| MadSci Network: Chemistry |
Why does a 5% weight-volume solution consist of 5g of NaCl dissolved in 100ml total volume rather than adding 5g NaCl to 100ml?
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For the same reason that a 1 mole per liter solution consists of 1 mole of
material dissolved in 1 liter total volume. It is convenient to control the
amount of material delivered by controlling the volume of solution delivered.
For example, 10 mL of the 5% w/v solution you describe will contain 0.5g of
NaCl.
You can also make w/w solutions and v/v solutions. A 5% w/w solution would have 5g of solute dissolved in 95g of solvent, and a 5% v/v solution would have 5mL of solute dissolved in 95mL of solvent. But the problem with these is that, while they're easy to mix, it's not so easy to predict just how much material one gets with a given volume. The "100-mL" w/w and v/v solutions won't necessarily come out to 100 mL! This has to do with entropy of mixing... for more, try our search engine.
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Try the links in the MadSci Library for more information on Chemistry.