Determination of Ksp CuIO3 PDF

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Sample lab report for CHM 113 ...

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CHM 113 Determination of the Solubility Product of Copper(II) Iodate Introduction The purpose of this lab is to determine the solubility product of copper(II) iodate. The chemical equation and equilibrium constant expression for the dissolution of the slightly soluble salt Cu(IO3)2 in water are: Cu(IO3)2(s)  Cu2+(aq) + 2 IO-(aq) Ksp = [Cu2+] [IO3-]2 where Ksp stands for the solubility-product constant of this salt. The quantities in brackets are the concentrations of the two ions in the saturated solution. In theory, direct measurement of the concentrations of Cu2+ and IO3- in a

saturated solution of copper(II) iodate should give the value of K sp (by substitution in the Ksp expression).In practice, the concentration of an ionic species is most likely too low for accurate measurement. Therefore, in this lab, we will determine the concentration of the Cu2+ ion in three different saturated solutions by using spectrophotometric analysis. The concentration of the IO3- ion will be found using a volumetric method. From this, we will calculate five independent values of Ksp. Procedure At the start of the experiment, I obtained a kit from the stockroom, which contained five centrifuge tubes and cuvettes. My partner and I cleaned and labeled the tubes 1-5. Then, we obtained 30 mL of a 0.2 M solution of CuSO4 and 30 mL of a 0.3 M KIO3 solution using graduated cylinders. We used two of the 5 mL Mohr pipettes, one for each solution, to transfer solutions into the five tubes. The exact amounts were as follows: Test tube # 1 2 3 4 5 Volume of 0.2 M 3.5 mL 4.0 mL 4.3 mL 4.6 mL 5.0 mL CuSO4 Volume of a 0.3 M 4.5 mL 4.0 mL 3.7 mL 3.4 mL 3.0 mL KIO3 After creating each sample, we capped them, mixed the contents thoroughly, and set them aside on a test tube rack to sit for about an hour. Then, we cleaned and prepared five cuvettes. We added the following amount of mixtures to create the solutions: Cuvette # 1 2 3 4 5 (blank) Conc. of 1 mL 0.160 M 0.080 M 0.040 M 0.016 M None CuSO4 Vol of 0.5 M NH3 3.5 mL 3.5 mL 3.5 mL 3.5 mL 3.5 mL Vol of water 2.25 mL 2.25 mL 2.25 mL 2.25 mL 3.25 mL

We calibrated the spectrophotometer using our blank, and then measured the absorbance of cuvettes 1-4 (at 610 nm). We plotted a calibration curve using these values. Then we rinsed and dried the cuvettes used. After an hour, our five samples had formed some precipitate. We added an equal amount of water (8.0 mL) to an empty sixth test tube, to balance the centrifuge. Then, we centrifuged each tube for about 2-3 minutes, while our TA supervised. After this process, we used a pipet to transfer 0.90 mL (of only the supernatant liquid!) from each of the centrifuged tubes into separate labeled cuvettes. Then we added 3.0 mL of 0.5 M NH3 solution and 2.4 mL of water to each cuvette and mixed thoroughly. Then we recalibrated the spectrophotometer with another blank and measured the absorbances of each tube at 610 nm. At the end of our experiment, all materials were rinsed and dried and all solutions were placed in the labelled waste containers. Equations M1V1 = M2V2 Ksp = [Cu2+] [IO3-]2 = [IO3-]2 / 2 Average Ksp = (sum of all Ksp values) / 5 M1V1 = M2V2 ∆ = final - initial Data/Observations

Con centrAtion of cu ^2+ versu s a bsorb a n ce 1.4 1.2 f(x) = 7.2 x + 0.03

Absorbance at 610 nm

1 0.8 0.6 0.4 0.2 0

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

Concentration of Cu^2+ in mol/L

Equation of Line of Best Fit: y= 7.2048x + 0.0258 where y: absorbance, x: concentration Measured absorbances for standard solutions (from calibration curve): Concentration Absorbance 0.16 M 1.170 0.08 M .622 0.04 M .313 0.016 M .131 Measured absorbance for experimental solutions: Test Tube # Absorbance 1 .602 2 .704 3 .686 4 .714 5 .750 Calculations

New New New New

For calibration curve: concentration = (0.160 M x 1.00 mL) / 6.75 concentration = (0.080 M x 1.00 mL) / 6.75 concentration = (0.040 M x 1.00 mL) / 6.75 concentration = (0.016 M x 1.00 mL) / 6.75

mL = 0.0237 M Cu2+ mL = 0.0119 M Cu2+ mL = 0.00583 M Cu2+ mL = 0.00237 M Cu2+

1. 2. 3. 4. 5.

For preparing the sample: Initial [Cu2+] = (3.5 mL CuSO4 x 0.200 M) / 8.0 mL = 0.0875 M Cu2+ Initial [Cu2+] = (4.0 mL CuSO4 x 0.200 M) / 8.0 mL = 0.1000 M Cu2+ Initial [Cu2+] = (4.3 mL CuSO4 x 0.200 M) / 8.0 mL = 0.1075 M Cu2+ Initial [Cu2+] = (4.6 mL CuSO4 x 0.200 M) / 8.0 mL = 0.1150 M Cu2+ Initial [Cu2+] = (5.0 mL CuSO4 x 0.200 M) / 8.0 mL = 0.1250 M Cu2+

1. 2. 3. 4. 5.

