**Summer Research Program for
Science Teachers & Partners in Science Program**

How can the concentrations of a solution be determined?

**Annelisse Falzone, Bronx High School of
Science - 1997**

**Aim**: *How can the concentrations of a solution be
determined*?

**Student Objectives**: 1. Students will prepare a set of
Co(II) standard solutions.

2. Students will measure %T (% transmittance) of all known standard solutions.

3. Students will measure the %T for at least one unknown solution and determine its concentration from the calibration curve (%T vs concentration).

4. Students will use a computer graphing application to graph the calibration curve (e.g. Criket Graph). [Content Standard A- Use of technology/mathematics] [Content Standard E- Understandings about science/technology]

**Materials**: 0.010M stock solution of (CoNO_{3})_{2}6H_{2}O,
5 labeled test tubes, test rack, graduated cylinder, and a Spec
20 spectrophotometer

**Discussion**: Colored ions either absorb or transmit
light from the visible spectrum.Teachers may use solutions
containing Co (II) complexes, Fe(II) complexes or Cu complexes. [Content Standard B- Properties of matter]

Beer-Bouger Law: **A= ***abc*

where, *A** = *absorbance

** a** = absorbance coefficient

** b** = cell thickness, 1cm

** c** = concentration of the solution

Assume that ** a** and

**A**_{2}** = ***a*_{2}*b*_{2}*c*_{2},
where **A**_{2} equals absorbance of the
unknown solution solution:

**A**_{2}** / A**_{1}** =
***c*_{2}** / ***c*_{1}**.**

The spectrometer should be set at the maxmium wavelength for
the compound in question ((CoNO_{3})_{2}6H_{2}O)
where maximum absorbance 510 nm is achieved while executing the A
or %T measurments on the standard solutions.

**Preparation of Standard Solutions **: Students will
dilute a stock solution of 0.01M CoNO_{3})_{2}6H_{2}O
solution prepared by the teacher.Students will then take their
graduated cylinders and measure 2ml of the stock solution and
dilute to the 10mL mark with distilled water. Transfer 6mL of the
diluted solution into a labeled test tube #1. Discard the
remaining 4mL into a waste bottle. Students will then pour out
into the graduated cylinder respective amounts(4mL, 6mL, and 8mL)
of the stock solution into the graduated cylinder and diluted
each of the amounts to the 10ml mark. Transfer 6mL of each
diluted solution into test tubes labeled #2, 3 and 4. Test tube
#5 contains 6mL of the stock solution.

**Preparation of the Spec 20** **or any visible absorption
spectrometer** : Use the instruction manual or set the machine
to zero %T by having the sample slit empty and closed adjust the
%T knob to zero if necessary.

Then fill a test tube with distilled water and place it into the sample slit and adjust the %T knob to 100 %T with the right knob if necessary.

**Data Table** :

test tube 1 2 3 4 5 Unknown

dilution factor 2:10 ? ? ? ? ----------

concentration(molarity) ? ? ? ? ? ?

%T ? ? ? ? ? ?

**Ask students/teams
to fill in this section before beginning the experiment. Have a
student /team put up their dilution factors on the board have
students explain their answers*.

Students will construct a calibration curve %T vs concentration. The unknown concentration can be found by finding the %T of the unknown and determining its concentration from the calibration curve. [Content Standard Unifying Concepts- Change, constancy, and measurement]

**Questions **: 1.What is the
concentration of your unknown solution at %T of (left to
teacher)? [Teaching Standard B- Orchestrate scientific discourse]

2. What is the percent error for your unknown compared to its true concentration?

3. How can we improve our dilution techniques ?

4. How can we improve this experiment?