[cherrytomb]

So, I'm doing the synthesis of salicylic acid from methyl salicylate and I'm trying to figure out the limiting reagent.
The rxn will go like this:
methyl salicylate + 2NaOH  salicylic acid
Does this mean I have to find the moles of methyl salicylate, and NaOH and see which is less? Do I have to consider the sulfuric acid too?
Here's what we did (in case you're confused):
So basically, we combined methyl salicylate and NaOH, refluxed, then added sulfuric acid. We vacuum filtered, recrystallized, vacuum filtered again, dried it and weighed it.
My final weight for salicylic acid was 0.284 g.
But my initial weight for methyl salicylate was 0.235 g (0.2 mL * 1.174g/ml).
My percent yield is going to be over 100%. Has this ever happened to anyone? Because I don't know what to do. Or it just means I made a mistake somewhere, which is going to be an unpleasant lab report to write...

[orgopete]

When I graded lab reports, I gave students deductions for a failure to calculate any molar values. My preferred way of doing an experiment is to determine the number of moles of all reagents (it is not needed for the solvent). In most organic chemistry reactions, the molar equivalents are in a one to one ratio. Therefore, simply calculating the number of moles of each reagent virtually makes it possible to determine the limiting reagent. Which one is the least?

For the sulfuric acid, you may have added an excess without measuring the volume and hence the number of moles directly. In that case, you won't need to calculate the number of moles of it. However, if you did, you will need to take into account that sulfuric acid creates two moles of hydronium per mole of sulfuric acid, thus the one to one ratio will not hold in that instance.

[cherrytomb]

Ok, so I'm pretty sure I know how to get the limiting reagent, but I still don't know what to do about my %yield. Do I just say I got 121% ? I still don't understand what my source of error is..

[democanarchis]

Your thought process here is completely wrong. You always calculate based on moles.

Basically, think about it like this. You started with 0.235g of methyl salicylate. How many moles of methyl salicylate is this (A) ? You ended up with 0.284g of salicylic acid. How many moles is this (B)?

If one mole of methyl salicylate reacts to form one mole of salicylic acid, then the moles of methyl salicylate you start with (A moles) will be the same amount of moles of salicylic acid you would expect to get if you got 100% conversion. This is called your theoretical yield. The actual moles (B moles) of salicylic acid you ended up with is termed your actual yield. To calculate % yield, you simply find what percentage of your theoretical yield your actual yield is.

If you still come out with a % yield greater than 100%, take a look at your experimental procedure. The most common reason to have a greater than 100% yield is simply that your product is still damp.

[cherrytomb]

 My melting point was 157-159 C, when the theoretical mp is 158-161 C (www.sigmaaldrich.com). That's pretty close. I think my product was pretty dry. So I'm supposed to do moles of salicylic acid over moles of methyl salicylate?

[cherrytomb]

OOOKKKK! Wait!! Sorry, I just got it!! Lol...I was so confused..
K, so I take 0.235g of methyl salicylate. molar mass = 152.15g/mol. That gives me 0.00155 mol. So how many grams of salicylic acid will 0.00155 moles give me, and that's my theoretical?? Is that what you're saying? Cuz that would make sense.
Ok, so then it should give me 0.214g of salicylic acid if molar mass = 138.12g/mol.
So theoretical is 0.214g, and actual mass is 0.284g.
But then I still get more than 100%..if I'm dividing actual by theoretical. wow, that sucks.

[democanarchis]

Yep, that looks right. Your % yield is indeed still greater than 100%. As basically all you are doing is base hydrolysis of an ester, you'd be expecting close to complete conversion anyway. Vacuum filtering won't completely dry your compound, usually to do that you'd leave it connected to a very high vacuum in a round bottom flask overnight, or dry it in a vacuum oven. I wouldn't worry too much about it. As long as you rationalise why you have a greater than 100% yield, you'll be fine.

[cherrytomb]

Ok, thanks!