Question

What is the melting point of a solution in which 2.5 grams of sodium chloride is added to 230 mL of water?

Answer 1

To get the melting point of a solution so, we will use this formula:

ΔT = - mKf

when:

m is molality of the solution

Kf is cryoscopic constant  of water  = 1.86 C/m

and ΔT is the change in melting point (T2 - 0 °C)

so, now we need to calculate the molality to substitute:

when the molality = moles NaCl / Kg of water

 and when moles NaCl  = mass / molar mass

                                       = 2.5 g / 58 g/mol

                                       = 0.043 mol
∴ Kg water = volume *density /1000

                   = 230 mL * 1 g/mL / 1000

                  = 0.23 Kg
∴ molality = 0.043 / 0.23 =0.187 M

by substitution:

T2-0°C = - 0.187 * 1.86

∴T2 = - 0.348 °C