Molu Mas of a Vedaile Liqu PROCEDURE for, mass in grams to the nearest 0.001 9.

Question

Molu Mas of a Vedaile Liqu PROCEDURE for, mass in grams to the nearest 0.001 9. and volume in milliliters to the gh a clean, dry 125-mL Erlenmeyer flask, along with a 5-cm × 5-cm square of aluminum foil. n a to 5 ml. of an unknown liquid from the instructor and place it in the flask. Record the ecord temperature in degrees Celsius to the nearest degree, pressure in torr to the nearest nearest milliliter. dnknown. Crimp the aluminum foil fairly tightly around the neck of the flask and punch a a 600-mL. beaker about 1/3 full of water. Clamp the Erlenmeyer flask to a ring stand and B. Obtain 4 5 number of the unknown. tiny hole with a pin in the center of the foil. C. Fill not so deep that the water reaches the lower edge of the foil. tion in the beaker and continue heating. er, as shown n Figure 17-1. Add more water until the flask is totally immersed, but water to boiling. Return the flask to its former posi- D. Remove the flask from the beaker and heat the E. Measure and record the temperature of the boiling water and the barometric pressure. of the flask). remove the flask from the water and allow it to cool to room temperature. You may find it nec- essutry to remove the flask from the beaker a few times to inspect it for remaining liquid, returning it promptly As soon as all liquid in the flask has vaporized (no condensed beads of liquid on the walls or if liguid is present. Do not heat the flask too long, because this will lead to erroneous results. G. As the flask cools, carefully wipe the outside, making sure that no water droplets have collected under the edge of the aluminum foil. When the flask is at room temperature, weigh the flask, foil, and con- densed liquid on a milligram balance and record the mass on your report sheet. H. To establish the volume of the gas, fill the flask completely with water and then measure its volume using a graduated cylinder. Record your reading to the nearest milliliter Aluminum foil Erlenmeyer flask Sample Figure 17-1. Apparatus for determining the molar mass of a volatile liquid.

Explanation / Answer

1a) A hot substance absorbs moisture more rapidly than a cold substance. Hence, if the hot flask (with its contents) is weighed, the flask will absorb moisture from the atmosphere and register a higher weight. The mass of the gas is obtained by subtracting the mass of the flask (with the condensate and aluminum foil) from the initial mass of the flask (with the liquid). Since the latter mass is high, the calculated mass of the vaporized liquid will be lower and hence, the molar mass of the gas will be calculated low.

b) The condensed water vapors add to the weight of the flask (and the condensate). Consequently, the mass of flask and its contents (after heating) will be high and the calculated mass of the liquid vaporized will be low. Therefore, the calculated molar mass of the volatile liquid will be low.

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