“Determination of the solubility product constant for a sparingly soluble salt”
ID: 1046505 • Letter: #
Question
“Determination of the solubility product constant for a sparingly soluble salt” Objective: To become familiar with equilibria involving sparingly soluble substances by determining the value of the solubility-product constant for a sparingly soluble salt. Apparatus and Materials: Spectrophotometer Cuvettes 5 mL pipets (2) 150 mm test tubes (3) No. 1 corks (3) 100 ml volumetric flasks (4) Buret Centrifuge Ring stand and buret clamp 0.0024 M K Cr2O 0.25 M NaNO 0.004 M AgNO Procedure: A. Preparation of a calibration curve: Note: This is a whole class activity. Each group should prepare only one of the following solutions in the volumetric flask. Using a buret, add 1, or 2, or 3, or 4, or 5 or 6 mL of standardized 0.0024M K-CrO to one of the clean, dry 100 ml volumetric flasks and dilute to the 100 mL mark with 0.25 M NaNO Calculate the CrO2 concentration in each solution. Calibrate the spectrophotometer with 0.25 M NaNO, by adjusting the transmittance to 100%. Measure the absorbance of each solutions at 375 nm and plot the absorbance (Y- axis) versus concentration in Molarity (X - axis) to construct your calibration curve. ) Accurately prepare two solutions in separate 75 mm test tubes by adding 5.00 mL of 0.004 M AgNO, to 5.00 mL of 0.0024 M K Cr,0 i) Stopper each test tube. Shake the solutions on a shaker for about 7 minutes intermittently to establish equilibrium between the solid phase and the ions in solution. Place both of the 75 mm test tubes in a centrifuge and centrifuge for 10 minutes. Discard the most of the supernatant liquid in a waste container and retain the precipitate. ili) To each of the test tubes add 2 mL of 0.25 M NaNO, Shake each test tube on a shaker for 7 minutes intermittently and centrifuge again for 10 minutes. Discard the supernatant liquid and retain the precipitate. v) Then add 2 mL of 0.25 M NaNO, to each of the test tubes and shake on a shaker for 7 minutes to establish an equilibrium between the solid and the solution. Some solid Ag,Cro, must remain in the test tubes. If none remains start over. After shaking the test tubes for about 10 minutes centrifuge the mixtures v) Transfer the clear pale yellow supernatant liquid from each of the two test tubes to two clean dry cuvettes Measure and record the absorbance of the two solutions. Using your calibration curve calculate the molar concentration of CrO2 in each solution
Explanation / Answer
Purpose of the lab
To determine the solubility of a sparingly soluble salt Ag2CrO4
2AgNO3 (aq) + K2CrO4 (aq) ---> Ag2CrO4(s) + 2KNO3(aq)
First step is calibration curve formation using Standard K2CrO4 solution and NaNO3. the plot of absorbance for CrO4^2- vs intensity is plotted.
The solutions of AgNO3 with K2CrO4 is made and the precipitate formed is washed with fresh NaNO3 solution.
The clear solution thus obtained has dissolved CrO4^2- from dissolution of solid Ag2CrO4 in solution.
The absorbance value corresponds to the amount of free CrO4^2- present in solution.
[CrO4^2-] = value determined from calibration curve using absorbance data
[Ag+] = 2 x [CrO4^2-]
Ksp = [Ag+]^2.[CrO4^2-]
Thus, we can easily determine solubility and Ksp of sparingly soluble salt by this experiment.
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