Lab Report 2

Lab 2: Ksp, the common ion effect and temperature Introduction:

In this lab students studied the effects of different temperatures on the solubility constant (Ksp) of a solid in water, in this case KHP. KHP is known to be a weak acid in water, water, this allowed the students to use titration to measure the amount of KHP dissolved in a saturated solution at different different temperatures. Using the measured measured values for solubility, solubility, students were also able to calculate the standard enthalpies and entropies of solution by van’t Hoff’s equation: 0

ln( K sp )  

 H sol

RT

0



S sol

R

Students also investigated the common ion effect e ffect both in comparing a solution of KHP and KCl through titration and through flame emission photomet ry. ry. The common ion effect inhibits the dissolution of some solutes by increasing the concentration of one of that solutes dissolved species using another, more soluble compound. The common ion effect is described by Le Chatelier ’s ’s principal that the position positi on of equilibrium will shift to compensate for changes in the concentration or pressure of chemicals on one side of a chemical reaction.

Procedure:

Part 1: Place a 10mL buret in a buret stand and rinse it with deionized water, then rinse again with NaOH. Close the valve on the buret and fill it to just below the 10mL mark, recording the volume. Rinse a 100mL beaker with DI water and transfer 0.40 ±0.05 g KHP to the

beaker. Record the exact weight of the KHP. Add 20-30mL of DI water and 2-3 drops of phenolphthalein indicator to the beaker and mix it with a magnetic stirrer. Titrate the solution

to a faint pink endpoint and record the volume in the buret (or the total volume NaOH used if the buret needs to be refilled). Perform a second identical titration, if the two titrations disagree by more that 3% perform a third. Refill the buret with NaOH. Using a micropipet, obtain a 1 mL sample of saturated KHP solution from one of the six heat baths and dispense it into a clean Erlenmeyer Flask. Add 20-30mL of DI water and 2-3 drops of phenolphthalein indicator to the beaker and mix it with a magnetic stirrer. Titrate the solution to a faint pink endpoint and record the volume in the buret (or the total volume NaOH used if the buret needs to be refilled). Titrate each temperature of saturated solution as above. Titrate the two room temperature solutions (the KHP only and the KHP and KCl) three times each and use 3mL each instead of one.

Part 2: Obtain 15mL of 0.005M KCl from the KCl dispenser. Prepare 50 mL of 7.5e-5 M KCl from the 0.005M solution and transfer to a test tube. Similarly prepare 15e-5, 22.5e-5, 30e-5, and 37.5e-5 M solutions. Transfer 1mL of 0.5M KCl+KHP solution to a 50mL volumetric flask and fill with DI water to the 50mL mark. Transfer 1mL of the diluted solution to a second 50mL flask and repeat. Transfer the doubly diluted solution to a test tube. To calibrate the flame emission photometer place the inlet tube into DI water and adjust the “blank” knob until the display reads zero. Then, aspirate the highest concentration solution (the 37.5e-5 M) and adjust the fine sensitivity knob to 375. Aspirate

each solution in order of increasing concentration and record the readings for each.

Results:

See Appendix 1 for all tables and graphs. Note that an obviously erroneous, contaminated, second 35° C trial is included in the laboratory notebook but the trial was discarded immediately and is not included in this report.

Discussion: In running the experiments described above the students found that higher temperatures correlate to higher K sp values, calculated the standard enthalpies and entropies of solution for KHP and found slight evidence for the opposite of the common ion effect, a result which was probably incorrect. Using samples from near 0° C up to about 65° C, the students found that

the K sp of KHP varies greatly over that range from 0.0149 all the way up to 0.564; almost forty times greater. Using van’t Hoff’s equation and graphing ln(K sp) with respect to the inverse of the absolute temperature, students calculated both the standard enthalpy of solution (53.2 kJ) and the standard entropy of solution (158 JK -1) from the slope and y-intercept of a best fit line, respectively. In a three-trial comparison of the solubilities of a solution of KHP and KHP+KCl, both at room temperature, the KHP+KCl solution was found to have a slightly higher solubility constant for KHP. This was contrary to what was expected because the presence of K + ions from the KCl should have decreased the solubility of KHP, however it appears that the opposite occurred. It remains unknown what mechanism caused the solubility of KHP to be higher in the mixed solution. It is likely that the error came from a small

amount of undissolved KHP being taken along with the solution each time as that would easily cause the observed error. The other piece of possibly erroneous data is the titration of the 55° C saturated solution which produced a K sp of 1.22 which was much larger than any other value. It is likely that there was a piece of undissolved KHP responsible in this situation too. Other than the unexpected results in comparing the two room temperature solutions the results were very close to expected. The K sp data follows the trend line reasonably well and the concentration data from the flame emission photometer forms an almost perfectly straight line. In other words, random error was kept to a minimum despite not having enough time to run a second trial for every solution.

