Question

You work in a small-scale manufacturing facility that uses phthalic acid as the starting material for a multi-step synthesis. Your company buys ~98% pure phthalic acid from a major manufacturer and purifies it in-house prior to its use in the multi-step synthesis. You are given the job to purify 50 g of the impure phthalic acid by recrystallization from water. Approximately how much water will you need to use to prepare the hot saturated solution? Upon cooling in an ice-bath, and subsequent filtration, how much pure phthalic acid would you expect to recover?

Solubility of phthalic acid

0.54 g/100 g of water @ 14 °C

18 g /100 g of water @ 100 °C

Answer 1

Solubility of phthalic acid

0.54 g/100 g of water @ 14 °C

18 g /100 g of water @ 100 °C

to prepare the hot saturated solution we need 100 g water for 18 g phthalic acid thus for 50 g phthalic acid we will require = 100 g water / 18 g phthalic acid * 50g phthalic acid

= 277.78 g water

Here 277.78 g water contains 50 g phthalic acid

Now determine the amount of phthalic acid in 277.78 g at @ 14 °C as follows:

0.54 g/100 g of water @ 14 °C *277.78 g water

= 1.500 g phthalic acid   in water

The pure phthalic acid which would we expect to recover = 50.0 g – 1.50 g

= 48.5 g phthalic acid