Question

I am really struggling with this chemistry problem. I don`t even know how to start..

The problem is about recrystallization:

Two crystalline compounds x and y have the same solubility ratio in water( the solubility is: 1 g/100 mL at 20 degree celsius and 10 g/100 mL at 100 degree celsius) In this example we predict that if the x and y compunds is in the mix, they have the same solubility ratio.

1. From a mix of 10 g x and 1 g y shall clean/pure x be extraxted. How much clean x can we get after a recrystallization?

2. How much clean x can we get after a recrystallization if the mix consist of 10 g x and 8 g y?

Would really appreciate some help:)

Answer 1

Both X and Y have same solubility ratio.

(1).  Here we take a mixture of 10 g X and 1 g Y

Volume of water needed to soluble 10 g X at 100 DegC = (100 mL / 10g) x (10 g) = 100 mL water.

Amount of X remained soluble in 100 mL of water at20 DegC = ( 1g X/ 100 mL)x100mL = 1g X.

Hence amount of X recrystallized = 10g - 1 g = 9 g

Amount of Y mixed = 1 g

Since X and Y have same crystallization ratio,

Amount of Y remained soluble in 100 mL of water at20 DegC = ( 1g Y/ 100 mL)x100mL = 1g Y.

Hence amount of Y recrystallized = 1g Y - 1 g Y = 0 g

Hence there is no Y in the recrystillized X and the X otained is pure which is equals to 9 g of X (answer)

(2). Here we take a mixture of 10 g X and 8 g Y

Volume of water needed to soluble 10 g X at 100 DegC = (100 mL / 10g) x (10 g) = 100 mL water.

Amount of X remained soluble in 100 mL of water at20 DegC = ( 1g X/ 100 mL)x100mL = 1g X.

Hence amount of X recrystallized = 10g - 1 g = 9 g

Amount of Y mixed = 8 g

Since X and Y have same crystallization ratio,

amount of Y remained soluble in 100 mL of water at20 DegC = ( 1g Y/ 100 mL)x100mL = 1g Y.

Hence amount of Y recrystallized = 8 g Y - 1 g Y = 7 g Y

Hence after recrystallization we can get 9 g X and 7 g Y but they are not pure. Hence pure X is 0 g (answer)