Re: Q about "concentrated acid" and molarity

From: Wilco Oelen (photo_at_woelen.nl)
Date: 02/09/05 

Date: 8 Feb 2005 23:50:02 -0800

catwoma@hotmail.com wrote:
> Hello,
>
> It's been 25 years since I took my last chem class, so please bear
with
> me. I need to make some 1 molar solutions of acids, and the
following
> 3 issues have me stumped:
>
> Question 1: Is "concentrated HCL" the same thing as pure HCL?
No, this is not the same. Concentrated HCl is water, with HCl dissolved
in it. The most concentrated acid you can have at room temperature
contains 37 - 38% of HCl by weight. This heavily fumes. What can be
purchased commercially as muriatic acid has a HCl percentage of 30 -
32%.
Concentrated H2SO4 is almost 100% H2SO4. The usual stuff, which can be
purchased commercially has an acid content of 95 - 98% by weight.

> If
> there is water in "concentrated HCL", then how the heck can one get
the
> molarity correct??? If "concentrated HCL" or "concentrated H2SO4"
has
> water in it, then where can I get info on the amount of water already
> in the acid?
If you want to know the correct molarity, then you need the density of
the acid of the concentration you have. I don't have these numbers at
hand, but with some effort, I'm sure, you can find them on Internet. If
I remember correctly, conc. HCl will be something like 1.1 - 1.2 g/l
and conc H2SO4 will be something like 1.8 g/l, but lookup yourself for
more accurate numbers.
Once, the density is known, and the concentration is known, then you
can compute the amount in grams/liter and from this you can compute the
molarity.

>
>
> Question 2: Is "fuming nitric acid" the same thing as "concentrated
> nitric acid"?
Depends on what is meant with concentrated nitric acid. Frequently, the
name "concentrated nitric acid" is used for a liquid, containing around
68% of HNO3 by weight, the azeotropic mixture of H2O and HNO3. This is
a clear, almost colorless (or very pale yellow) liquid.
Fuming nitric acid has a higher HNO3-content, but usually also contains
dissolved NO2/N2O4, which make the liquid fume, giving off red/brown
fumes. There also seems to exist white fuming nitric acid, which is
almost 100% HNO3 without the NO2/N2O4 dissolved in it, but I've never
come across that and I'm not sure, how stable that is.

>
>
> Question 3: When one makes a 1-molar solution of, say, sodium
> chloride, does one:
>
> a) Add one mole of NaCl to one liter of water? OR, does one....
>
> b) Add one mole of NaCl to the partially-full flask, and then TOP IT
> OFF to one liter?
You have to do (b). Add one mole to let's say 800 ml, dissolve
completely and then add water while mixing constantly, until you get
1000 ml.

>
>
> There is a big dif' between a) and b).
>
>
> Thanks in advance for any clarification on these three questions.
(and
> yes, I'll be wearing an apron, gloves, and using a fume hood).

One extra warning (just in case you didn't know yet): If you are mixing
concentrated H2SO4 with water, you'll get a LOT of heat!!! Never add
water to acid, buit SLOWLY and constantly stirring, add acid to water.
The same holds for fuming nitric acid!

Success - hope this helps you,

Wilco

>
> -K.

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