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Question How can one predict the volume of a solution, given the quantities of solvent and solute? Answer The answer to this question may be more difficult than you were expecting. As a general rule of thumb when planning reactions I always plan for 1 ml of volume increase for each gram of solid solute. Sometimes it's more, sometimes it's less, but that's a decent approximation. However, to get into the reality of the situation, you have to understand partial molar volumes. The calculations can be somewhat daunting. Unfortunately when I finished college my physical chemistry texts were the first and only thing to go so I cannot help with literature references, just a few hyperlinks which is something I hate to do. What I do remember though is that in my physical chemistry classes the introductory exercise involved mixing ethanol and water. If someone asked you exactly how much 100% ethanol you had to mix with water to get 200 ml of a 50% (by volume) solution, the intuitive answer would be 100 ml of water and 100 ml of ethanol. Problem is that if you mix 100 ml of ethanol with 100 ml of water you get less than 200 ml of solution. Water molecules are polar and undergo van der Waals bonding, they orientate themselves at given angles in solution in a framework. The ethanol molecules get in there and find their ways in-between the water molecules and stop them from bonding in the way they want to bond. Think of a building, now think of that same building as a pile of rubble. It takes of less space that way because the structure of the building is gone. Some substances dissolved in water increase the order, the water molecules are highly organized around cations and the like, this causes a greater than expected increase in the volume. Other things like ethanol decrease the order and subsequently cause a contraction in the total volume. This extends to all solvents, both polar and non-polar. Aside from very similar solute/solvent mixtures, volumes are not exactly additive. There is no real quick and easy way to determine these things without specific calculations dependant on the identities of the solvent and solute being examined. Like I said, it's harder than one would expect. Regardless, here are some resources, none of which but the last seemed that helpful: Mixtures; Partial Molar Quantities; Ideal Solutions MOLAR VOLUMES IN AQUEOUS SOLUTIONS (10/21/03) |
