Explore BrainMass

Cyrstal Violet concnetration

This content was STOLEN from BrainMass.com - View the original, and get the already-completed solution here!

The question is : What is happening to the crystal violet concentration as the absorbance decreases?

Choices: a) increases
b) decreases
c) nothing happens to the crystal violet concentration



This experiment involves the determination of the rate expression of a chemical reaction. A rate expression is an equation expressing the rate of a reaction in terms of the concentrations of the substances taking part in the reaction. In general, the rate expression for any reaction is

Rate = k[A]x[B]y...

where A and B are reactants and x is the order with respect to A, y the order with respect to B, and so on. The reaction to be studied is that of "crystal violet" with sodium hydroxide. Crystal violet is a compound that is purple in basic solutions and colorless in acidic solutions.

The absorbance of the crystal violet will be monitored over time with the computer interfaced colorimeter.

In order to study kinetics you have to be able to measure the concentration of some component in the reaction being studied at any given point during the reaction. This is typically done via one of several common methods:
Spectroscopy - (The method we will be use) If one species absorbs a particular frequency (color) of light, then the more of that species present in the solution the more of that light it will absorb (and the less that is left the less light will be absorbed). Hence by measuring the amount of light absorbed by the sample solution the fraction of reagent remaining (or product produced) can be calculated.
Conductivity - If the reaction being studied involves ionic reagents or products that can conduct electricity in solution, then the more reagent/product present the more electricity the solution will conduct. By recording the conductivities of the solution over the course of the reaction one can calculate the amount of reagent at any given time.
Manometry - If the reaction involves the production or consumption of a gaseous product/reagent and is performed in a sealed container, then as the reaction progresses the gas evolved/consumed will raise/lower the pressure inside the reaction vessel. The pressure, therefore, can be used to determine the amount (concentration) of reagent or product at any given time.
Direct Chemical Analysis - this involves taking small samples (aliquots) out of the reaction at regular intervals during the reaction and freezing/cooling each aliquot to stop the reaction. Each aliquot is then chemically reacted in some manner to measure the amount of reagent remaining. This is probably the most tedious and involved method, but can be very accurate.

Once the order with respect to crystal violet is determined, the rate constant and half-life can also be calculated. Ultimately you will be able to measure the effect of crystal violet concentration on the rate of the reaction.
(1) Reference: 2nd Edition, Chemistry with Computers, Dan Holmquist and Don Volz, Vernier Software and Technology, 2000.


Use a 10-mL graduated cylinder to obtain 10 mL of 0.020 M NaOH, sodium hydroxide solution. Use another 10-mL graduated cylinder to obtain 10 mL of 2.0 x 10-5 M crystal violet solution.

Crystal violet is a dye and can permanently stain your clothing.
Pour very carefully and clean up all spills.

Prepare the computer by making sure that the colorimeter is plugged into the interface box as seen as the photo to the right.

If everything looks connected, open the Chem1146 folder on the desktop of the computer and click on the Crystal Violet experiment file.
This is how your bench top should be set up before you begin working: the colorimeter (black box) is connected to the LabPro Interface (dark blue), which is connected to the computer and also plugged into an electrical outlet.

Before you can make measurements you must
Calibrate the Colorimeter
(Click Here)

© BrainMass Inc. brainmass.com December 20, 2018, 2:52 am ad1c9bdddf

Solution Summary

The solution contains explanation for suitable option