Conclusion:

K sp trends with respect to temperature followed closely with expectations, as did the concentration data from the flame emission spectrometer. Other than an unexpected result titrating the 55° C saturated solution and a slight unexplained reversal of the common ion effect, error was minimal. Standard enthalpies and entropies of solution were calculated with a reasonable degree of accuracy and were within expected ranges.

APPENDIX 1 Solution

Calibration 1

Mass KHP added (g)

Volume water added (mL)

0.399

25.0

Total volume NaOH added to titrate (mL) 19.79

Calibration 2 0.401 25.0 19.95 Table 1: the titration of two calibration samples agrees very closely and demonstrates the precision of the titration apparatus to be within 3%.

Solution

Actual temperature (° K)

0° C 0° C 35° C 45° C 45° C 55° C 65° C Room temperature KHP only

275.7 276.7 309.2 317.7 317.8 331.1 337.5 295.0

Room temperature KHP and KCl

295.0

Total volume NaOH added to titrate (mL)

1.22 0.42 4.90 5.39 4.94 11.06 7.51 4.23 3.83 4.54

6.81 7.00 6.60

Table 2: raw data collected from the titration of the listed solutions. Increasing solubility with temperature is immediately obvious.

Solution

0° C 0° C 35° C 45° C

Total volume NaOH added to titrate (mL)

1.22 0.42 4.90 5.39

Saturated [HP-]

Ksp of KHP

0.122 0.042 0.490 0.539

0.0149 0.00176 0.240 0.291

45° C 55° C 65° C Room temperature KHP only

4.94 0.494 0.244 11.06 1.11 1.22 7.51 0.751 0.564 4.23 0.423 0.179 3.83 0.384 0.147 4.54 0.453 0.205 Room 6.81 0.227 0.199 temperature 7.00 0.233 0.204 KHP and KCl 6.60 0.220 0.193 Table 3: processed titration data leading to and including calculated K sp values which, again show increasing solubility with temperature and the strange reversal in the common ion effect.

Solubility of KHP varying with temperature with refernce to the van't Hoff equaiton 1 0 0.0029 -1 ) p-2 s K ( n-3 l

0.003

0.0031 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037

Solubilities forKHP

y = -6404.4x+19.039 

-4 -5 -6

1/T (K^-1)

Figure 1: shows a trend-line on a graph of K sp versus temperature, manipulated to fit the van’t Hoff equation. From this the standard enthalpy and entropy of solution could be calculated.

ΔH0soln (kJ) KHP

ΔS0soln (JK -1) 53.2

158

Table 4: shows the calculated standard enthalpy and entropy of solution for KHP

Solution

Flame Emission Photometer Readout

7.5e-5 M KHP Solution

85


151


222


303


Calibrated to 373

1:2500 diluted sample of 0.5 M KCl+KHP

359

solution

Table 5: shows raw data from the flame emission photometer

FlameEmissionPhotometerreadout 400 350

t u o d300 a e R250 r e200 t e m150 o t o100 h P

50 0

0.00E+0 5.00E-0 1.00E-0 1.50E-0 2.00E-0 2.50E-0 3.00E-0 3.50E-0 4.00E-0 0

5

4

4

4

4

4

4

4

ConcentrationofKHPinmoles

Figure 2: all of the data point’s proximity to the trend-line demonstrates the impressive precision of the instrument. Solution

Concentration per number in readout


8.82e-7


9.93e-7


10.14e-7


9.90e-7


10.05e-7

Table 6: shows the conversion of the arbitrary numbers on the photometer readout to useful concentrations

Concentration K +

1:2500 diluted sample of 0.5 M KCl+KHP solution

35.07e-5 M

Table 7: shows the calculated value for the concentration of KHP in the mixed sample.

Lab Report 2

Recommend Documents