Question

1. By what amount is the boiling point elevated when 153 g propanetriol (C3H8O3) is added to 1.5 L acetic acid? (daa = 1.32 g/mL) (Acetic acid: Kb = 3.07 degrees C/m and BP = 118.1 degrees C)

2. How would your response to question 1 change if acetic acid were replaced with ethyl acetate? Explain why this is so. (Ethyl acetate: Kb = 2.583 degrees C/m and BP = 77 degrees C)

3. How would your response to question 1 change if the propanetriol were replaced with an equal molar amount of sucrose (table sugar)? Explain why this is so.

Answer 1

1. By what amount is the boiling point elevated when 153 g propanetriol (C3H8O3) is added to 1.5 L acetic acid? (daa = 1.32 g/mL) (Acetic acid: Kb = 3.07 degrees C/m and BP = 118.1 degrees C)

Solution :-

Lets first calculate the mass of the acetic acid using the volume and density

Volume = 1.5 L * 1000 ml = 1500 ml

Mass = volume * density

         = 1500 ml * 1.32 g per ml

         = 1980 g

Convert gram to kg

1980 g * 1 kg / 1000 g = 1.98 kg

Now lets calculate the moles of the propanetriol (C3H8O3)

Moles = mass / molar mass

Moles of propanetriol (C3H8O3) = 153 g / 92.0932 g per mol = 1.661 mol

Now lets calculate the molality of the solution

Molality = moles / kg solvent

              = 1.661 mol / 1.98 kg

              = 0.839 m

Now using the molality and the molal boiling point constant lets calculate the change in the boiling point.

∆Tb = Kb * m

Where ∆Tb = change in the boiling point , m= molality and Kb =molar boiling point constant

Lets put the values in the formula

∆Tb = Kb * m

         = 3.07 degrees C per m * 0.839 m

        = 2.58 ^oC

Therefore the boiling point of the solution will be elevated by 2.58 ^oC

2. How would your response to question 1 change if acetic acid were replaced with ethyl acetate? Explain why this is so. (Ethyl acetate: Kb = 2.583 degrees C/m and BP = 77 degrees C)

Solution :- If we replace the acetic acid with the ethyl acetate then the change in the boiling point will be lower because the Kb value of the ethyl acetate is lower than the Kb value of the acetic acid. Therefore the value of the elevation in the boiling point would be lower than 2.58 ^oC

3. How would your response to question 1 change if the propanetriol were replaced with an equal molar amount of sucrose (table sugar)? Explain why this is so.

Solution :- If the propanetriol is replaced with the sucrose then the elevation in the boiling point will be lower because the molar mass of the sucrose is higher than the molar mass of propanetriol . therefore the sucrose solution will have the lower molal concentration than the propantriol solution and since the molal concentration is directly proportional to the elevation in boiling point. The sucrose solution with lower molal concentration will have lower elevation in the boiling point of the solution.

Therefore the elevation in the boiling point will be lower than 2.58 ^oC