SYNTHESIS OF POTASSIUM DIHYDROGEN PHOSPHATE

Name: ______Partner: ______

Date: ______Section/Instructor: ______

Instructions: Complete the following form being sure to show the requested calculations and attach the requested graphs (and spreadsheets).

Purpose:

Solution Concentrations: H3PO4 ______KOH ______

Determine the following information from your results:

  1. From the complete titration curve determine the equivalence point volumes of the titrations. Comment on and discuss these three equivalence point volumes [is the third volume three times the first? etc.]. Attach your graph (complete titration curve).
  1. From the complete titration curve determine the target pH for the synthesis of KH2PO4. What is the volume of KOH added at this pH value? Label this point on your graph.
  1. What is the actual (final) pH value for the synthesis of KH2PO4? What is the actual volume of KOH added during this step? Attach the titration curve for this step.
  1. (a) Determine the equivalence point volume from the titration curve from week two. Attach the titration curve for this step.

(b) Determine the moles of NaOH using the volume determined above.

(c) The purityof the salt, expressed as a percent, should be determined using the moles of NaOH required to titrate the “pure KH2PO4” crystals to the equivalence point.

Notes: moles of “pure KH2PO4” = moles of NaOH

% purity =100% (mass of “pure KH2PO4”)/(mass of “crude KH2PO4”).

(d) Determine the uncertainty in this purity using proper propagation of the uncertainties for each of the following:

volume of NaOH used: ______± _____ mL (± ____ %)

conc. of NaOH used: ______± _____ M (± ______% )

mass of “pure KH2PO4”: ______± ______g (± ______% )

Show work for propagation of uncertainties (use relative uncertainties and convert back to absolute uncertainty:

purity of “KH2PO4”: ______± ______%

  1. Calculate the percent yield of your crystals, expressed as:

100*(actual crude yield)/(theo. yield)

We’ll not propagate uncertainties on this yield . . . researchers typically do not do so!

Show any calculations after the table:

Volume of KOH / Moles of KOH / Moles of KH2PO4 / Mass of KH2PO4
(theoretical)

Determination of percent yield of the reaction (show calculations next to the table):

Mass of actual crude product / Theoretical yield / Percent yield
  1. How would you rationalize a yield in excess of 100%?
  1. Was there a difference in the melting points of the crude and pure? Report your numbers and explainwhat you can conclude.
  1. (a) Determine the first and second ionization constants (Ka1 and Ka2) for phosphoric acid from your complete titration curve and compare them to the accepted values. Label the points on your graph from which you have done this and show the appropriate math in the space below.

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(b) Determine the percentage errors ofpKa1 and pKa2 by comparing them to the “accepted” values listed at the start of this experiment.