Initial Initial Initial Initial Initial

1. 2. 3. 4. 5.

After Centrifuging: [Cu2+] in cuvette = [Cu2+] in cuvette = [Cu2+] in cuvette = [Cu2+] in cuvette = [Cu2+] in cuvette =

(0.602 (0.704 (0.686 (0.714 (0.750

1. 2. 3. 4. 5.

[Cu2+] [Cu2+] [Cu2+] [Cu2+] [Cu2+]

(0.01187 (0.01397 (0.01360 (0.01417 (0.01491

1. 2. 3. 4. 5.

Solving for Ksp: ∆[Cu2+] = 0.0875 M – 0.08309 M = 0.00441 M Cu2+ Δ[Cu2+] = 0.1000 M – 0.09779 M = 0.00221 M Cu2+ Δ[Cu2+] = 0.1075 M – 0.09520 M = 0.01230 M Cu2+ Δ[Cu2+] = 0.1150 M – 0.09919 M = 0.01581 M Cu2+ Δ[Cu2+] = 0.1250 M – 0.10437 M = 0.02063 M Cu2+

1. 2. 3. 4. 5.

∆[IO3-] ∆[IO3-] ∆[IO3-] ∆[IO3-] ∆[IO3-]

1. 2. 3. 4. 5.

Final Final Final Final Final

[IO3-] [IO3-] [IO3-] [IO3-] [IO3-]

in in in in in

= = = = =

= = = = =

(4.5 (4.0 (3.7 (3.4 (3.0

sample sample sample sample sample

mL KIO3 x 0.300 M) / 8.0 mL = 0.16875 M IO3mL KIO3 x 0.300 M) / 8.0 mL = 0.1500 M IO3mL KIO3 x 0.300 M) / 8.0 mL = 0.13875 M IO3mL KIO3 x 0.300 M) / 8.0 mL = 0.1275 M IO3mL KIO3 x 0.300 M) / 8.0 mL = 0.1125 M IO3-

= = = = =

– 0.0252) / 48.6 – 0.0252) / 48.6 – 0.0252) / 48.6 – 0.0252) / 48.6 – 0.0252) / 48.6

= 0.01187 = 0.01397 = 0.01360 = 0.01417 = 0.01491

M Cu2+ M Cu2+ M Cu2+ M Cu2+ M Cu2+

M x 6.30 mL) / 0.90 mL = 0.08309 M Cu2+ M x 6.30 mL) / 0.90 mL = 0.09779 M Cu2+ M x 6.30 mL) / 0.90 mL = 0.09520 M Cu2+ M x 6.30 mL) / 0.90 mL = 0.09919 M Cu2+ M x 6.30 mL) / 0.90 mL = 0.10437 M Cu2+

M = 0.00882 M IO3M = 0.00442 M IO3M = 0.02460 M IO3M = 0.03162 M IO3M = 0.04126 M IO3-

2 2 2 2 2

x x x x x

0.00441 0.00221 0.01230 0.01581 0.02063

[IO3-] [IO3-] [IO3-] [IO3-] [IO3-]

= = = = =

0.16875 M – 0.00882 M = 0.15993 M IO30.15000 M – 0.00442 M = 0.14558 M IO30.13875 M – 0.02460 M = 0.11415 M IO30.09919 M – 0.03162 M = 0.06757 M IO30.11250 M – 0.04126 M = 0.07124 M IO3-

1. 2. 3. 4. 5.

Ksp Ksp Ksp Ksp Ksp

= = = = =

½ ½ ½ ½ ½

(0.15993 (0.14558 (0.11415 (0.06757 (0.07124

M)3 = 0.00205 M)3 = 0.00154 M)3 = 0.00074 M)3 = 0.00015 M)3 = 0.00018

Average Ksp = (0.00205 + 0.00154 + 0.00074 + 0.00015 + 0.00018) / 5 = 0.000934 Discussion/Conclusion The standard (theoretical) value of Ksp is 1.4 × 10-7 and the value we calculated is 0.000934. The data recorded in our experiment only slightly supported our goal of calculating the solubility constant of copper(II) iodate. Our experimental value is far from our theoretical. Our value is not exact because our experiment did include some error. A possible source of error could have been not properly measuring out exact quantities. This can occur by not standing eye-level when reading the volume. Also, students may have not accounted for the tip of the Mohr pipets when measuring the samples, since the beginning of the measurement (zero) is not exactly at the tip. This would have caused larger samples than needed. Another error that could have easily occurred is that students didn’t wait long enough for precipitate to form in majority of the test tubes, which could have compromised the absorbance values measured after the centrifugation. An error in our data occurred in our absorbance values. As the test tube #’s went up, there was a trend that the absorbance would go up as well. However for #3, the absorbance decreased and then increased again for #4 and #5. My partners and I were not sure what happened here. We may have mislabeled the tubes. Also, we forgot to rinse every pipet between the pipetting of different solutions....