Chemlab: Chemistry 3/5


Natural Salt Solutions 1: Ion Exchange

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Prelab Problems
1. Write the balanced chemical reaction that describes the displacement of Al3+ ions by Ca2+ ions from a cation exchange resin. How many equivalents of positive charge will be present in one mole of each ion?

2. A large volume of a dilute CuSO4 solution is passed through a cation exchange column in the hydrogen form. The initial CuSO4 solution tests neutral with pH paper. At first a blue band forms and grows at the top of the column. The effluent at this stage is colorless and tests strongly acidic. Eventually the entire column is blue and the effluent becomes the same color as the charging solution. Explain this sequence of events using chemical equations where appropriate. At the last stage of the experiment, what is the pH range (acidic, neutral, or basic) of the blue effluent?

3. When 5 mL of 0.015 M CuSO4 solution is added to a column, a blue band measuring 1.0 cm forms. What is the capacity of this column, in milliequivalents, if the total column height is 5.0 cm? When the heights are measured, you estimate the values to the nearest 0.1 cm. What are the absolute and relative uncertainties of the total column height and the height of the blue band? Definitions of absolute and relative uncertainty are given in the section at the start of your lab manual manual called Computing Uncertainty in Laboratory Data and Results.

4. Draw a graph of how you expect pH of the effluent to change with time, as NaOH is added to a column in the hydrogen form. Label the pH axis with the words "acidic", "neutral", and "basic". Label the time axis with "start" and "stop". Explain the shape of your graph.

5. As NaOH is added to the column, the pH of the effluent is monitored carefully. After 2 mL of 0.4 M NaOH has been added, the effluent pH changes dramatically. Estimate the total column capacity in milliequivalents from this information.
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