Sciencemadness Discussion Board

Sulfuric acid and water trapping.

Nevermore - 29-11-2003 at 14:41

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?

blip - 29-11-2003 at 20:13

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>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;96 g H<sub>2</sub>SO<sub>4</sub>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp ;&nbsp;96 g H<sub>2</sub>SO<sub>4</sub><br>
96 % H<sub>2</sub>SO<sub>4</sub> = <s>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;& amp;nbsp;&nbsp;</s> = <s>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;& amp;nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</s> = <br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;100 g H<sub>2</sub>SO<sub>4</sub> soln&nbsp;&nbsp;&nbsp;96 g H<sub>2</sub>SO<sub>4</sub> + 4 g H<sub>2</sub>O<br><br>


&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 96 g H<sub>2</sub>SO<sub>4</sub><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<s>&nbsp;&nbsp;&nbsp;&nbsp;& amp;nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;& amp;nbsp;</s><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;98.07848 g/mol H<sub>2</sub>SO<sub>4</sub><br><s>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</s> = <br>
&nbsp;&nbsp;&nbsp;&nbsp;96 g H<sub>2</sub>SO<sub>4</sub>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp ;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;4 g H<sub>2</sub>O<br>
<s>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;& amp;nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</s> + <s>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;& amp;nbsp;&nbsp;&nbsp;&nbsp;</s><br>
98.07848 g/mol H<sub>2</sub>SO<sub>4</sub>&nbsp;&nbsp;&nbsp;18.01528 g/mol H<sub>2</sub>O<br><br>


&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;0.97880799 mol H<sub>2</sub>SO<sub>4</sub>
<s>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;& amp;nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;& amp;nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</s> =<br>
0.97880799 mol H<sub>2</sub>SO<sub>4</sub> + 0.22203374 mol H<sub>2</sub>O<br><br>


&nbsp;&nbsp;&nbsp;0.97880799 mol H<sub>2</sub>SO<sub>4</sub><br>
<s>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;& amp;nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&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>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;& amp;nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</s> = 4.4083 H<sub>2</sub>SO<sub>4</sub> &middot; H<sub>2</sub>O<br>
&nbsp;0.22203374 mol H<sub>2</sub>O<br><br>


&nbsp;0.22203374 mol H<sub>2</sub>O<br>
<s>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;& amp;nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</s> = H<sub>2</sub>SO<sub>4</sub> &middot; 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> &middot; 0.22684 H<sub>2</sub>O
</tt>

Nevermore - 30-11-2003 at 01:55

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 :o
i suppose im missing something? :(


[Edited on 30/11/2003 by Nevermore]

unionised - 30-11-2003 at 04:31

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.

Nevermore - 30-11-2003 at 04:43

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.

vulture - 30-11-2003 at 06:15

Only the first hydration step of sulfuric acid is hygroscopic enough to overpower HNO3's hygroscopiscity.

Nevermore - 30-11-2003 at 13:56

thanks vulture, so i should consider only one mole H<sub>2</sub>O for mole sulfuric.
i will make some calculations..

blip - 3-12-2003 at 20:15

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?