: A number of analytical methods can be used to measure the molar mass of a compound; the choice of the analysis depends on the properties of the compound. For example, the molar masses of “large” molecules, such as proteins, natural drugs, and enzymes found in biochemical systems, are often determined with an osmometer, an instrument that measures changes in osmotic pressure of the solvent in which the molecule is soluble. For smaller molecules a measurement of the melting point change of a solvent in which the molecule is soluble can be used. Recent developments in mass spectrometry have expanded its use to include not only molar mass measurements but also the structures of high molar mass compounds in the biochemical fields. For volatile liquids, molecular substances with low boiling points and relatively low molar masses, the Dumas method (John Dumas, 1800–1884) of analysis can provide a fairly accurate determination of molar mass. In this experiment, with the aid of an advanced procedure of Dumas method, a volatile liquid will be evaporated at a measured temperature T and pressure P, into a measured volume V of an Erlenmeyer flask. The number of moles of the gas (nvapor ) and its molecular weight will then be determined using the ideal gas law.

In this equation, R is the universal gas constant, P is the barometric pressure in atmospheres, V is the volume in liters of the vessel into which the liquid is vaporized, and T is the temperature in kelvins of the vapor. The mass of the vapor, mvapor, is determined from the mass difference between the “empty” flask and the vapor-filled flask. mvapor = mflask +vapor - mflask The molar mass of the compound, Mcompound , is then calculated from the acquired data. Procedure: 1- Clean a 125-mL Erlenmeyer flask and dry it either in a drying oven or by allowing it to air-dry. Do not wipe it dry or heat it over a direct flame. 2- Cover the dry flask with a one - hole stopper and the insert a capillary tip piece of glass tubing into the stopper. 3- Weigh together a clean, dry, 125 mL Erlenmeyer flask, and a stopper. Record the total weight in your data table. 4- Pour roughly about 3 mL of the liquid into the flask. Cover the mouth of the flask with and insert the stopper. 5- Obtain a suitable beaker from your instructor and half fill it with water. Record the temperature of the boiling water and the atmospheric pressure in the data table. 6- Immerse the flask containing the volatile unknown liquid in the boiling water so that most of the flask is beneath the hot water as shown in figure 1.

Figure 1: Apparatus for molecular 7- Gently heat the water until it reaches a gentle boil. (Caution: Most unknowns are flammable; use a hot plate or moderate flame for heating.) When the liquid in the flask and/or the vapors escaping from the holes in the capillary tip are no longer visible, continue heating for another 5 minutes. Read and record the temperature of the boiling water. 8- Turn off the burner and carefully remove the flask from the water using a towel and allow cooling to room temperature. Then dry the flask completely.

9- Weigh the flask with stopper and condensed vapor. Record the weight in the data table. 10- To determine the volume of the flask, fill it to the brim with water and insert the capillary – filled stopper, and then measure the volume of water using a graduated cylinder. Record the volume in the data table.

Name: Experiment 7 Date: Molecular weight of a volatile liquid I.D: LAB SEC: REPORT SHEET Unknown number ( ) Trial 1 Trial 2 1- Mass of empty flask + stopper (g) 2- Mass of flask + stopper + condensed vapor (g) 3- Mass of condensed vapor (g) 4- Temperature of boiling water ( ºC) 5- Temperature of boiling water ( K) 6- Atmospheric pressure (mmHg) 7- Atmospheric pressure (atm) 8- Total volume of 125 mL flask used

Calculation: Trial 1 Trial 2 1. Mass of vapor, mvapor (g) 2. Moles of vapor, nvapor (mol) 3. Molar mass of compound (g/mol) 4. Average molar mass (g/mol) **Show your calculation

QUESTIONS 1) In step 3, the mass of the flask is measured when the outside of the flask is wet. However, in step 9 the outside of the flask is dried before its mass is measured. Will the molar mass of vapor in the flask be reported too high, too low, or unaffected? Explain? 2) The pressure reading from the barometer is recorded higher than it actually is. How does this affect the reported molar mass of the liquid . . . too high, too low, or unaffected? Explain. 3) For which one of the following liquids, the molecular weight cannot be determined by the method described in this experiment? a- glycerin (180 ºC) b- Benzene (72 ºC) c. dichloromethane (40 ºC) Answer: ( )

4) Perform the following conversions: a. Convert 760 torr to atm. b. Convert 1.81 atm to mmHg. 5) A 125-mL Erlenmeyer flask has a measured volume of 152 mL. A 0.199-g sample of an unknown vapor occupies the flask at 98.7°C and a pressure of 754 torr. Assume ideal gas behavior. What is the molar mass of the vapor?


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