ChemLab - Chemistry 3/5 - Analysis of Seawater

Natural Salt Solutions 2: Analysis of Seawater

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Before Lab
Complete the five prelab problems for this experiment. Write the prelab sections in your notebook, including the objective, reference, prelab procedure or flowchart, and sample calculations or analysis flowchart. The last section should include a description of how you will calculate the concentration of the NaOH solution and the cation equivalents per liter of sea water.

In Lab
While doing the experiment, record any deviations from your prelab procedure in your notebook. Carefully record your titration data and results for the standardization of NaOH. Before leaving the lab, calculate the cation concentration in equivalents per liter for all of your seawater trials. Record the data from others in your lab group, for use in your uncertainty analysis. Finally, check that you understand each of the discussion points below.

Uncertainty Analysis
In this week's experiment, you made repeated measurements of the molarity of the NaOH solution, and will use a statistical analysis to evaluate your uncertainty. Statistical methods of uncertainty analysis are summarized in the section at the beginning of the manual, called Calculating Uncertainties in Laboratory Data and Results. First, average your values of the molarity. During the lab period, collect all the individual values of the molarity from all the students in your lab group and average them. Calculate the standard deviation and 95% confidence interval for this larger sample. For the lab group average molarities, use the 95% confidence interval as the absolute uncertainty and calculate a relative uncertainty. All molarity values should be included, unless you know there was a mistake in the experiment, like a missed endpoint. Compare the lab group average and uncertainty from the 95% confidence interval to your own results. Do they fall within the confidence interval?

For the calculation of the cation equivalents per liter of seawater, you will perform a similar statistical analysis of your results. Calculate the average, standard deviation and 95% confidence interval for your results and also for the results of those in your lab group who analyzed seawater from the same location that you did. Don't eliminate a result from the calculation of the mean unless you know it was botched and can state why.

After Lab
This is the first of two "formal" reports that will be done this term. Prepare a word-processed reoprt containing sections 1, 2, 5 and 6 from the introduction of the manual. Don't forget a title page. The first major section of your formal report should be a section about the theory behind the experiment. You should discuss the background of the ion-exchange column, sea water and the experiment in the theory section. Your results section should include your individual values of the standard NaOH molarity and their average. Also report the average of your lab group's molarity results with the 95% confidence interval for the entire group's results. An example molarity result would be 0.124 ą 0.001 M (95%, N=30).

Your results should also include the calculation of the number of cation equivalents per liter in your seawater sample. Quote the mean value of all your satisfactory titration results. Also report the average cation equivalents per liter from of all students in your lab group who used seawater from the same location as your sample, with a 95% confidence interval.

In your discussion, review your results and discuss their significance. You should address the following points

• Summarize the final result of the experiment, the cation equivalents per liter, and compare to the chemical description of seawater presented in the Appendix. What did you learn about the seawater you analyzed?

• Compare your result for the concentration of NaOH to the posted value. Assuming that the posted molarity is the "true" value, discuss the accuracy of your individual results and the average and confidence interval of your entire lab group. Does the posted value fall within the confidence interval of the group's average?

• Discuss the precision of your results for the cation equivalents per liter of seawater. How does your relative uncertainty compare to the less than 1% expected, based on the precision of the equipment, assuming there were no blunders or systematic errors? Do your results fall within the confidence interval for your lab group?