concentrations

[hairyjoe]

i feel really stupid posting this, but hey, im new to the board. When someone says "you need to distill to 98% conc" what does that mean? is that 98% of the chemical by volume, mass moles, molarity? please help me understand this

[wantsomfet]

I suggets you learn the basics before you try to do anything.
A 98% HNO3 concentration would be 98% HNO3 and 2% H2O.

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[hairyjoe]

yes i know 98% HNO3 would be 2% H2O, but 2% by what? moles, volume, grams? please help me out here

[CragHack]

it doesn't matter what measurement you are using. It could be 98 moles of HNO3 and 2 moles of H2O. that would work. it could be 98 grams and 2 grams. the proportions have to be 98:2 if you want a 98% concentration.

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[a_bab]

Usualy, a 98% solution of HNO3 means that if you have, let's say, 500 grams of this solution, it will contain 98*500/100=490 grams of PURE nitric acid and 10 grams of water. So, the concentrations are given in weight, unless there are others specificated (molar, molal, volumetric, etc.) The biggest mistake is to consider that 98% means 980 ml of nitric acid (or other) and 10 ml water/litre of solution...


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[Lagen]

CragHack: 98:2 weight ratio and 98:2 molar ratio are two different things. They're the same if, and only if, the molar weights of the two compounds are the same. Hairyjoe: always assume all proportions and concentrations to be on WEIGHT basis, unless specified otherwise. (But: In this very field - particularly in various pyrotechnic mixes - you must watch out for people giving ratios by volume, without notice... Better ask them, before mixing any powders...)

[hairyjoe]

THANK YOU Lagen and a-bab, i was starting to rethink my initial assumption that people on this message board had brains. You two have upheld my assumption.

[CragHack]

my bad lagen, i guess i didn't consider volume when trying to simplify things. 98 pounds of one thing and 2 pounds of another could be 98% and 2% under the scope of weight but not necesarily volume. thanks for the correction.

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"If you must, do it with intelligent people, at least they know how to talk to the cops."