To determine the freezing points of distilled water, 1m glucose, 1m NaCl, 1m CaC

Question

To determine the freezing points of distilled water, 1m glucose, 1m NaCl, 1m CaCl2. Use the freezing point of the pure water as a baseline to determine deltaTf for each of the solutions (glucose,NaCl,CaCl2) On the basis of these values, calculate the freezing point Coefficient (Kf) for water in each case and compare your average Kf value to the accepted value. This confuses me. I would like to know what to do here. additional info. for calculating molality we used 0,05kg water. So glucose was 180g/mol> 9g NaCl 58.5g/mol > 2.9g CaCl2 147g/mol > 7.4g these are the values we used.

Explanation / Answer

deltaTf = Kf*b*i

Kf = cryoscopic contant (The accepted value for this is 1.853 K*kg / mol)

b = molality (mols per kg of solvent)

i = # of particles per molecule of solute.

Now I am assuming that you measured the new freezing point after you added the solutes to each sample.

That means that you know that value for deltaTf in that sample because you measured it. (It is just 0 minus whatever the freezing temperature of the sample was)

So, let's think about the NaCl sample:

9 g NaCl

58.5 g/mol

9 / 58.5 = 0.154 mol

So the molality is 0.154 / 0.05 = 3.08

Now, because NaCl dissolves into Na+ and Cl-, the value for i is 2.

You measured the deltaTf, so you have:

deltaTf(measured) = Kf*(3.08)*(2)

Thus you can solve for Kf:

Kf = deltaTf(measured) / [3.08*2]

This gives you an expiremental value for Kf.

You can repeat these same steps for the other samples to get more expiremental values for Kf. Then you can average these together and compare your expiremental value for Kf with the accepted value.

Good Luck!

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