Question

A sample of benzoic acid contaminated with NaCl and sand is to be recrystallized:

Benzoid Acid: Solubility in water at 100 degrees Celsius in g/100 mL = 6.8, at 0 degrees Celsius in g/100 mL = 0.2. NaCl: Solubility in water at 100 degrees Celsius in g/100 mL = 39.1, at 0 degrees Celsius in g/100 mL = 35.7.
Sand:Solubility in water at 100 degrees Celsius in g/100 mL = Insoluble, at 0 degrees Celsius in g/100 mL = insoluble

The sample initially contained 4.0 g of benzoic acid, 2.0 g of NaCl and 1.0 g of sand and was treated with 50 mL of boiling water, filtered by gravity and then cooled in ice to obtain a precipitate which was isolated by suction filtration. How much benzoic acid should be formed?

Answer 1

At 100 C what dissolves?
6.8 x (50 mL/100 mL) = 3.4 g goes into solution for benzoic acid.

NaCl is
39.1 x (50 mL/100 mL) = 19.5 g so the entire 2 g NaCl goes into solution.

Sand of course stays out.

Filtered. You now have 3.4 g benzoic acid and 2.0 g NaCl in the filtrate.

What comes out at 0C?
0.2g benzoic acid x (50/100) = 0.1 g is in solution. The remainder ppts which is 3.4g - 0.1 g = ?

NaCl?
That's 35.7g x (50/100) = about 18 g in soln so no NaCl ppts.