In this experiment, the change in the freezing point of cyclohexane is measured as impurities are added, in the first case naphthalene and then an unknown chemical. The molecular weight of the unknown is determined from the measured lowering of the freezing point.
**Before coming to the lab**, write a program with Matlab that can convert measured resistance of a thermistor (the temperature in the sample is measured with a thermistor instead of a thermometer) to temperature and makes a plot of the temperature as a function of time given readings of the resistance vs. time. Bring a portable computer to the lab if you have one. It is important to make the plot of temperature vs. time during the experiment to see when enough data has been collected to determine accurately the freezing temperature. A thermistor is made of a metal oxide with rapidly varying electric resistance as temperature is changed. The resistance is measured and needs to be converted to temperature. The relationship between the temperature and the resistance (log(R) stands for base 10 logarithm of the resistance in units of Mega Ohm) is
T = A + B log(R) + C log(R)^2 + D log(R)^3
where A = 22.4863, B = -43.008, C = 1.82007, and D = 2.11612 and x^2 means x squared.
Otherwise, the procedure is basically the same as described by NGS. First of all, read the description of the experiment in NGS, and then read the additional comments below.
The sample is placed in a low temperature bath consisting of a mixture of cracked ice and dilute ethanol (a 1:10 ethanol/water mixture). Fill the container with ice and do not use more liquid than is needed to just cover the ice. There should be room for at least a cm rise in the level of the fluid as the instrument is immersed in the bath. It is important to monitor the condition of the bath during the experiment. Also, it is important to mix the ice and ethanol every once in a while. To do this, lift the metal rod up and down every 5 minutes or so. Use a regular thermometer to measure the temperature of the bath once the ice/ethanol solution have reached equilibrium.
It is even more important to stir the cyclohexane constantly with the small rod. It is also good to scrape the side of the glass tube with the small rod while stirring to clean off any crystals that may form at the glas/liquid interface. Otherwise the cooling can become so slow that the measurements take excessively long to perform. Make a measurement of the temperature every 30 seconds or so.
The amount of cyclohexane needs to be large enough so that 2-3 cm of the lower part of the thermistor is immersed. But, make sure the level of the cyclohexane does not reach above the level of the bath (that would make it difficult to keep unifrom temperature). The best thing is to weigh the cyclohexane directly into the glass tube that represents the inner part of the instrument. It can be standing in a beaker on the balance. The density of cyclohexane is 0.8 g/ml. Use the coarse Mettler balance (with precision of 0.01 g). Make sure the inner part of the instrument is clean and dry. Clean only with acetone, not with water. You can shorten the time it takes to make the measurments by cooling the inner part of the instrument (without the insulation and outer glass tube) in the bath until the cyclohexane freezes. Then, heat it in your palm just above the melting temperature, assemble the instrument, immerse in the bath and start measuring. If the fully assembled instrument is immersed in the bath starting at room temperature, the cooling down to the freezing point takes a very long time.
An appropriate amount of naphthalene is about 0.2 g per 25 g of cyclohexane. The same is true of the unknown chemical. Measure the weight of the impurity on the more precise balance, the Mettler AE200. Use small glasses with plastic covers to weigh the sample. First, the glass is weighed roughly (to have an idea of how much the sample weighs after it has been added), then the glass + sample are weighed carefully and, finally, the glass is weighed carefully after the sample has been added to the cyclohexane. This way any sample remaining on the walls of the glass does not get counted in.
The samples take rather long time to dissolve in the cyclohexane. A magnetic stirrer can be added to help dissolve the naphthalene.
It is important to monitor the shape of the cooling curve during the measurements. Type in the temperature vs. time data into a computer during the measurement and plot the curve using the Matlab. This is important to see when the freezing occurs and to determine how long to continue the measurements.
The following minimal number of measurements need to be made:
Why is ethanol added to the bath? Report the temperature of the bath and explain why it deviates from the temperature of a pure water/ice bath (which is at zero degrees Centigrade).