Question

A recrystallization of 3.00 g of benzoic acid is to be performed using 80 ml of water. The solubility

      of benzoic acid in water is 0.680 g/10.0 ml at 100^oC and 0.0290 g/10.0 ml at room temperature.

      3a) What is the minimum volume of boiling water needed to dissolve the 3.00 g of benzoic acid?

      3b) How much of the benzoic acid (in grams) will theoretically recrystallize after heating to boiling

          and then cooling to room temperature? What is the percent recovery for Crop 1?

      3c) After collection of the first crop the filtrate is evaporated until only 20.0 ml of solution is left.

        Upon cooling to room temperature, how much benzoic acid (in grams) will theoretically

          be recovered as Crop 2? What is the percent recovery for Crop 2?

      3d) Calculate the total %recovery obtained from the two crops.

Answer 1

Answer.

                (3a). The solubility of benzoic acid in boiling water = 0.0680 g/ml

                                So, for 3.00 g the required amount of boiling water = 3.00 g/0.068 (g/ml) = 44.12 ml

                (3b). The solubility of benzoic acid in water at room temperature = 0.0029 g/ml

So, Amount of benoic acid dissolved in 80 ml water at room temperature = 80*0.0029 g = 0.232 g

So, the amount recrystallied = (3.00 - 0.232) g = 2.768 g

Percentage recovery = 100*2.768/3.00 % =92.27%

(3c). For the second crop, Total dissolved material in 80 ml = 0.232 g, now the volume reduced to 20 ml and temperature brought down to room temperature.

Dissolved benzoic acid = 0.0029*20 g = 0.058 g

Re-crystallized after 2^nd crop = (0.232 - 0.058) g = 0.174 g

So, percentage recovery = 100*0.174/3.00 % = 5.80 %

(3d) Total recovery = 2.768+0.174 g = 2.942 g

Percentage yield = 100*2.942/3.00 =98.06%