freezing-point depression凝固点降低.docx

Freezing-point depression and determinationof the molecular weight of the solute:Formula:MB=kf*1000*wB/?Tf*wAkf=1.855 K*kg*mol-1 ; wB is the mass of the solute; wA is the mass of the solvent; ?Tfis the value of the freezing-point depression.When you cool a solvent, the temperature of this solvent should decrease equally until the crystal formed. When the crystal formed, it will give off the crystallization heat to cancel out the heat loss of the system, and at this point, the temperature of this system will not change until all the liquid solidified, then the temperature goes down equally. See the line (a)In fact, the vapor pressure of the just formed crystal is bigger than the vapor pressure of the liquid at the same temperature, so we will always see this over-cooling phenomenon. See line (b)And if super over-cooling, see line (c)If we cool down a solution, because the formation of the solvent crystal, the concentration of this solution will increase, so the freezing-point of this system is gradually decrease along with the formation of the crystals. And the cooling curve should look like the line (d)And of course, the over-cooling exists, see line (e)As a matter of fact, we get the line (f)Procedure:Part I1/ Add ice and salt and water into that 2000 mL beaker. Keep the temperature of the ice/water mixture at about -3oC. (too high, the cooling process will be very slow; too low, the solution will have this super over-cooling, and the MW you get is smaller than the actual one)2/ Add about 25 mL pure water in the sample tube, immerse it into the ice-salt bath directly to cool down quickly. (Take it out when crystal formed, wait for the crystal just melt,wipe the outer side with paper and then put the sample tube in the air-bath tube). Stirring gently, write down the temperature(when should you start? one at every 40s, when you see the temperature suddenly goes up, you could write it down every 20s), draw the cooling curve. Take the sample tube out, use your hand