chemistry pre lab work (need in 5 hours)

PRE-LAB – MOLAR MASS OF ISOPROPYL ALCOHOL USING DUMAS METHOD

The objective of this laboratory experiment is to calculate the molar mass of isopropyl alcohol (rubbing alcohol) by measuring the density of its vapor using the Dumas method. Once the density of the isopropyl alcohol's vapor is known, that value can be used in a formula rearranged from the Ideal Gas Law and isopropyl alcohol's molar mass can be calculated.

BACKGROUND INFORMATION

One of the simplest methods for measuring the density of the vapor of a substance (i.e., its vapor density) is the method devised by Dumas. In the Dumas method, an approximate volume of the substance being studied is put into a tared (previously weighed) flask that is then sealed except for a pinhole opening. The flask is heated to a temperature above the boiling point of the substance in a boiling water bath. The vapor formed from the boiling substance pushes the air and any extra vapor out of the flask through the pinhole opening. At the moment that the substance is completely vaporized, the flask will be filled with essentially pure vapor that is at the atmospheric pressure of the lab and the temperature of the water bath. The heating of the flask is then stopped and the flask quickly cooled to room temperature so that the vapor required to fill the volume of the flask will condense to a liquid. The air that enters through the pinhole during the cooling will prevent the volume of vapor from escaping. The flask is re-weighed so that the mass of vapor required to fill the flask at that specific pressure and temperature can be calculated.

The Ideal Gas Law is PV = nRT. The n stands for the number of moles of the substance and it can be replaced in the equation by its equivalent, g  molar mass. When the formula is solved for molar mass, we find that it contains g  v or mass divided by volume, which is actually its equivalent density. We will then have a formula rearranged from the Ideal Gas Law from which we can calculate the molar mass of a substance based upon its density and atmospheric conditions at the time the density was measured. You will be collecting in this experiment the data necessary to do these calculations to determine the molar mass of isopropyl alcohol.

PROCEDURE

Set up water bath by heating up about 400 mL of tap water in a 600 mL beaker. Obtain a piece of aluminum foil about 1 to 1 square inches and fold it over the top of the clean, dry Florence flask. Weigh the Florence flask with the cap on, record the mass to the nearest 0.0001g. Make a tiny hole (as small as possible) in the center of the foil cap. Obtain about 3 mL of isopropyl alcohol, pour it into the flask and replace the cap securely.

Clamp the flask at the very top and hold the clamp in your hand. When water

starts boiling, suspend the flask in the water bath. The rounded portion of the flask should be completely submerged. Try to keep the foil cap dry, if possible. Keep heating the water bath and watch for the disappearance of all the isopropyl alcohol liquid. Tilt the flask gently to help you see the point at which the liquid disappears. Heat for two minutes longer and simultaneously measure and record the temperature of the water bath.

Do not reduce the temperature of the flask by removing it from the bath or by letting the bath cool during the vaporization of the liquid because air will reenter the flask. When the liquid has been gone 2 minutes, stop heating, and remove the flask from the beaker. DO NOT remove the foil cap. Cool the flask under a stream of water from the tap taking care not to get the aluminum foil wet. Dry the outside of the flask well. Weigh it on the analytical balance and record it as the weight of the flask, cap and condensed liquid. Fill the flask completely full of water and measure and record its volume by pouring the water into a large, graduated cylinder (if you did not do so at the beginning of the experiment.). Next, read the barometer in the room to obtain the barometric pressure and record this value.

From the measured pressure, volume, temperature and weight of isopropyl alcohol when it filled the flask, calculate the moles of isopropyl alcohol. Divide grams by moles to get the molar mass.

Obtain the true value for the molar mass from your instructor. Calculate the percent error from this value.

CALCULATIONS FOR MOLAR MASS

grams g

Molar Mass = =

mole n

PV = nRT

PV

n =

RT