# Help with a problem please?

1. Aug 29, 2006

### xc630

Hello I desperately need help with this chemistry problem. I know this is a physics site but it was the closest I could get to for chemistry. I would appreciate any help at all.

Acetaminophen (C8H9O2N) is produced by the following three step synthesis:

I. C6H5O3N (s) + 3 H2 (g) + HCL (aq) --(arrow) C6H8ONCl (s) + 2 H2O (l)

II. C6H8ONCl (s) + NaOH (aq) --(arrow) C6H7ON (s) +H2O (l) + NaCl (aq)

III. C6H7ON (s) + C4H6O3 (l) --(arrow) C8H9O2N (s) + HC2H3O2 (l)

Note the numbers after letters are subscripts

The 1st 2 reactions ahve percent yields of 87% and 98 % and the overall reaction yields 3 mol of acetaminophen product for every 4 mol of C6H5O3N reacted. The questions I need are what is the percent yield by mass for the overall process and what is the ercent yield by mass of step III? Thanks for any help:)

2. Aug 29, 2006

### dmoravec

well the overall yield (from start to finished) can be computed the same as your first two reactions but using the following generic formula (although this reaction did not happen as written it is in effect the yield you wish to calculate)

$$C_6H_5O_3N~+~stuff \rightarrow C_8H_9O_2N~+~stuff$$

the third reaction yield can be computed once the overall reaction yield is known by some simple algebra

3. Aug 29, 2006

### mrjeffy321

The Homework help section would be a better place for this.

The first step in solving this would be to assume 100% efficiency and figure out what the molar ration of moles of C6H5O3N reacted to moles of C8H9O2N produced. Look at the sub-ratios of reactants to products in each of the sub-reactions. From this, you should be able to calculate the ideal ratio of C6H5O3N : C8H9O2N.

Now look at the question...it says that for every 4 moles of C6H5O3N reacted, 3 moles of acetaminophen are produced....a ratio of 4 : 3, or 1 : 3/4. How does this differ from your ideal predictions earlier?
Using this information, you can calculate the overall efficiency of the 3 reactions.

You are given the efficiencies of the first 2 reactions and you just calculated the overall efficiency, from this you can calculate the efficiency of the 3rd reaction.

Overall efficiency = efficiency_1 * efficiency_2 * efficiency_3
4 variables, 3 of which you know, the other variable you can solve for.

4. Aug 29, 2006

### Staff: Mentor

5. Aug 29, 2006

### xc630

Hi I still do not understand how to find the percent yield in general as the yields of the frist two reactions were given by the problem. Can someone please explain. All I was told was that percent yield was actual yield over theoretical. Thanks.

6. Aug 29, 2006

### mrjeffy321

Did you find the theoretical, 100% max efficiency; ratio of moles of C6H5O3N reacted to moles of C8H9O2N produced? Follow the process, step by step, watch how in each sub-reaction, the ration of reactants to products (the product we care about) is always 1 to 1.
So in the end it should take 1 mole of C6H5O3N to form 1 mole of C8H9O2N. But in reality, it does not. The question tells you outright that it takes 4 moles of C6H5O3N to form 3 mole of C8H9O2N...what efficiency is this?

7. Aug 30, 2006

### xc630

so the efficiency would be 75%? this is the precent yield for the overall process correct?

8. Aug 30, 2006

### xc630

so the efficiency would be 75%? this is the precent yield for the overall process correct?

9. Aug 30, 2006

### mrjeffy321

Correct, overall process is 75% efficient.

Mathematically, we can figure out that the overall efficiency is the product of the sub-reactions' efficiency,
Overall efficiency = efficiency_1 * efficiency_2 * efficiency_3
So there are 4 variables, but only 1 unknown at this point so you are able to solve for efficiency_3.

10. Aug 30, 2006

### Gokul43201

Staff Emeritus
xc : Please note that according to the guidelines, we expect you to first show what effort/progress you've made (or any thoughts you have) on the problem before we can help you.

dmoravec : Please do not help with standard textbook/homework problems unless the OP shows their work first.

Links to the posting guidelines are found in my signature.

11. Aug 31, 2006

### xc630

I never learned overall effeciency equals the product of sub reactions' effeiciencies. So I solved for x and and got 87.9 percent or 88 percent efficiency for the third reaction. Thank you mrjeffy321