Question

i dont understand why the molecular weight i calculated (42.012 g/mol) for my unknown liquid (acetone) is less than the actual molecular weight (58.08 g/mol). what errors could i have made in my lab that would account for the difference.

Molecular Weight of a Volatile Liquid In this experiment, an amount of liquid more than sufficient to fill the flask when vaporized is placed in a flask of measured volume and mass. The flask is then heated in a boiling water bath so that the liquid vaporizes completely. The vapor drives air out of the flask and fills the flask at barometric pressure and the temperature of the water bath, both of which can be measured. The pressure is low enough and the temperature is high enough for the gas to behave ideally. The flask is weighed after cooling to condense the vapor. You will then calculate the molecular weight using the equation M , where M is the molecular weight (molar mass), m is the mass of the volatile liquid, R is the gas constant in appropriate units, T' is the absolute temperature, Pis the pressure, and V is the volume of the container. -145360824971.45) MATERIALS (1.013) (1325) 125 ml flask volatile organic liquid (methanol, ethanol, acetone...) 600 mL beaker (1000 ml works great) tongs thermometers barometer analytical balance 250 ml graduated cylinder Red food dye 45097.go 01.047 erty 90°C PROCEDURE coot water SOO me or Fill your beaker about two-thirds full of water and begin to heat to boiling. Add several drops of red food dye to slightly color the water in the water bath. Try to keep the temperature just under 100 °C. Mass the empty Erlenmeyer flask with foil cover and rubber band Add 3 mL of an organic liquid to the flask. Place the foil over the mouth of the Erlenmeyer flask, seal with rubber band and use a pinto make a small hole in the top of the cover. Clamp the flask to a ring stand and immerse the flask up to its neck in boiling water. Have the flask tilted at a slight angle so you can see when all the liquid has evaporated. The flask should be at least 0.5 cm from the bottom of the beaker. The liquid will begin to vaporize immediately. Measure the temperature of the water by placing the bulb of the thermometer on a level with the middle of the flask. Record Heat until all liquid evaporates and excess vapor is no longer expelled. You will need to heat for at least 1 minute after you perceive that all the liquid has evaporated. You can try to test by holding a cool watch glass near the pin hole in the foil cover. Remove the flask from hot plate and allow to cool on counter top until it no longer radiates heat. Dry the flask as before and reweigh the flask with the condensed organic liquid. Discard the liquid down the drain. Record the barometric pressure. This equals the total pressure of gasses in the flask throughout the experiment. Also measure room temperature. Calculate the weight of the unknown present in the flask. Find the volume of the flask. Fill the flask with water then dry the outside of the flask. Measure the volume of water by pouring it into a graduated cylinder. Calculate the molecular weight of the unknown. 9. 10. 11. 12. arouz 0 000374 (1.013) (132) 50.08

Answer 1

The less molecular weight is due to the following four reasons.

(i) The mass of acetone may be less than the actual.

Explanation: Molecular weight is directly related to the mass (m).

(ii) The temperature may be less than the actual.

Explanation: Molecular weight is directly related to temperature (T).

(iii) The pressure may be more than the actual.

Explanation: Molecular weight is inversely related to pressure (P).

(iv) The volume may be more than the actual.

Explanation: Molecular weight is inversely related to volume (V).