Question

2. Assume that a sample of 10.00 g of a solid unknown is dissolved in 25.0 g of water. Assuming that pure water freezes at 0.0 oC and the solution freezes at -5.58 oC, what is the molal concentration of the solution

Answer 1

Answer:

m = 3 moles/kg

Explanation:

This is a problem of freezing point depression, and the formula or expression to use is the following:

ΔT = i*Kf¨*m (1)

Where:

ΔT: Change of temperature of the solution

i: Van't Hoff factor

m: molality of solution

Kf: molal freezing point depression of water (Kf = 1.86 °C kg/mol)

Now, the value of i is the number of moles of particles obtained when 1 mol of a solute dissolves. In this case, we do not know what kind of solution is, so, we can assume this is a non electrolyte solute, and the value of i = 1.

Let's calculate the value m, which is the molality solving for (1):

m =  ΔT/Kf (2)

Finally, let's calculate ΔT:

ΔT = T2 - T1

ΔT = 0 - (-5.58)

ΔT = 5.58 °C

Now, let's replace in (2):

m = 5.58/1.86

m = 3 moles/kg

This is the molality of solution.

The other data of mass, can be used to calculate the molecular mass of this unknown solid, but it's not asked in the question.