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How to set up equation for drying agents, and calculating % recovery

  • #1

Homework Statement


Benzoic acid/biphenyl mixture is mixed with diethyl ethe(CH3CH2-O-CH2CH3). The solution is placed in a separatory funnel and sodium hydroxide is mixed in.

The bottom layer, H2O, NaOH and benzoic acid is then placed in a flask where it is mixed with HCl, chilled and then suction filtrated. Solid sample left over.

The top layer which consists of biphenyl, H2O trace and ether is poured into a flask. Anhydrous potassium carbonate (K2CO3) is mixed into the solution. It is then distilled. Solid sample left over.


For percent recovery, the initial mass weighed was the combination of benzoic acid and biphenyl mixture. Do I use the combined weight and divide that into the weight of the pure compound each? Or do I combine it?

I started the relevant equations and wanted to double check to see if I was on the right track and am not missing a relevant equation.



Homework Equations



Percent recovery = (mass of pure product recovered/ mass of crude product used) x 100

Benzoic acid - C6H5COOH

Biphenyl - C12H10

The Attempt at a Solution



Percent recovery of benzoic acid - (0.9g/2.08g) x 100 = 43%

Percent recovery of biphenyl - (27.11g/2.08g) x 100 = 1303..... this is where I run into a problem. 27.11g seems extremely high.


Equations

Bottom layer :
C6H5COOH + NaOH -> H2O + NaC7H5O2

NaC7H5O2 + H2O + HCl -> C6H5COOH (solid product) + H20 + NaCl

Top layer:
C12H10 + C4H10O ->
A little stuck here
 

Answers and Replies

  • #2
Borek
Mentor
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Percent recovery of what? 27.11 g is a mass of what?
 
  • #3
The mass of the benzoic acid/biphenyl mixture was 2.08 g.
Biphenyl product was placed in a pre-weighed round bottom flask. I didn't take the measurement for the pre-weighed stuff, it was copied from a partner. Pre-weighed rbf was 61.61g. When I weighed it after leaving it alone, it was 88.72g. Something sounds very wrong.
 
  • #4
I've been trying to convert the benzoic acid/biphenyl to moles to find the limiting reactant, then comparing the number of moles in order to find percent recovery. But I cannot get the equation or the mass used to find theoretical yield since there is only one mass (2.08 g)
 
  • #5
Borek
Mentor
28,401
2,800
Percent recovery = (mass of pure product recovered/ mass of crude product used) x 100
Which means: total mass of both recovered substances over initial mass.

Biphenyl part is an unreadable mess to me. But as you are using random numbers don't be surprised you are getting random results.
 
  • #6
The issue is, when I do Percent recovery right away. I get crazy answers. (0.9[benzoic acid] + 27.11[biphenyl])/2.08 x 100 = 1346 %

Mass of RBF + boiling stones= 61.61g
Mass of RBF + bioling stones + biphenyl = 88.72g
Mass of biphenyl = 27.11g

The biphenyl itself was very little in substance and were hard crystals basically. Starting to wonder if the initial mass is correct.

I then tried finding the moles and moved on to find theoretical yield, but there was a combined mass. So I didn't know the individual masses of each. Stoichiometry has not been brought up anywhere in the course so far so I'm starting to wonder if that was unnecessary.
 
  • #7
Borek
Mentor
28,401
2,800
27.11 is for sure wrong. But so far you have not even explained what was the procedure you followed. You distilled something for no apparent reason, then you have weighted something else, and finally you subtracted some other number you found in your partner lab book. Sorry to be so blunt, but do you know what GIGO means?
 
  • #8
I was trying to brief earlier. The first two paragraphs only talk about the start of the experiment and the lower layer. Reading the third will probably clarify everything.

Benzoic acid/biphenyl mixture was a white powdery solid. It was 2.08g. It was dissolved in 30mL of diethyl ether in an erlenmeyer flask. The solution was then transferred into a separatory funnel. 20mL of diethyl ether was used to rinse the flask and the contents were added to the funnel. Then 20mL NaOH was added to the funnel. The funnel was shaked and inverted periodically to let out gas. The lower layer (benzoic acid, NaOH, h20) was let out into an erlenmeyer flask. An extra 20mL of NaOH was added to the solution in the funnel and the bottom layer was extracted again. The top layer(biphenyl, ether) was then poured out into another erlenmeyer flask.

Lower layer - 9ml of HCl was added, then the solution was chilled for 10 minutes. Precipitate was collected by suction filtration. Precipitate was stored for a week.

Upper layer - 4g of anhydrous potassium carbonate was added. It was swirled gently to prevent the drying agent to become saturated with water. There is a dry solution above the solid drying agent after 20 minutes. The dry solution was filtered through a funnel into the preweighed RBF flask. The erlenmeyer flask was rinsed with 4-5 ml of diethyl ether. A distillation assembly was set up and the solvent was removed by distillation. Once the sample was solidified, it was stored away for a week to be weighed. The masses are in my previous post.

GIGO, garbage in garbage out?
 
Last edited:
  • #9
Borek
Mentor
28,401
2,800
Yes about GIGO.

OK, now it got clear. Still:

I didn't take the measurement for the pre-weighed stuff, it was copied from a partner.
Are we talking about the same flask? If so, one of the masses is wrong (measurement error, notation error, whatever). If not... it doesn't even make sense to think about why it doesn't make sense :tongue2:
 
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  • #10
Oh my. I guess everything else is okay. Thanks for your patience. I appreciate it!
 

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