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A student in the laboratory finds three bottles with no labels. Each bottle contains a...

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deedar | eNotes Newbie

Posted March 15, 2013 at 11:31 PM via web

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A student in the laboratory finds three bottles with no labels. Each bottle contains a white solid powder. Using qualitative analysis, design an experiment the student could perform to determine which elements are present in each of the bottles. Be sure to include possible outcomes.

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ncchemist | eNotes Employee

Posted March 16, 2013 at 2:20 PM (Answer #1)

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Let's say that the three bottles contain Fe(NO3)3, BaCl, and AgNO3.  These are iron (III) nitrate, barium chloride, and silver nitrate.  All three of these are white solids that dissolve in water.  We could come up a with a series of precipitation tests to determine which cation is in which bottle.  Based on the results of these tests, we will be able to determine the identities of the solids.

We know that silver chloride is highly insoluble in water.  So if we reacted an aqueous solution of each of the chemicals with a dilute solution of hydrochloric acid (HCl), the one with Ag+ in it will form AgCl and precipitate as a solid.  The other two samples will give no chemical reaction.  So we have identified which powder is AgNO3.

We know that barium sulfate is highly insoluble in water.  So if we reacted an aqueous solution of each of the chemicals with a dilute solution of sulfuric acid (H2SO4), the one with Ba2+ in it will form BaSO4 and precipitate as a solid.  The other two samples will give no chemical reaction. So we have identified which powder is BaCl.

We know that iron (III) hydroxide is insoluble in water. So if we reacted an aqueous solution of each of the chemicals with a dilute solution of sodium hydroxide (NaOH), the one with Fe3+ in it will form Fe(OH)3 and precipitate as a solid. The other two samples will give no chemical reaction. So we have identified which powder is Fe(NO3)3.

So we have now designed a qualitative analysis experiment in which we have matched up the identities of the three starting solids.

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