Elking_around
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Concentration of a solution based on molar mass
Hello
I've recently learned that in order to make a 36.5% HCl solution, you would need 36.5g of HCl gas per L, since that's its molar mass. (or 36.45g/L)
I was wondering if that was the same for concentrated solutions in general, for instance, a 1L 50% formic acid solution would contain 22.51g of the
acid. Formic acid being 46.025g/mol.
I know this is a pretty simple question, I'm just not entirely sure as I haven't seen any online examples with this level of obviousness.
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UC235
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None of that is correct. There is no relation between percentage concentrations of a solution and molar mass. HCl is a coincidence that the
percentage and molar mass are similar. First, you need to specify what type of %, w/w%, (weight weight), w/v% (weight volume) or v/v% (volume
volume) all of which are fairly common. Commercial concentrated ~36.5% HCl is w/w% and is about 12M or roughly 44% w/v%, requiring ~440g per liter of
solution.
ABV for alcoholic beverages is a v/v%.
50% formic acid again would depend entirely on which type of % you mean. w/v% would be 500g/liter and about 11M
[Edited on 18-3-2023 by UC235]
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Elking_around
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Oh right, what your saying makes a lot more sense, thank you for the detailed reply. I highly appreciate it!
I do have a follow up question related to your example of the commercially available solution of 12 moles HCl,
How can a HCl solution equal to ~36.5% w/w% but also be 44% w/v% if 1Kg of water is equal to 1L of volume. Shouldn't it also be ~36.5% for w/v% ?
I don't understand how it could be 12 moles for w/v% but then potentially be 10 moles for w/w% ?
[Edited on 19-3-2023 by Elking_around]
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Sulaiman
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The density of about 36.5% w/w HCl is about 1.18
1kg contains 365g HCl (= 365/36.46 = 10 moles)
(1kg = 1/1.18 = 0.85 litre) (10/0.85 = 12 moles/litre [Molar] )
So equivalent to just over 43% w/v
(365g/0.85l = 430 g/l = 0.43 kg/l = 43% w/v)
my 2c on w/w vs w/v
If for your applications measurement of liquids by volume is more suitable than by weight
then use w/v% (kg/l)
else use w/w% (kg/kg)
For general chemistry molarity is common (I guess) due making up solutions in volumetric flasks,
which are calibrated to contain, not to deliver, accurate volumes.
Molality usage requires accurate weights of solvent delivered to make up solutions.
eg If digital scales were previously easily available
then we would probably use molality more than molarity.
PS... In industry it is often easier to weigh liquids than to measure their volume.
and yes, a solution can have a Molality of >100%
[Edited on 19-3-2023 by Sulaiman]
CAUTION : Hobby Chemist, not Professional or even Amateur
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Elking_around
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Thank you Sulaiman, weighing liquids definitely seems simpler but your explanation is more than I could've wished for. Taking density into account
when working with volume is something I should've thought about..
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Texium
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Quote: Originally posted by Sulaiman  | eg If digital scales were previously easily available
then we would probably use molality more than molarity |
I highly doubt that. Maybe on an industrial scale
it’s easier to measure by weight, but in the lab it’s much easier to measure liquids by volume and make up solutions by molarity so that they can
easily be measured by volume. On very small scales, it’s far easier to use a micropipette than it is to weigh out mg amounts, especially of liquids
since your precision is limited by the mass of a drop. On larger scales, it’s easier to measure with a graduated cylinder or a syringe.
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j_sum1
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As a nitpick, I dislike the % w/v notation. I would much prefer to say 430 g/L. Percentage implies like units which is clearly not the case here.
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DraconicAcid
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| Quote: Originally posted by j_sum1 | | As a nitpick, I dislike the % w/v notation. I would much prefer to say 430 g/L. Percentage implies like units which is clearly not the case here.
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Agreed. w/v is used, but I loathe it. If it's a percentage, then your actual units should have been cancelled out.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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