[SumerianKing]

Synthesis of Aspirin. Acetylsalicylic Acid.
This is not too hard but I’m confused about the calculations. So I would appreciate the help so much!

1>(SYNTHESIS)First it was the basic procedure; weighed 5g of Salicylic acid and added 7.5cm3 of Ethenoic Acid and then added 3 drops of Sulphuric Acid. Then it was heated, vacuumed and weighed. So the CRUDE PRODUCT I retrieved was: 8.04g

2>(RECRYSTALISATION) Now next is the recrystalisation of this product. Added some Methylated spirit to the CRUDE PRODUCT, and then heated, filter vacuumed and weighed. So the weight of the PURIFIED SAMPLE WAS: 4.96g

NOW I need to calculate the CRUDE yield and the PURIFIED yield.

*CRUDE> Now I retrieved 8.04g and I started with 5.0g how am I meant to find the Yield when it would give me way over 100%? As the recovered amount is was over the 5.0g i started with??

*PURIFIED> Now here I simply done 4.96g/5.00g x 100= 99%    .. Now is this the right way to work out the yield for the purified sample??? or am I meant to divide 4.96 by the crude sample weight(8.04g)??

It is very confusing but i am mostly worried about the Crude yield. Am i working this out right, what am i missing?  I will appreciate all help.
Thank you. 

[discodermolide]

Synthesis of Aspirin. Acetylsalicylic Acid.
This is not too hard but I’m confused about the calculations. So I would appreciate the help so much!

1>(SYNTHESIS)First it was the basic procedure; weighed 5g of Salicylic acid and added 7.5cm3 of Ethenoic Acid and then added 3 drops of Sulphuric Acid. Then it was heated, vacuumed and weighed. So the CRUDE PRODUCT I retrieved was: 8.04g

2>(RECRYSTALISATION) Now next is the recrystalisation of this product. Added some Methylated spirit to the CRUDE PRODUCT, and then heated, filter vacuumed and weighed. So the weight of the PURIFIED SAMPLE WAS: 4.96g

NOW I need to calculate the CRUDE yield and the PURIFIED yield.

*CRUDE> Now I retrieved 8.04g and I started with 5.0g how am I meant to find the Yield when it would give me way over 100%? As the recovered amount is was over the 5.0g i started with??

*PURIFIED> Now here I simply done 4.96g/5.00g x 100= 99%    .. Now is this the right way to work out the yield for the purified sample??? or am I meant to divide 4.96 by the crude sample weight(8.04g)??

It is very confusing but i am mostly worried about the Crude yield. Am i working this out right, what am i missing?  I will appreciate all help.
Thank you. 

You need to work in moles, not grams. 1 mol salicylic acid gives 1 mol acetylsalicyclic acid if the yield is 100%.
Don't worry about the crude yield, it can easily be >100% when the sample is not dry.

[kalos]

This is totally wrong. You can't calculate yields through the masses. You have to calculate the moles. When you calculate a yield you want to know how much of your starting material (meaning: how many molecules) has been actually converted into your product. If you started with 100 molecules of Salicylic Acid and in the end you got 60 molecules Acetylsalicylic Acid, then you have a yield of 60%. The weight of one molecule of your product does not equal the weight of your starting material. You have to understand how to calculate the moles! One mole means just a certain amount of molecules (look up the Avogadro constant). It's like a dozen for 12 units of something.

Therefore you need to calculate how much moles 5 g Salicylic acid are and how much moles 4.96 g Acetylsalicylic Acid are. Then you can compare those and give a percentage.
To me it doesn't make any sense to calculate a crude yield.

[Arkcon]

Well, to start with Crude yield, you're wrong that the observed product yield should be compared to one reactant.  The product is made of the two reactant, minus known by-products.  You have to account for that.  And no, that's not the sums of the masses of the two reactants.  As a start, can you write a balanced chemical reaction.  You can it in word form, for example: salicylic acid, or C₆H₄(OH)COOH, instead of C₇H₆O₃.

