And Define the Equilibrium Constant As Kai

Equilibrium

Chemistry 150

Introduction

Indicators are substances whose solutions change color due to changes in pH. These are called acid-base indicators. They are usually weak acids or bases, but their conjugate base and acid forms have different colors due to differences in their absorption spectra.

Indicators are typically weak acids or bases with complicated structures. For simplicity, we represent a general indicator by the formula HIn, and its ionization in a solution by the equilibrium,

HIn↔ H+ + In-,

and define the equilibrium constant as Kai,

[H+][In-]

Kai = ------

[HIn]

In this experiment we will determine the equilibrium constant (Keq) for the indicator Bromothymol blue using a spectrophotometer and a pH meter. Keep in mind that Bromothymol blue is blue when in the basic form (In-) and yellow when in the acidic form (HIn).

Pre-lab Questions

1. What is the original concentration of bromothymol blue in solution 1?

2. What color do you expect solution 1 to be? Solution 5?

3. What is the concentration of the basic form of bromothymol blue in solution 1?

4. What is the concentration of the basic form of bromothymol blue in solution 5?

5. What is the relationship between [BTB]original, [BTB-]eq and [BTB]eq ?

6. If the solution reads pH 7.66, what is the [H+]eq ?

Procedure

Part 1

(1)Obtain a spectrophotometer and turn it on.

(2)Prepare 5 standard solutions in medium test tubes using disposable pipettes.

Solution0.00025M0.10M 0.10M

Bromothymol blue (mL)K2HPO4 (mL)KH2PO4 (mL)

------

11.004.000.00

21.003.001.00

31.002.002.00

41.001.003.00

51.000.004.00

(3) With test tube #1 in the sample holder, find the wavelength that would give the maximum %T. It should be around 480nm. (look out for the wavelength filter located on the left lower corner on the digital spec 20.)

(4)Once the maximum wavelength is identified, insert a test tube with distilled water and set %T to be 0% using the left lower knob.

(5)Insert test tube #1 again and adjust the %T reading to 100% using the right lower knob.

(6)Read %T for S2 to S5.

Part 2

(7)Read %T for three unknown solutions: A,B, and C.

Part 3

(8)Transfersolution A intoa 50-mL beaker and read the pH.

(9)Repeat (8) for unknown solutionsB and C. Make sure to rinse the beaker and the pH probe with distilled H2O in between readings.

Calculations

Use Excel to graph a calibration curve, %T vs. [BTB-]eq using solutions 1 through 5.

Make an ICE table for the three unknown solutions: A, B, and C.

Calculate an average Kai.

Post-lab Assignment

Write an abstract.

Present all data neatly and creatively in data tables (including the calibration curve data and all calculated values).

Provide one set of sample calculations, including at least one ICE table.