ScopeGuy2 - 1-1-2016 at 18:04
I don't understand how they get a 10 percent solution from this can some one explain it to me?
10% potassium hydroxide in double distilled water
PREPAIR :
Potassium hydroxide pellets: 11.7 g
Bidistilled: → 100 ml
Transfer 11.7 g of potassium hydroxide in a volumetric flask of 100 ml in- resistant glass
temperature lations (the dissolution of potash releases heat) and make up to the
mark with water (it must therefore be about 90 ml). Then stir thoroughly but gently,
to avoid dissolving too much carbon dioxide
I thought for a 10 percent solution it would be
A 10% aqueous solution of KOH in water is made by taking 10 grams of KOH and dissolving it in water up to 100 ml
I don't understand where 11.7 grams came from
DraconicAcid - 1-1-2016 at 18:34
It's ten percent by mass. Ten g KOH plus 90 g water. The method described will give a 10.5% solution, which is probably close enough.
careysub - 1-1-2016 at 18:52
The notion of an X% solution is not unambiguous in use (although molarity is unambiguous).
It is common to see products for example with a statement like "X% w/v" which means weight/volume - a 10% w/v solution would be prepared by adding 10
g to less than 100 mL of solute, then after dissolution adding solute to make 100 mL. Mixtures of liquids might be marked "x% v/v", similarly "X% w/w"
mixtures may be encountered.
Deathunter88 - 1-1-2016 at 20:17
I think it may be due to the fact that KOH is hygroscopic and thus absorbs water from the air. Your procedure may account for the 10% water in most
KOH, coupled with the fact that even reagent grade KOH is only about 90-95% pure. Just an idea...
BromicAcid - 1-1-2016 at 22:07
Likely as Deathunter88 said, most KOH out there is tech grade, only about 85% pure (see attached capture of purities of listings at Sigma Aldrich).
To get that last several percent of water out of there can take a monumental effort.
There are a couple things that this could be though. As careysub said, the solution is 10% but it could be mass vs mass or mass vs volume or volume
vs volume (unlikely) it could even by 10 mol %. Really a % solution can be very confusing. But let's look at what we know.
If it is mass vs volume then you would expect 10 grams of KOH in 100 mL of solution. If that holds then they would have to be assuming that your KOH
is 85% pure which is exactly where tech grade KOH falls in terms of purity.
10 g KOH expected / 11.7 grams of KOH needed = 85.4% purity assumed
I was going to work out the other situations but since this fits I will stop here and go with them assuming you are working with standard grade KOH
and that your solution is mass vs. volume (m/v)%.
aga - 1-1-2016 at 23:56
Your solid chemical may already contain crystal water so you'd have to account for that as well if you wanted to be really really accurate.
e.g. good old copper sulphate CuSO4 has a molecular weight of 159.6g/mol.
Those nice blue crystals are actually CuSO4.5 H2O, giving a total weight of 249.7g/mol, with crystal water making up the extra
90.1g/mol.
ScopeGuy2 - 2-1-2016 at 00:45
Thank you everyone. I spent an hour trying to figure it out and failed. I never thought to think about the KOH being about 85 percent pure. That
has to be it. It all makes perfect sense now. Thank you.
morganbw - 2-1-2016 at 04:44
If you have pure KOH and a scale, add 10 grams KOH to a weighed beaker, then bring the tared weight up to 100 grams with water.
Or 10 grams KOH plus 90 ml water = very close/
You will not have 100 ml due to the fact that the density will increase. You wind up with less than 100 ml.
I think you will wind up a few ml short of reaching 100 ml, however you will have a 10% w/w solution.
[Edited on 2-1-2016 by morganbw]
[Edited on 2-1-2016 by morganbw]
careysub - 2-1-2016 at 08:21
Many inorganic and organic substances are hygroscopic.
Any time you deal with any preparatory procedure it is essential to examine the possible hydration states of the chemicals involved, even if only to
rule out the likelihood of them containing water. This should be routine.
DraconicAcid - 2-1-2016 at 09:03
I have never understood why any chemist would do a weight per volume percent. They won't add up to 100%, and the units don't cancel.