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I have problem with my experiment. My experiment is acid hydrolysis of ethyl benzoate. The acid that I used is sulfuric acid. My problem is when I get the precipitate (impure benzoic acid) I have to filter it from the solution (of course) and I need to remove the sulfuric acid from my sample or else my filter paper will not dry. But the major problem is sulfuric acid takes longer time to dry. It could takes even month. I've tried leave it for a month and it still wet. My lecturer asks me to rinse my sample + filter paper with cold water (distilled water). And when I did my sample which is the precipitate is LESS. Maybe because I used large amount of cold water. Is there are suggestions or methods to remove the sulfuric acid?

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  • $\begingroup$ Hello and welcome to Chemistry.SE! At this point I suggest you take the short tour of the site and then familiarize yourself with the homework policy by reading through this discussion. Good luck! $\endgroup$ – airhuff Feb 15 '17 at 3:29
  • $\begingroup$ How do you know that you had less precipitate after the cold water rinse? Just by observation? How much less did it seem to be? $\endgroup$ – airhuff Feb 15 '17 at 3:32
  • $\begingroup$ Also, do you have a feel for how much benzoic acid is there or what your theoretical yield should be? $\endgroup$ – airhuff Feb 15 '17 at 3:39
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    $\begingroup$ Acid hydrolysis requires a few drops of sulfuric acid. This is easily removed by washing with a small amount of cold water... What exactly are you struggling with? $\endgroup$ – Zhe Feb 15 '17 at 4:22
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    $\begingroup$ Maybe extract with ethanol or DEE and then recrystalise. $\endgroup$ – Martin - マーチン Feb 15 '17 at 5:03
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Sulfuric acid is an excellent dessicant and is also very non-volatile itself (it boils at 337 °C). In other words it would likely still have been there in yet another month.

Your instructor was correct to suggest that you rinse the sulfuric acid with some cold water. Benzoic acid does have some non-negligible solubility in water (1.7 g/L at 0 °C). Unfortunately, it is even more soluble in most organic solvents so cold water may well be your best bet. The solubility in 25 °C water is 3.4 g/L, twice the solubility that it has at 0 °C, so definitely keep it cold.

Additionally, use as little water as necessary to get the sulfuric acid out. If you are working with a gram or so of benzoic acid, I would start with a dropwise addition of about 10 mL. If it’s still gooey, or still appears wet the next day, you need to repeat the rinse. It’s best if you can use a vaccum filter system of course, and spread the material thin and even before rinsing.

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  • $\begingroup$ I have converted your MathJax to html in this instance:  ° for degree Celsius; because it loads faster. Otherwise you might want to use MathJax for the whole expression, like $\pu{0 ^\circ C}$, $\pu{0 ^\circ C}$. What you have used might break on various occasions. $\endgroup$ – Martin - マーチン Feb 15 '17 at 5:12
  • $\begingroup$ Yea, I was just going over your edits, much thanks. The  ° also doesn't have the silly italicized look of the way I did it. I've head that "it might break" statement before and am just curious...is that a browser-specific thing, or server side or SEO stuff? $\endgroup$ – airhuff Feb 15 '17 at 5:16
  • $\begingroup$ MathJax is complicated in itself. It renders client based, and depending on your own configuration it uses installed or web fonts. Basically it is quite unpredictable how it looks. And then we have the apps... something, somewhere is certainly going to break... So far the worst I have experienced are potential line breaks between MJ and text when using mixed styles, i.e. CO$_{2}$ could have the two on the next line. Better wrap the whole thing to be safe, it also looks better (or at least less intrusive). [It was worse before graduation.] $\endgroup$ – Martin - マーチン Feb 15 '17 at 5:36

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