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coolyo90 · Jan 31, 2010, 01:18 PM

A sample of 10.00g of a solid unknown was dissolved in 25.00g of water.
Pure water freezes at 0.0 degrees C and the solution freezes at -5.58oC

How can I find the molal concentration of the solution? And the molar mass of the solid?

Unknown008 · Feb 1, 2010, 11:21 AM

The molar concentration of a solution is the amount of moles per unit volume.

You therefore needs to find the molar mass of the solid to find its concentration in the amount of water you used.

Check your books, you surely have examples there. If not, post back.

jaz12345 · Jul 6, 2010, 12:31 PM

Is there anyway you can explain how to do this problem step by step

Unknown008 · Jul 8, 2010, 02:11 AM

Find, find the change in temperature.

$\Delta T = T_{solution} - T_{pure\ solvent}$

Find the molal concentration from the change in freezing point and the freezing point depression constant.

$m = \frac{\Delta T}{-K_f}$

From there, you can find the number of moles of solute using:

$Moles\ of\ solute = Molality \times Mass\ of\ solvent$

Now that you know how many moles there are, use proportions to find the molar mass of the solute.

The value of K_f for water is 1.853 (Kkg)/mol

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