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Spectrophotometric Stoichiometry

We used spectrophotometry to determine concentrations of solutions by their absorbtion. Our experiment used known concentrations of copper II sulfate to create a best fit line and then we determined the concentration of an unknown copper II solution (0.24M). Using the best fit line, we found the resulting concentrations after a KOH + CuSO4 precipitation reaction. A 0.5g KOH in 25mL water and 25mL 1M CuSO4 resulted in 0.38M Cu2+ ions in the supernatant. The other 1.8g KOH sample gave a 0.14M Cu2+ solution. These values affirmed the hypothesized result.


Spectrophotometry relies on the attenuation of light as a function of concentration. Absorbance, measured by the spectrophotometer, is related linearly to the concentration. The experiment involved determining the linear relation and applying it to an unknown sample and later using the relation to determine concentrations after a reaction.

The reaction used copper II sulfate and potassium hydroxide to create a precipitating reaction. We hypothesized that we would have 0.41M copper solution from the 0.5g KOH sample and 0.18M copper solution from the 1.8g KOH sample. Because this reaction uses copper II ions, the spectrophotometer was set to 620nm and the known solutions were made from stock copper II sulfate. The copper ions should absorb light at this wavelength thus our absorbance measurement on the spectrophotometer will change according to the concentration.

Materials and Methods

We prepared several copper II sulfate concentrations according to the directions in the lab handout. They were shaken a bit to mix the solution. After calibrating the spectrophotometer with a cuvette of distilled water, we measured the absorbance of each of the samples that we had prepared. The cuvette was dried each time and a new sample loaded. After the known solutions were tested, an unknown was tested to find out its concentration.

Initially, the above procedure yielded absurd results where two of the known solutions (0.500M and 0.333M) read the same absorbance. We suspected an error in the dillution procedure. When the cuvette was filled with water and measured, aberrant readings which fluctuated wildly appeared suggesting that the machine was defective. When the procedure was repeated with a different spectrophotometer, there were no glaring problems.

The second part of the experiment involved dissolving a measured quantity of KOH in 25mL of water. This reaction is exothermic and during the addition of water, the solution was constantly stirred. To this solution, 25mL of stock 1M copper II sulfate was added. The addition of the stock solution started a reaction. The solutions were stirred so that the reaction could occur as evenly as possible. The mixture was then filtered to remove the precipitate. The clear bluish liquid was then placed in the spectrophotometer to determine its concentration.


All masses were determined to the nearest hundreth of a gram. All volumes were measured to the nearest tenth of a milliliter. The spectrophotometer gave two significant figures. Those values are dimensionless because there is ratio between the intensities of light eliminate the units for that measurement.

We found the 1.8g KOH sample to leave a 0.14M copper II sulfate solution. The 0.5g KOH sample gave a 0.38M copper II sulfate solution. The unknown solution was determined to be 0.24M copper II sulfate solution.

Our results should be reliable within the range of concentrations tested. Outside of these ranges, other factors can play a greater role. For example, with a 0.001M copper II sulfate solution, the glass of the cuvette would start to be significant. Also, we were working with small enough samples that contaimination is easy.


Our hypothesis was affirmed by the data. The reactions products fell within a reasonable error margin from our predictions. The 1.8g KOH sample's absorbance was outside our tested area so should be either repeated several more times or further known solution data should be collected. Immidiate application for this technique includes measurements for water-borne contaminants at major polluting sites and instant quality control measurements for industrial processes.

The relation between concentration and absorbance is linear. This means simply plotting molarity versus absorbance will result in a straight line. This line should pass through zero because at zero concentration, all the light should go through the distilled water cuvette. The equation for our data is A=1.65M where A is absorbance and M is molarity. (Part I Question 1) Our unknown sample has a molarity of 0.24 based on the best fit line. (Part I Question 2)

Our percent error was about 24% for the 0.5g KOH sample and 3.8% for the 1.8g KOH sample. Our absorbance readings were less than what they should have been. Although we were careful to clean the water out of the cuvette, residual water could have dilluted our samples. Even a drop could have significantly affected our reading as our test only used 3-4 mL. The extrapolation for the low concentration 0.5g KOH sample brings on some added uncertainty. It is possible that the potassium sulfate product also in the solution starts to play a role as its concentration gets higher relative to the copper II sulfate. The difference between the time of day and the execution of the procedure over two days could have cause additional errors. These error should be relatively small: water expansion, reaction speed, thermal effects within the spectrophotometer, etc. Accuracy of the lab could be improved by running more samples, by getting a better spectrophotometer, or by running the experiment on multiple wavelengths. (Part II Question 3)