We’re being asked to determine the moles of CO(NH2)2 present in 131 g water to get a solution with a freezing point of –4.02 ˚C.
Recall that the freezing point of a solution is lower than that of the pure solvent and the change in freezing point (ΔTf) is given by:
The change in freezing point is also related to the molality of the solution:
i = van’t Hoff factor
m = molality of the solution (in m or mol/kg)
Kf = freezing point depression constant (in ˚C/m)
Recall that the molality of a solution is given by:
For this problem, we need to do the following:
Step 1: Calculate for ΔTf.
Step 2: Determine the molality of the solution.
Step 3: Calculate the mass of CO(NH2)2 present.
How many moles of CO(NH 2)2 are present in 131 g of water, if the freezing point of the solution is −4.02 °C? kf (water) =1.86 C • kg/mol.
1. 0.263 mol
2. None of these
3. 0.280 mol
4. 0.297 mol
5. 0.288 mol
6. 0.272 mol
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