Nevermore
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Sulfuric acid and water trapping.
It will be very useful for me to know how much water can H2SO4 trap, for example how many moles of H2O can be trapped by 96-98% H2SO4, or maybe how
many % of water can be trapped by 96% H2SO4..
Does anyone have idea?
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blip
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This is probably not correct, but I am giving it a shot. Sorry for wasting time if it is. It seems to have an HTML error, at least in IE, somewhere
that I can't seem to find.  Assuming that the sulfuric acid completely
hydrates itself to the heptahydrate, this is what I calculated:
<tt>
96 g
H<sub>2</sub>SO<sub>4</sub>  
; 96 g H<sub>2</sub>SO<sub>4</sub><br>
96 % H<sub>2</sub>SO<sub>4</sub> =
<s> &
amp;nbsp; </s> =
<s> &
amp;nbsp; </s> = <br>
100 g
H<sub>2</sub>SO<sub>4</sub> soln 96 g H<sub>2</sub>SO<sub>4</sub> + 4 g
H<sub>2</sub>O<br><br>
96 g H<sub>2</sub>SO<sub>4</sub><br>
<s> &
amp;nbsp; &
amp;nbsp;</s><br>
98.07848 g/mol
H<sub>2</sub>SO<sub>4</sub><br><s>
</s> = <br>
96 g
H<sub>2</sub>SO<sub>4</sub>  
; 4 g H<sub>2</sub>O<br>
<s> &
amp;nbsp; </s> +
<s> &
amp;nbsp; </s><br>
98.07848 g/mol H<sub>2</sub>SO<sub>4</sub> 18.01528 g/mol
H<sub>2</sub>O<br><br>
0.97880799 mol
H<sub>2</sub>SO<sub>4</sub>
<s> &
amp;nbsp; &
amp;nbsp; </s>
=<br>
0.97880799 mol H<sub>2</sub>SO<sub>4</sub> + 0.22203374 mol H<sub>2</sub>O<br><br>
0.97880799 mol H<sub>2</sub>SO<sub>4</sub><br>
<s> &
amp;nbsp; </s> = 0.81510158
bleh!...<br>
1.20084173 mol H<sub>2</sub>SO<sub>4</sub> + H<sub>2</sub>O<br><br><br>
0.97880799 mol H<sub>2</sub>SO<sub>4</sub><br>
<s> &
amp;nbsp; </s> = 4.4083 H<sub>2</sub>SO<sub>4</sub> ·
H<sub>2</sub>O<br>
0.22203374 mol H<sub>2</sub>O<br><br>
0.22203374 mol H<sub>2</sub>O<br>
<s> &
amp;nbsp; </s> = H<sub>2</sub>SO<sub>4</sub> · 0.22684
H<sub>2</sub>O<br>
0.97880799 mol H<sub>2</sub>SO<sub>4</sub><br><br>
7 mol H<sub>2</sub>O - 0.22684 mol H<sub>2</sub>O = 6.77316 mol H<sub>2</sub>O that can be absorbed per unit of
H<sub>2</sub>SO<sub>4</sub> · 0.22684 H<sub>2</sub>O
</tt>
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Nevermore
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woa! 6 mol of H<sub>2</sub>O for mol of H<sub>2</sub>SO<sub>4</sub> is really alot!
i wonder why nitrations with my poor % HNO<sub>3</sub> in excess sulfuric doesn't give out the yeld i wish 
i suppose im missing something?
[Edited on 30/11/2003 by Nevermore]
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unionised
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It depends.
If you get 2 beakers, one with H2SO4 and the other with clean water and put them in a sealed container then water will diffuse from the water to the
acid. This will continue to happen until the water is gone. If you then put some more water in that beaker, it too will difuse into the acid. (This
might get a bit slow, but the equillibrium will always favour the water moving to the acid).
If you repeat the experiment, but replace the water by saturated salt solution then the water will dilute the acid until it reaches (looks up data in
CRC book) about 37% by weight.
How much water the acid takes up depends on what it is taking it from.
Why assume a heptahydrate?
A monohydrate may make some sense, but even that is a bit arbitrary.
For the monohydrate; 18 g (ie 1 mol) of water hydrate 98 g of acid to give 116 g of solution. that means that the acid takes up 18/98 ie about 18.4 %
of it's weight of water. Since you start with only 96% acid the water absorbtion will be roughly 96% of that figure ie about 17.5 to 18 %
This is only an approximation, but it is more accurate than the assumption of a monohydrate.
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Nevermore
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I think that deserves more study and test trials, since i have 55% HNO<sub>3</sub> and i wish to use it fro nitrations..
so, i suppose, if i use double the amount of SA necessary for the reaction i should be able to dehydrate the NA enough to make it act like
concentrated nitric.
however i've never been able to get interesting yelds with this tecnic.
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vulture
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Only the first hydration step of sulfuric acid is hygroscopic enough to overpower HNO3's hygroscopiscity.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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Nevermore
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thanks vulture, so i should consider only one mole H<sub>2</sub>O for mole sulfuric.
i will make some calculations..
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blip
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I assumed the heptahydrate because that's the highest I've seen and:
| Quote: | | Assuming that the sulfuric acid completely hydrates itself to the heptahydrate... |
That doesn't account for situations where the H<sub>2</sub>SO<sub>4</sub> actually has to do some "work" to
wrench out more water molecules. Nevermore didn't put any constraints on his question.
So my calculations were correct, or at least mostly so?
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