Summer Research Program for Science Teachers

Jacqueline Kennedy Onassis High School

Summer 2000

How can we determine the concentration of an unknown solution?

Instructional Objectives:

a.      Students will create a standard curve, plotting the relationship between the concentration of a solution and the amount of light that it is able to absorb. [Content Standard Unifying Concepts- Change, constancy, and measurement] [9-12 Content Standard B- Properties of matter]

b.  Students will be able to use the standard curve they have  created to determine the concentration of an unknown solution. [9-12 Content Standard A- Formulate explanations using evidence]

Materials:

CBL System

TI Graphing Calculator [9-12 Content Standard E- Understandings about science and technology]

Vernier Colorimeter

One cuvette

Five 20 x 150 mm test tubes

Tissues (preferrably lint free)

30 mL of 0.40 M NiSO4

5 mL of NiSO4   of unknown concentration

two 10 ml pipets (or graduated cylinders)

pipet pump or pipet bulb

distilled water

test tube rack

two 100mL beakers

stirring rod [Teaching Standard D- Make accessible science tools]

Procedure:

1.     Using the table below, use the distilled water provided to dilute the NiSO4  and make 5 solutions with known concentrations.

 Trial Number 0.40 M NiSO4 (mL) H20 (mL) Concentration (M) 1 2 8 0.08 2 4 6 0.16 3 6 4 0.24 4 8 2 0.32 5 10 0 0.40

2.     Calibrate the colorimeter.  Prepare a blank by filling a cuvette ¾ full of distilled water.  With the light source turned off, enter this absorbance value obtained as 0%  transmittance.  With the wavelength knob in the Red LED position (635 nm), enter the absorbance value obtained as 100% transmittance.

3.     In this same manner, collect absorbance data for each of the five standard solutions.  When the percent transmittance value for each solution is displayed , enter the molar concentration for that solution.

4.     Using your calculator, construct a graph of absorbance vs. concentration.  Then perform a linear regression on your data.  [9-12 Content Standard A- Use mathematics to improve scientific communication] If the data you have obtained are consistent with Beer’s Law (a direct relationship between absorbance and concentration), the regression line should closely fit the five data points and should pass through (or near) the origin of the graph.

5.     Obtain about 5 mL of the unknown solution of NiSO4 .  Find the absorbance for the unknown solution.  Then use your calculator to interpolate along the regression line on your Beer’s Law curve.

6.  Use the TI Graph link cable and program to transfer the graph of absorbance vs. concentration (including the interpolated unknown concentration) to a laptop computer.  Print a copy of the graph.