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

  1. Oct 19, 2013 #1
    1. The problem statement, all variables and given/known data
    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.

    2. Relevant equations

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

    Benzoic acid - C6H5COOH

    Biphenyl - C12H10

    3. 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.


    Bottom layer :
    C6H5COOH + NaOH -> H2O + NaC7H5O2

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

    Top layer:
    C12H10 + C4H10O ->
    A little stuck here
  2. jcsd
  3. Oct 19, 2013 #2


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    Staff: Mentor

    Percent recovery of what? 27.11 g is a mass of what?
  4. Oct 19, 2013 #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.
  5. Oct 19, 2013 #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)
  6. Oct 20, 2013 #5


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    Staff: Mentor

    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.
  7. Oct 20, 2013 #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.
  8. Oct 20, 2013 #7


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    Staff: Mentor

    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?
  9. Oct 20, 2013 #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: Oct 20, 2013
  10. Oct 20, 2013 #9


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    Staff: Mentor

    Yes about GIGO.

    OK, now it got clear. Still:

    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:
  11. Oct 20, 2013 #10
    Oh my. I guess everything else is okay. Thanks for your patience. I appreciate it!
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