Chemistry Practice Problems Freezing Point Depression Practice Problems Solution: How many moles of CO(NH 2)2 are present in 131 g o...

# Solution: 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 mol2. None of these3. 0.280 mol4. 0.297 mol5. 0.288 mol6. 0.272 mol

###### Problem

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

###### Solution

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:

$\overline{){{\mathbf{\Delta T}}}_{{\mathbf{f}}}{\mathbf{=}}{{\mathbf{imK}}}_{{\mathbf{f}}}}$

where:

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.

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