Question

What is the approximate temperature of a 0.1 molar solution of benzoic acid in acetone that is heated in a boiling water bath?

Answer 1

Ans. Step 1: Calculate molality of solution

Given, solution = 0.1 M benzoic acid in acetone or 0.1 mol acetone in 1.0 L acetone.

It’s assumed that there is no change in volume after addition of benzoic acid. Also, the solution is initially at room temperature (25.0⁰C).

Moles of benzoic acid in 1.0 L solution = 0.1 mol

Volume of of acetone (solvent) = 1.0 L

Mass of acetone (solvent) = Volume x density

                                                = 1.0 L x (0.78576 kg/ L)

                                                = 0.78576 kg

Now, molality of 0.1 M benzoic acid in acetone = Moles of benzoic acid / Mass of acetone

                                                = 0.1 mol / 0.78576 kg

                                                = 0.127 mol/ kg

                                                = 0.127 m

Step 2: Elevation in boiling point of a solution is given by-

dT_b = i Kb m                        - equation 1

            where, i = Van’t Hoff factor = [i = 1 for non-electrolyte solute].

                        K_b = molal boiling point elevation constant of solvent, acetone = 1.71⁰C/m

                        m = molality of the solution

                        dT_b = Boiling point of solution - Boiling point of pure solvent

                        Boiling point of acetone = 56.05⁰C

Putting the values in equation 1-

            dTb = 1 x (1.71⁰C/m) x 0.127 m = 0.217⁰C

Now,

            Boiling point of solution = Boiling point of pure solvent + dT_b

                                                            = 56.05⁰C + 0.217⁰C

                                                            = 56.72⁰C

Therefore, boiling point of resultant solution = 56.72⁰C

Boiling point is the minimum temperature at which the liquid phase transits into vapor phase. It remains constant till all the liquid is converted into vapor. So, whatever the temperature of water bath, here temperature of solution remains constant at 56.27⁰C until all of it is vaporized.