| Introduction | | | | Many people, including me way back when, do |
| Freezing point depression is a colligative property | | | | freezing point the hard way. By this I mean that we |
| meaning that the amount of change in the freezing | | | | fabricated our own apparatus with test tubes, glass |
| point depends only on the number of particles | | | | thermometers, a piece of wire, and an acetone plus |
| dissolved in the solution. The freezing point depression | | | | dry ice bath. We placed our sample into the tube, |
| does not care what the particles are; it is only | | | | immersed the thermometer with the wire looped |
| concerned with how many are there. Because of this, | | | | around it then sticking out as a handle, and then froze |
| the measurement of the change in freezing point | | | | the solution in the dry ice acetone mixture. Once |
| between a pure solvent, and a solvent with a known | | | | frozen, we removed our apparatus and painstakenly |
| amount of substance in it gives an accurate, and | | | | plotted the rise in temperature moving the wire up and |
| absolute, value of the number of molecules dissolved in | | | | down to keep the temperature constant. With this plot |
| the solution. Knowing the mass of the solute and the | | | | we estimated freezing point the best we could, |
| freezing point of the pure solvent provides enough | | | | cleaned our device, and went on to the next sample. |
| information to accurately calculate the Molecules per | | | | The process was slow, kind of fun, but fairly inacurate |
| unit mass of the solute. | | | | and not very cost effective for a commercial lab. |
| Discussion | | | | Enter the cryoscope; an instrument that does all this, |
| Each solvent has its own freezing point and with it, its | | | | automatically and on small samples at that. With |
| own molal freezing - point constant. The molal freezing | | | | microliter size samples up to a milliliter or so, a |
| point constant is the depression in degrees Centigrade | | | | cryoscope accurately measures freezing point |
| of the solvent when 1 mole of solute is dissolved in 1 | | | | depression to thousandths of a degree centigrade on |
| kilogram of the solvent (Chemical Technician's Ready | | | | a calibrated, basically push button instrument. A single |
| Reference Handbook, pg 435). Freezing point | | | | value is returned from which molecular weight is easily |
| constants are published values available for most pure | | | | calculated. Hazardous solvents, such as benzene, can |
| solvents. | | | | readily be used with little risk of analyst exposure. |