Examkin

Question

Difficulty: MediumFiltration, Evaporation, and Crystallization

A solid mixture containing insoluble calcium trioxocarbonate(IV) (CaCO3\text{CaCO}_3) and soluble hydrated magnesium tetraoxosulfate(VI) (MgSO4⋅7H2O\text{MgSO}_4 \cdot 7\text{H}_2\text{O}) is thoroughly stirred in distilled water and filtered. To obtain pure hydrated magnesium tetraoxosulfate(VI) crystals from the filtrate without decomposing the salt or losing its water of crystallization, which procedure must be performed?

  1. A
    Evaporating the filtrate completely to dryness over a Bunsen flame
  2. Concentrating the filtrate by gentle heating to saturation, followed by slow coolingAnswer
  3. C
    Fractionally distilling the filtrate to collect the dissolved salt as the first distillate
  4. D
    Filtering the filtrate a second time through a finer filter paper at room temperature

Answer

Concentrating the filtrate by gentle heating to saturation, followed by slow cooling to allow crystal formation.
Concentrating the filtrate until it becomes saturated and then allowing it to cool slowly yields pure crystals of hydrated magnesium tetraoxosulfate(VI) (MgSO4⋅7H2O\text{MgSO}_4 \cdot 7\text{H}_2\text{O}) while preserving its water of crystallization.

Step-by-Step Solution

1
Analyze the components of the filtrate
The filtrate contains dissolved hydrated magnesium tetraoxosulfate(VI) (MgSO4⋅7H2O\text{MgSO}_4 \cdot 7\text{H}_2\text{O}) in water after insoluble CaCO3\text{CaCO}_3 is removed by filtration.
Filtration separates the insoluble solid residue (CaCO3\text{CaCO}_3) from the soluble salt solution.
2
Determine the appropriate recovery method for hydrated salts
Crystallization is required instead of evaporation to dryness.
Evaporating to dryness thermally decomposes hydrated salts or drives off essential water of crystallization, destroying the crystalline structure.
3
Apply the crystallization procedure
Partially evaporate the solvent to prepare a hot saturated solution, then allow it to cool slowly to precipitate pure hydrated crystals.
Solubility of the salt decreases as the saturated solution cools, prompting pure crystals to separate out.

Key Concept

Crystallization vs Evaporation to Dryness for Hydrated Salts
Estimated Time:1m 0s
Rate this question

Topics

Filtration, Evaporation, and Crystallization