LAB NAME MOLECULAR MASS OF A VOLATILE LIQUID 3

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LAB NAME MOLECULAR MASS OF A VOLATILE LIQUID 3 LAB NAME____________________________

Molecular Mass of a Volatile Liquid

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It is often useful to know the molecular mass of a substance. This is one of the properties that helps characterize the substance. If the substance is a volatile liquid, one common way of determining its molecular mass involves the ideal gas law, PV=nRT. Since the liquid is volatile, it can easily be converted to a gas. While it is in the gas phase, its volume, temperature and pressure are measured. The ideal gas law will then allow the calculation of the number of moles of the substance present:


n = LAB NAME MOLECULAR MASS OF A VOLATILE LIQUID 3 


where n is the number of moles of gas, P is pressure, V is volume, R is the ideal gas constant, and T is the temperature on the Kelvin scale. If pressure is given in atm, then R = 0.0821 atmL/molK. The number of moles of gas is related to the molecular mass, M, by the expression:


n = LAB NAME MOLECULAR MASS OF A VOLATILE LIQUID 3 


The mass of the gas is found by first cooling the gas so that it condenses back into a liquid, and then determining the mass of the condensed liquid. The equations above can be combined into one equation that can be solved directly for molecular mass:


M = LAB NAME MOLECULAR MASS OF A VOLATILE LIQUID 3 


In this experiment to determine the molecular mass of a volatile liquid, some of the liquid is placed into a small flask. The flask is closed with aluminum foil that has a small hole in it. The flask is heated in boiling water. The liquid vaporizes, the vapors fill the flask and excess vapor leaves through the hole. Since the flask is open to the air, the pressure of the vapor will be the same as the atmospheric pressure. The gas temperature will be that of the boiling water. The volume of the gas, which is the volume of the flask, can be easily found. The mass of the gas must also be determined. To do this, the flask is quickly cooled so that the vapor condenses back into liquid, and the mass of the flask, aluminum foil and liquid are found using a sensitive balance.


Chemicals


Unknown volatile liquid






Equipment


Flask Beaker, 400 or 600 mL aluminum foil and rubber band

Thermometer Hot plate ring stand

ice water bath analytical balanceLAB NAME MOLECULAR MASS OF A VOLATILE LIQUID 3


(Note: you will be using a hot plate instead of a Bunsen burner due to the volatility of the unknown liquid)


Procedure


1. Prepare a hot water bath by placing a half filled 400 or 600 mL beaker on a hot plate. Be sure the flask fits inside the beaker.


2. Make a small hole in a piece of aluminum foil. Find the mass of a dry flask, aluminum foil and rubber band using the analytical balance.


3. Pour about 0.5 mL (about 10 drops) of the unknown volatile liquid into the flask. Cover the flask with the aluminum foil and secure with the rubber band. Place the flask in the hot water bath. Try to have as much of the flask immersed as possible.


4. Heat the beaker of water to boiling to vaporize the liquid. The expanding vapor will flush the air out of the flask. As the liquid continues to vaporize, the excess vapor will escape out of the hole. The vapor will fill the flask at the boiling water temperature and at the room pressure. Keep the flask in the beaker for at least three minutes after all of the liquid has vaporized. Dry flask with a sponge to be sure all the liquid has vaporized. Measure the temperature of the boiling water.

5. Quickly cool the flask in an ice bath. Dry off the flask and find the mass of the flask, aluminum foil, rubber band, and condensed liquid.


6. Clean the flask and fill it up to the top with water and us a graduate to measure the volume of the flask.


7. Record the barometric pressure.


Data Letter of Unknown___________

Mass of empty flask, aluminum foil and rubber band


Temperature of boiling water bath


Barometric pressure


Mass of flask, aluminum foil, rubber band and condensed gas


Volume of the flask



Calculations


1. Determine the mass of the condensed liquid.


2. Find the mass of the water in the flask. From the mass of the water contained in the flask and its density, calculate the volume of the flask.


3. Convert atmospheric pressure into atmospheres.


4. Use the mass of condensed vapor, pressure, temperature of the boiling water bath and volume of the flask to calculate the molecular mass of the liquid. Calculate the percent error once given the unknown substance.



Discussion


1. Was the vapor really “ideal”? If not, how would this affect the calculated molecular mass? Be specific – for example, too high because…


2. Did all the vapor condense into the liquid? Again, if not, how would this affect the calculated molecular mass?


3. Why is it not necessary to be precise when the liquid is measured out into the flask?


4. It was important that the flask be completely dry before the unknown liquid is added so that water present would not vaporize when the flask was heated. A typical single drop of liquid water has a volume of approximately 0.05 mL. Assuming the density of liquid water is 1.0 g/mL, how many moles of water is in one drop of liquid? What volume would this amount of water occupy when vaporized at 100°C and 1 atm?

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Tags: liquid 3, of liquid, liquid, molecular, volatile