Colorimetry GT Chemistry 5/13/15

Drill Think back to the Kool-Aid lab… HW: What happened to the color of the solutions as the molarity increased? How would this observation impact the amount of light transmitted HW: Study for Solutions Test on Monday Wear closed-toe shoes on Friday

Objectives IWBAT Apply Beer’s Law to determining the concentration of an unknown solution

Agenda Drill Beer’s Law Notes/Sample Data Colligative Properties Review Exit Ticket

Beer’s Law Notes, Intro to Colorimetry

Beer’s Law Beer’s Law quantifies the relationship between color and concentration Formula: A = ebc A = absorbance e = molar absorptivity (unique for a given compound) b = path length c = concentration

We can use Beer’s Law to Determine the concentrations of unknown solutions

How do we do this? We use a technique called spectroscopy

Spectrophotometer

How Do We Do This in lab? We make several solutions with known concentrations We determine which wavelength or color of light the solution responds to best We measure that transmittance and/or absorbance for the known solutions and the unknown solution We graph the knowns and interpolate for the unknown

Sample Scenario

Sample Scenario A series of standard solutions containing a red dye was made by diluting a stock solution and then measuring the percent transmittance of each solution at 505 nm (greenish blue). This wavelength was selected by examining its absorption spectrum. If the solution looks red, it is absorbing red's complementary color of light, which is greenish blue. The results, after conversion to absorbance, are shown below. http://www.harpercollege.edu/tm-ps/chm/100/dgodambe/thedisk/labtech/example.htm

Sample Scenario A series of standard solutions of known concentration were made by diluting a stock solution of water with red dye. The wavelength of light that will absorb the most is selected. It is the complementary color, so a red solution will absorb in the blue-green range, at 505 nm This wavelength was selected by examining its absorption spectrum.

Sample Scenario The absorbance for each solution is measured. Solution Blank 0.00 Standard # 1 0.24 Standard # 2 0.50 Standard # 3 0.72 Standard # 4 0.99 Unknown Sample 0.39

Step 2 – Calculate Unknown Concentration using Beer’s Law, A = ebc Solution Absorbance Concentration Blank 0.00 0.00 M Standard # 1 0.24 0.15 M Standard # 2 0.50 0.30 M Standard # 3 0.72 0.45 M Standard # 4 0.99 0.60 M Unknown Sample 0.39 ???? M Note: e = 1.6 1/(cm•M), b = 1.0 cm

Step 2 A = ebc 0.39 = (1.6 1/(cm•M))(1.0 cm)•c c = 0.24 M

Step 3 – Create a Concentration vs Absorbance Standard Curve and find line of best fit

Step 4 – Determine the concentration of the unknown using the line of best fit

Look at that! It matches! Why do we do it both ways? The “best-fit line” method is more accurate—always trust your data over a “given” e-value. Sometimes path length (b) is not accurately measured

Let’s do some review… Effect of a Solute on BP and FP problems—let’s see ‘em on the boards  Any questions about separation techniques? Solution Review WS

Closure What is the purpose of creating a standard curve when using a spectrophotometer? What steps do you take to determine the concentration of an unknown?