In Another math/chemistry question - Factual Questions - Straight Dope Message Board, the perpetual problem of a % solution being given without specifying how that percentage was derrived has come up. (w/v%, w/w% or v/v%).
I’m a microbiologist by training, and though we were taught w/w%, I’ve never seen it actually used. If the solute is a solid, it’s safe to assume it’s w/v%, and if it is a liquid, v/v% is always specified.
What fields use w/w%?
Looks like it is used in metallurgy, or at least steel.
Environmental regulatory standards are written in terms of mg (pollutant)/kg (solution), which is numerically equivalent to parts-per-million (ppm).*
The unit “mg (solute)/kg (solution)” can easily be converted to a percentage, which turns out to be a w/w%.
Parts-per-million (ppm) is computed as the mass of solute/mass of solution (times 10[sup]6[/sup]). Expressed as a percentage, this would be a w/w%.
Weight/weight %, of course, is computed as mass of solute per mass of solution (times 100).
Also, if I’m not mistaken, aren’t pharmaceutical dosages computed as mg (drug)/kg (body weight)? Expressed as a percentage, this would be a w/w%.
*They are also written in terms of ug/kg and parts-per-billion (ppb).
Of course, you’re right. :smack: I have seen it used before in this way, just not for any actual solution concentrations. I’ve never had to deal with ppm or ppb.
I’ve used w/w% in chemistry when it comes to miscable liquids.
How about construction (i.e. concretes)?
Really? I’ve never seen it. Of course whenever I’ve dealt with % solutions, the two are effectively equivalent because nobody is interested in any type of precision beyond an order of magnitude. When the procedure says, “wash with 10% HCl” we could care less if it’s by volume or weight. Personally, I pour 100-200 ml of Conc. HCl and fill it to 1 L while holding my breath and pointing the bottle away from me because technically, I shouldn’t add water to acid at all.
Actually, usually the procedure just says “wash w/ HCl” and we assume it’s 10%.
Concentrated acids are labeled and sold % w/w. The density of such solutions is much higher than water, making the approximation “100ml = 100g” highly inaccurate.
That’s an interesting point. Ironically, if you see a bottle of 10% HCL on a chemists bench, it is most likely a solution made with 10% by volume Conc. HCL and water. If a more precise solution is desired, it will be labeled in molarity not %.
In (gold and the like) mining, ore sampling is done in ppm of gold/ton of ore, which is effectively a w/w% usage, as robby points out.
You do know there’s a very good reason for that rule, right? Maybe it’s not such a risk for HCl, but please don’t ever try this with concentrated sulfuric acid.
For those following along at home, mixing acid and water produces considerable heat. Adding water to an excess of acid can cause it to get very hot very quickly, possibly splattering acid on you. Adding acid to water does not cause this problem.
Interesting. I never noticed the fine print on those bottles until now. On a microbiologist’s bench, you’re more likely to see acid + base solutions labeled in normality. When I need to replenish my 1N HCl stock, I’ll dilute concenrated HCl (12N or 37%w/w) 1:12.
Yes.
The water content of soils is (Mass Water)/(Mass Solids). That’s required knowledge for soil compaction, among other things.