Washing Precipitates It is almost always necessary to wash precipitates to free
ID: 515869 • Letter: W
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Washing Precipitates It is almost always necessary to wash precipitates to free them from ions that might cause confusion in later steps. To do this, add 1 or 2mL of distilled or deionized water to the precipitate, stir, centrifuge, and discard the wash water. Sometimes the directions will require a specific reagent in the wash water. Checking the pH To check the pH of a solution, put a piece of litmus paper or pH paper on a clean glass plate or watch glass. Dip the stirring rod into the solution in the test tube, and touch the stirring rod to the paper (see of Figure 2). Do NOT dip the test paper into the test tube. This may cause some of the indicator dye to dissolve in the solution, and the indicator color may confuse subsequent tests. Storing Solutions To keep a solution until the next laboratory period, stopper the test tube with a rubber stopper. If a precipitate is present, put a few drops of distilled or deionized water on it before the test tube. Be sure to record a list of substances that are present in each test tube. Do not rely on memory! In this experiment you will analyze a solution that can contain any combination of six different cations and a solution that can contain any combination of six different anions. First of all, you will analyze a cation and an anion solution that are "known" to contain all of the ions to learn the techniques for the analysis. Then you will analyze an "unknown" solutions to determine which ions are present and which are absent. Use the flow charts at the end of the experimental procedure to answer the following questions. In each question, a test is carried out to determine the presence or absence of the ions that are listed. State if the test results indicate if each possible ion is present, absent, or undetermined. 1. Test for Ag^+, Cu^2+, Fe^3+. Some 6 M HCl is added to a solution that may contain the three ions. A white precipitate forms. Ions present: _____ Ions absent: _____ Ions undetermined: _____ 2. Test for Mn Fe^3-, and Cu^3+ Some 3% H_2O_2 and 6 M NaOH are added to a pale blue solution. A dark precipitate forms, which totally dissolves when 6 M H_2SO_4 is added. Addition of 6 M NH_3 to the solution until it is basic results in a deep blue solution containing a dark precipitate. The dark precipitate completely dissolves in H_2SO_4 Ions present: _____ Ions absent: _____ Ions undetermined: _____ 3. Test for Cu^2+, Al^3+, and Zn^3+ Addition of 6 M NH_3 to a solution known to contain one or more of the above ions causes the formation of a deep-blue colored solution containing a gelatinous precipitate. Ions present: _____ Ions absent: _____ Ions undetermined: _____Explanation / Answer
1)
Ions present : Ag+
Ions undetermined : Cu2+, Fe2+
2)
ions present : Mn 2+
ions undetermined : Cu2+, Fe2+
3) ions present : Cu 2+
ions undetermined : Mn2+, Fe2+
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