What is filtration under reduced pressure?

Filtration under reduced pressure is a technique for separating a solid product from liquid impurities in a reaction mixture.

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What are the steps involved in filtering a reaction mixture under reduced pressure?

  1. The reaction mixture is poured into a Büchner funnel with a piece of filter paper inside. The Büchner funnel is fitted to a flask which is connected to a vacuum line.
  2. When suction is applied, the impure liquid passes quickly through the filter paper and into the flask, leaving behind dry crystals of product.

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What is recrystallisation?

Recrystallisation is a technique for purifying an organic solid.

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What are the steps involved in recrystallisation?

  1. Add a minimal amount of hot solvent to the impure solid until it dissolves.
  2. Leave the saturated solution to cool; crystals of the product will form and impurities will remain in solution.
  3. Remove the crystals by filtration under reduced pressure and wash with ice-cold solvent.
  4. Dry the crystals.

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Why is the choice of solvent important in recrystallisation?

The solvent must be one where the solid is very soluble when hot, but nearly insoluble when cold. If the solid isn't soluble enough in the hot solvent, it won't dissolve. If it's too soluble in the cold solvent, it will stay in the solution even after cooling.

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What is the purpose of washing the crystals with ice-cold solvent during recrystallisation?

Washing the crystals with ice-cold solvent helps to remove any remaining impurities from the crystals, ensuring that the final product is as pure as possible.

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What is the purpose of determining the melting point of an organic solid?

To identify whether the organic solid is pure.


A pure solid melts at specific temperature but an impure solid melts over a range of temperatures.

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What is the melting range of a substance?

The melting range is the difference between the temperature at which the sample starts to melt and the temperature at which melting is complete.

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How does the melting point help in determining the purity of a substance?

A pure substance melts at a specific temperature or has a narrow melting range of 1-2°C.

If the melting range is wide or if the melting point is lower than the known melting point of the pure substane, the substance contains impurities.

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What are the steps involved in accurately measuring the melting point of an organic solid using an oil bath?

  1. Pack a small quantity of the solid into a capillary tube.
  2. Continuously heat the tube in an oil bath, monitoring the temperature with a precise thermometer.
  3. Note the temperature range from the onset of melting to complete melting.
  4. Repeat the process to confirm the accuracy of the melting point range.

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