Project:

Lab Report:

There is a bottle of an organic liquid in a recently vacated laboratory. Unfortunately the label was misplaced and it is only known that it comes from a short list of compounds. You are asked to figure out the identity of this unknown liquid. Since the compounds on the short list all have quite distinctive molecular weights, you are convinced that if you can find out the molecular weight of this unknown liquid all problems will be solved. Fortunately you notice that the liquid is very volatile and vaporizes at a temperature not too high, so you decide it may be easier to convert the liquid to its vapor and then measure the molecular weight of the vapor, using the gas laws. Design an experiment and carry out this task.

1. Calculate the molecular weight of a volatile liquid using the ideal gas law. Background:

Most gases follow the ideal gas law when the temperature is not too low (much higher than their boiling points) and the pressure is not too high (around the atmospheric pressure). This condition ensures that the distances between the gas molecules are much larger than the size of the  gas molecules and that the kinetic energy of the gas molecules is much greater than their binding energy so they are completely free to move about. As a result, the interaction between the molecules of an ideal gas is negligible, and the identity of the gas molecules has no bearing on the overall physical state of the gas, i.e., all ideal gasses behave the same way. For example, 1 mole of any ideal gas at STP (Standard Temperature and Pressure, which is 0°C (273.15 K) and 1 atm) occupies 22.4 L. The ideal gas law

PV  = nRT

where P is the pressure of the gas sample, V is the volume, n is the number of moles of the gas molecules, R is the ideal gas constant, and T is the temperature of the gas sample in Kelvin, summarizes several previously discovered gas laws,

PV  = constant

when T and n areheld constant

(Boyle ‘ s Law)

V  = constant

T

V  = constant

n

when P and n are held constant when T and P are held constant

(Charle‘ s Law)

( Avogadro‘ s Law)

Using the ideal gas law, if any three of the four quantities, P, V, n and T, are determined, the fourth quantity is also determined, and the physical state of the gas sample is completely determined.

Since

n = m , where m is the mass of the gas sample and M is the molecular weight of the gas, the

M

ideal gas equation can also be used to determine the molecular weight of a gas. This method is called

Dumas Method.

Because a volatile liquid can be easily converted to its vapor which will follow all the gas laws, the Dumas Method may be used to determine the molecular weight of a volatile liquid by measuring its vapor.

Analysis of experimental data through calculations. (Report in table format in the results section of the lab report)

Report the experimental condition (volume, temperature, pressure) for each trial conducted.

Report the mass of the condensed vapor in the flask and determine the molecular weight of the unknown vapor for each trial conducted.

Perform error analysis on the obtained data.

Analysis of data and calculations through discussion. (This belongs in the Di scussion section in the lab report)

Refer back to the project, did you answer the question?

How were you able to obtain the molecular weight of the vapor using the deal gas law? Explain the relationship between density and molar mass in the ideal gas equation.

Physical Properties of substances to consider:

General Chemistry Concepts:  Boyle’s law, Charles law, Avogadro’s number, ideal gas law

Techniques:

Laboratory: heating mixtures

Computer:  apparatus setup using chemsketch

Special Apparatus and Chemicals:

Group Experimental Design Pre-lab

In this project you will design an experimental procedure based on Dumas Method to determine the molecular weight (molar mass) of an unknown volatile liquid. This procedure should be written stepwise with the initials of the student who is performing that step in the procedure.  Here is an example:

1. Weigh substance- AW (these are the initials of student performing task)
2. Get the volume of substance – EE

In designing your experiment, please try to answer the following questions:

1. What quantities do you need to measure in order to get the molar mass of the unknown liquid? Hint: derive an equation that relates the molar mass of a gas to its mass, volume, pressure, temperature, etc;
2. How do you plan to measure each of these quantities? The detail of your plan will constitute your experimental design. For example, how do you vaporize the liquid? Do you need to vaporize all the liquid? How do you plan to measure the mass of the vapor? The volume and pressure? The temperature? How many different ways can you have to measure these quantities? How do you ensure the gas sample you are measuring contains only the vapor of the liquid and not the air?
3. What equipment and techniques do you need for each measurement? Try to draw the apparatus and experimental set-up.
4. Can you envision anything that may go wrong in your design or in conducting your experiment? What are the possible sources of error? What are you going to do if that happens, etc? You may wish to come back to these questions and compare your observations with what you have planned after you have finished your experiment and started writing your report, especially the Discussion