Once you do that, you'll be using moles, like discodermolide: just said.

And like kalos: just said.  Damn, but I type slow.

[SumerianKing]

Thanks for your help guys.
Ok this is what i got...

Salicylic acid (RMM:138) + Ethenoic anhydride (7.5cm3) >>> Acetylsalicylic acid (RMM:180) + Acetic acid (RMM60)


RMM OF Salicylic acid: 138
Moles= 5.0g (What i started with) / 138 = 0.036 Moles
>Now as its a 1:1 ration both sides will be 0.036 moles!!
So the Acetylsalicylic acid also has 0.036 moles
Is that right?

[SumerianKing]

OH and also!!As my actual yield is 5.0g
Now i find the theoreticle by doing
RMM:180 x 0.036moles= 6.48g

So now i do 4.96g / 6.48 x 100 = 76%

Is that right?

So what shall i do with my crude yeild as 8.04//5.00 = 160%

[discodermolide]

OH and also!!As my actual yield is 5.0g
Now i find the theoreticle by doing
RMM:180 x 0.036moles= 6.48g

So now i do 4.96g / 6.48 x 100 = 76%

Is that right?

So what shall i do with my crude yeild as 8.04//5.00 = 160%

Write it down, it was asked for? It's not too important.

[SumerianKing]

Oh ok thanks!

Am i right for the purified product being 76% ?

[fledarmus]

Purified yield looks right - the crude yield isn't though. It should be 8.04/6.48. Yes, it will be more than 100%, because you have a lot of impurities adding to the mass.

[Borek]

You can't calculate yields through the masses.

Really?

[kalos]

You can't calculate yields through the masses.

Really?

Of course you need the masses, but only to calculate n. You can't do it directly as SumerianKing wanted.

[Borek]

You can't calculate yields through the masses.

Really?

Of course you need the masses, but only to calculate n.

Really?

Seems to me like you are suggesting yield is mol/mol. Why not mass/mass?

[SumerianKing]

Yeh the crude product is stupidly 160%
Shall I write in my analysis that this is due to the amount of impurities in the crude product and that also it wasnt placed inside the vacuume oven?

>ALSO does anyone know where i can get the literature values for the starting salicylic acid and for the recrystalised product. They seem to be NO WHERE!
Thanks 

[kalos]

I've explained the concept of yield before. It's a comparison between the number of molecules of starting material and the number of molecules of product. If you start with 100 molecules and each molecule has a mass x and you get 100 molecules product (100% yield) with each molecule having a mass y there are the following cases:

x>y: There is going to be a yield less than 100%
x=y: this is seldomly the case, usually only when you have rearrangements, you'd have a yield of 100%
x<y: There's going to be a yield greater than 100%.

You see the comparison of masses is mostly worthless. It says nothing about how well your reaction worked. It's like trading with cars through weight instead of mass. Nobody trades x tons of cars, but everybody trades with a certain number of cars. The only way you can make a meaningful comparison is by comparing the number of molecules that has been converted to the product.

[SumerianKing]

I've explained the concept of yield before. It's a comparison between the number of molecules of starting material and the number of molecules of product. If you start with 100 molecules and each molecule has a mass x and you get 100 molecules product (100% yield) with each molecule having a mass y there are the following cases:

x>y: There is going to be a yield less than 100%
x=y: this is seldomly the case, usually only when you have rearrangements, you'd have a yield of 100%
x<y: There's going to be a yield greater than 100%.

You see the comparison of masses is mostly worthless. It says nothing about how well your reaction worked. It's like trading with cars through weight instead of mass. Nobody trades x tons of cars, but everybody trades with a certain number of cars. The only way you can make a meaningful comparison is by comparing the number of molecules that has been converted to the product.

Thank you for your consideration my friend. I have worked it out and its

 Now i find the theoreticle by doing
RMM:180 x 0.036moles= 6.48g

So now i do 4.96g / 6.48 x 100 = 76%