Solubility Product Constant Ksp of KHT
Introduction:
A saturated solution contains the maximum amount of solute possible at a given
temperature in equilibrium with any excess undissolved solute present. The
general equation for an ionic solubility equilibrium involving a salt composed of
an ion with a 1+ charge and an ion with a 1- charge and the equilibrium
expression are shown below.
AB (s) ⇄ A+ (aq) + B- (aq)
Keq = Ksp = [A+] [B-]
In this experiment, you will determine the solubility in water of a slightly soluble
salt, potassium hydrogen tartrate (KHT), a monoprotic carboxylic acid. You will
use the solubility to calculate the solubility product constant for a filtered,
saturated solution of KHT at room temperature. KHT has the formula KHC4H4O6.
The solubility of potassium hydrogen tartrate (KHT, 188.18g/mol) is 1.0g/162mL
at 25C and 1.0g/16mL at 100C.
KHT (s)

HT - (aq)
KHC4H4O6(s)

HC4H4O6
+
-
(aq)
K + (aq)
+ K + (aq)
Objective : Determine the solubility and solubility product constant by titration.
Prelab: In addition to writing your procedure, write any needed data tables
into your notebook and complete the following.
(PL1) Calculate the solubility, in M, of potassium hydrogen tartrate at 25C and
at 100C.
(PL2) Write an expression for Ksp of potassium hydrogen tartrate.
(PL3) Calculate the theoretical solubility product, Ksp, of potassium hydrogen
tartrate at 25C and at 100C.
(PL4) Write a balanced equation for the titration of potassium hydrogen tartrate
with sodium hydroxide.
Procedure :
(1) Weigh out ~2.0 g of finely powdered potassium hydrogen tartrate into
250.mL beaker. Record the exact mass in your lab notebook.
(2) Add 150.mL of distilled water and stir well. There should still be solid present,
even after stirring well. You must stir this solution for ~20 minutes to ensure
saturation.
(3) Using vacuum filtration, filter the solution into a clean, dry Erlenmeyer flask
with a side arm. The solution in the flask is called the filtrate. If you see any
solid in your filtrate, filter it again.
Note: After removing excess solid KHT by filtration, you will determine the
amount of dissolved KHT by titrating the hydrogen tartrate ions (HT-) with a
standard sodium hydroxide solution to the endpoint. The funnel and flasks you
use in the filtration must be dry since any water added to the filtered, saturated
solution will lower the concentration of dissolved KHT.
(4) Record the temperature of the filtrate.
(5) Pipet out 25 mL of the filtrate into a 250.mL Erlenmeyer flask.
(6) Put ~3 drops of phenolphthalein indicator into the flask.
(7) Prepare a buret with NaOH solution. (Make sure to write down the exact
NaOH concentration.)
(8) Titrate with NaOH until you get a permanent, faint pink solution.
(9) Repeat (5) to (8) for a second and third determination.
Post lab: In addition to reporting your data results, conclusions, learning,
problems, and errors, complete the following.
Calculations :
(C1) Calculate the moles of NaOH used to reach the endpoint.
(C2) Calculate your solubility of potassium hydrogen tartrate in M.
(C3) Calculate your solubility product, Ksp, of potassium hydrogen tartrate.
(C4) Calculate your percent error for Ksp and molar solubility.
Questions:
(Q1) What are you assuming the filtrate is: an unsaturated, saturated,
oversaturated, or supersaturated solution?
(Q2) What species are present in the filtrate? What do you want excluded from
the filtrate?
(Q3) What would happen to the solubility and Ksp (that you calculated) if you
started with 0.000001 g of solid potassium hydrogen tartrate instead of 2.0 g but
you didn’t know it?
(Q4) What would happen to the solubility and Ksp (that you calculated) if there
was solid KHT present in your filtrate?
(Q5) What is the difference between the endpoint and equivalence point? What
assumption are you making in order to do your calculation in (C1) above?
Summary Table:
Include in your results table: the theoretical and your molar solubility of KHT,
theoretical and your Ksp of KHT, and percent errors for each.