[mmman]

When an experimental procedure states add 5% sodium bicarbonate, or 10% NaOH, or 10% HCl, etc. what is the meaning of that? Do they usually mean mass/vol, moles/moles, mass/mass or what? Why isn't there standard procedure to state what the ratio is? It seems like it would make life so much easier.

[Borek]

w/w or w/v. Typically it doesn't matter exactly what you use as long as your are reasonably close.

Personally I would go for w/w, assuming 1 mL = 1 g. Even if it is not what the original procedure calls for (say, when they say 10% NaOH they want 10g in 100 mL, not 10g in 90 mL) the difference is typically negligible.

[Arkcon]

I always go for w/w, unless w/v is specified.  If it doesn't work, then they should have specified w/v.  That's my argument, anyway.

[Babcock_Hall]

If the solute is a solid I usually assume w/v where v is volume of solution.  But this issue is a pet peeve of mine.  I once wrote up a document for people I work with pointing out that different sub disciplines sometimes have different understandings about what is meant.

[Dan]

I always assume w/w unless otherwise specified. Any commercial solution you buy listed as a % concentration is w/w (unless otherwise specified). % concentration should be unitless.

[mmman]

Ok thank for the replies ya'll. I assumed moles/moles in a synthesis of prilocaine I was doing and it failed miserably. Now I know probably why.