The purpose of this lab is to determine the molecular mass of a volatile liquid.
•Place a cork with a small hole in it inside of a test tube and find the weight of the two together using a sensitive balance. •Pour 0.5 mL of the unknown volatile liquid into the test tube, insert the cork, and place the tube into boiling hot water while keeping the cork above water level. As the gas evaporates, excess gas will be released throughout the whole in the cork. Keep the test tube in the hot water for at least three minutes after all the liquid has vaporized and measure the temperature of the water. •Quickly cool the test tube in an ice bath, dry it off and find the mass of the test tube, cork, and condensed liquid. •Then, fill the test tube to the top with water, insert the cork and find the mass of the test tube, water, and cork. Calculate the mass of the volatile liquid and record the barometric pressure. III. Data
The following data table shows the information collected while determining the molecular mass of a volatile liquid. The reason for collecting the data was to determine the molecular mass of a volatile liquid. The first three rows show mass measurements collected using a sensitive balance in order to figure out the mass of the volatile liquid and mass of the water. The fourth row shows the temperature of the boiling water determined in order to figure out the temperature of the gas formed from the volatile liquid. The temperature was collected using a digital thermometer. The fifth row shows the atmospheric pressure of the room the lab was conducted in. The atmospheric pressure was determined to find the pressure of the gas formed from the volatile liquid.
Mass of test tube and cork7.42 g
Mass of test tube, cork and volatile liquid7.47 g
Mass of test tube, cork and water14.39 g
Temperature of boiling water101.4 C
Barometric Pressure1.0153 atm
In order to determine the molecular mass of the volatile liquid, the following calculations were made.
1.Mass of Condensed Vapor
Mass = (mass of test tube, cork, and condensed liquid) - (mass of test tube and cork) 0.05 g = 7.47 g – 7.42 g
2. Volume of Test Tube
Volume = [(mass of test tube, cork, and water) - (test tube and cork)] x (density of water) x (1L/1000mL) 0.00697 L = [(14.39 g -7.42 g)] x (1 mL/1 g) x (1 L/1000mL)
3. Molecular Mass of Volatile Liquid
M = mRT/PV
217.23 g/mole = [0.05 g (0.08206 L atm/mole K)(101.4 °C + 273.15)]/[1.015 atm x 0.00697 L]
4. Percent Error
Percent Error = (expected - measured)/expected x 100% 577.99 % = | (32.04 g/mol – 217.23 g/mole)|/ 32.04 g/mole x 100%
All of the calcaulated quantaties are summarized in the following table: Calculation Table
Mass of Condensed Vapor 0.05 g
Volume of Test Tube0.00697 L
Molecular Mass of Volatile Liquid217.23 g/mole
Percent Error577.99 %
It is determined that the molecular mass of the unknown volatile liquid is 217.23 g/mole. The unknown volatile liquid was placed into a test tube and immersed into boiling water to evaporate excess vapor. When all the liquid had been evaporated, the test tube was placed into a cooling bath to condense the gas into a measurable form. The known molecular mass of the volatile liquid is 32.04 g/mole. The percent error of this experiment was calculated to be 577.99 %.
VI. Sources of Error
This great difference between the calculated molecular mass and the true molecular mass could have been caused by water vapor getting into the test tube while it was cooling in the ice bath. While the outside of the test tube was dried, water vapor could have gotten inside the test tube through the hole in the cork. This water vapor could have condensed in the test tube and increased the mass of the test tube, cork and volatile liquid measurement. This increased measurement, since it is on top of the equation to find the molecular mass, would have produced a slightly higher molecular mass that the true. Also, the temperature of the gas was assumed to be the temperature of the boiling water. However, if the test tube wasn't heated evenly and part of it wasn't at the same temperature as the boiling water, then the actual temperature of the gas would have been lower than the temperature of the boiling water. Since the temperature is on the top of the equation to find the molecular mass, a falsely high temperature measurement would have resulted in a slightly high molecular mass measurement.
VII. Discussion of Theory
Throughout this experiment various many scientifical theories are applied. Some of these are vaporization, volatility, condensation, molecular mass. The student uses vaporization to vaporize the unknown liquid in order to condense it later and then to find its mass so that it can be plugged into the calculations seen above to obtain the results that he/she obtained. Volatility in the context of this experiment is a measure of the tendency of a substance to vaporize. As the student observed the unknown liquid was actually volatile thus allowing the student to condense it and later obtain its molecular mass. Lastly, through the experimental process the student was able to find the molecular mass through several stoichiometric calculations as shown above. VIII. Discussion Questions
1.How can the ideal gas law be used to determine the molecular mass of a liquid?
The liquid is heated and converted to a gas and the temperature, pressure, and mass are determined. The data is substituted into the Ideal Gas equation.
2.Was the vapor really “ideal?” If not, how would this affect the calculated molecular mass?
The gas is not ideal because the temperature is only slightly above boiling and some of the molecules in the vapor will cling together causing the pressure of the vapor to be less than the ideal pressure. When the data is substituted in to the gas law the molecular mass calculated will be high.
3.Did all of the vapor condense into the liquid? Again, if not, how would this affect the calculated molecular mass?
Not all of the vapor will condense because even at cold temperatures there is still some vapor pressure. This causes the mass to be low and the calculated molecular mass to be low.
4.Why is it not necessary to be precise when the liquid is measured out into the test tube?
This is because you only need to have enough liquid for its vapor to fill the tube. Any excess will vaporize and escape through the hole in the cork,.