pressure
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One of the most important aspects of solvent selection for distillation is ensuring that the solute with the lowest vapor pressure will be at the bottom, or stationary, phase. In order to determine which solute would have the lowest vapor pressure, you need to know what each one’s vapor pressure is relative to water. Here are some examples: CHCl3 (methyl chloride) has a lower vapor pressure than H2O because it has more hydrogen atoms and less oxygen atoms in its molecule than water does. HCl (hydrochloric acid) vapor pressure is lower than H20 because it has fewer hydrogen atoms in its molecules. NH_ _ N_ _ HO (ammonium hydroxide) has a higher vapor pressure than water since ammonia is more volatile and less dense than water, while the salt dissolved in this solution will have an even higher vaporization point. CHCOOH (acetic acid) can be expected to have a far greater vaporization point; as the molecule contains both oxygen and carbon – which are two of the most volatilizable elements on earth. This makes CHCOOH especially suitable for use in vacuum distillation applications, where low boiling solvents are required to

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