What is the minimum volume of boiling water needed to dissolve 0.200g of sodium

Question

What is the minimum volume of boiling water needed to dissolve 0.200g of sodium benzoate: The solubility of sodium beiKoate in boiling water is 1 gram per 1.4 mL. About how much sodium benzoate will remain dissolved when the water is cooled to 25°C ? The solubility of sodium benzoate at 25°C is 1 gram per 1.8 mL. Calculate the maximum mass of solid sodium benzoate that can berecovered when the cooled solution is filtered. In some cases, mired solvents are used for recrystallization. What would be the purpose of using a mixed solvent? Explain why the melting point of pre phrhalic acid is higher than that of pure naphthalene, when the two compounds have nearly the same molecular weight.

Explanation / Answer

At boiling point:

Volume of water required to dissolve 1 g of sodium benzoate = 1.4 mL

Volume of water required to dissolve 0.200 g of sodium benzoate = 0.200/1 x 1.4

= 0.28 mL


At 25 deg C:

Mass of sodium benzoate dissolved in 1.8 mL of water = 1 g

Mass of sodium benzoate dissolved in 0.28 mL of water = 0.28/1.8 x 1

= 0.156 g


Mass of sodium benzoate recovered = 0.200 - 0.156

= 0.044 g


7. Mixed solvents can be used to increase the solubility of impurities while decreasing the solubility of the compound of interest

=> decreases contamination and increases the recovery yield of the compound of interest

=> increases the efficiency of the recrystallization


8. a. Phthalic acid is a polar compound with O-H bonds while naphthalene is a non-polar compound

=> intermolecular forces in phthalic acid (hydrogen bonding, dipole-dipole interactions and dispersion forces) are stronger that those in naphthalene (dispersion forces only)

=> more energy required to break intermolecular bonds in phthalic acid for melting

=> phthalic acid has higher melting point

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